a b s t r a c ta r t i c l e i n f o
Published by Elsevier B.V.
Keywords:
1. Introduction
Fluvial sediments can contain exceptional records of climate change
at timescales ranging from less than a year (e.g., drought or ﬂood
events) to multimillennial (e.g., glacial-interglacial cycles). Because alluvial
rivers are relatively common on most of Earth's land masses, ﬂuvial
sediments are among the most accessible terrestrial records of
responses to natural and human-inﬂuenced external forcings. Alluvial
deposition and erosion are highly inﬂuenced by changes in climate,
which exerts controls on water and sediment availability, and base
level, which affects the amount of energy available to a ﬂuvial system
for transporting its water and sediment loads. This study focuses on ﬂuvial
system responses to large-scale changes in sea level and climate
over the most recent glacial-interglacial cycle. An island setting was
chosen because all of its major drainages ﬂow directly into the ocean,
so we can be reasonably conﬁdent that a direct coupling exists between
sea-level changes and river system responses, eliminating (or at least
minimizing) the inﬂuences of indirect forcings or local base levels.
The concept of base level, ﬁrst introduced by John Wesley Powell
(1875) and later reﬁned by Davis (1902); Mackin (1948); Schumm
(1993), and others, has been critically important to understanding the
physical and sedimentological responses of ﬂuvial systems to climate
change. A simple deﬁnition of base level is the lowest elevation to
which a river can erode its bed, the ultimate base level being sea level
(Powell, 1875). Of course, sea level itself is not static, having ﬂuctuated
many times over the course of Earth's history. During the Quaternary,
for example, sea level changed by more than 100 m, driven by mass
transfer between oceans and glaciers during glacial-interglacial cycles
(Murray-Wallace and Woodroffe, 2014). Other causes of relative sealevel
changes include tectonically or isostatically driven uplift or subsidence
of the land surface. When base level changes, a river may adjust
⁎ Corresponding author.
its length, width, depth, channel roughness, sinuosity, gradient, and/or
pattern in order to achieve the amount of energy needed to move its
water and sediment loads downstream (Mackin, 1948; Schumm,
1993). If channel pattern and/or hydraulic geometry changes are not
possible, or are insufﬁcient to adjust for the base-level change, the
river will aggrade or incise its bed.
In general, rivers tend to incise when base level is lowered and to
aggrade in response to a rising base level. However, rivers typically respond
to perturbations in a complex manner, in some cases even
appearing to respond in ways that are opposite of expected, making
simple cause-effect relationships difﬁcult to determine. The reason for
this is that rivers are controlled by allogenic (external) and autogenic
(internal) forcings that may be interacting on concurrent or different
temporal and spatial scales. For example, a wave of incision initiated
by falling sea level may rejuvenate tributaries, causing deposition in
the main channel. The temporarily stored sediment raises the local
base level of the tributaries, and as the tributaries readjust, their sediment
load decreases, so the main channel is once again able to transport
the accumulated sediment downstream. As sediment is ﬂushed from
the main channel, the tributaries are again rejuvenated, and the cycle
repeats (Schumm, 1973; Slingerland and Snow, 1988), resulting in a
pulsed sediment delivery that is expressed in multiple downstream depositional
events in response to the single external forcing. One product
of this may be alluvial beds that are more localized in their spatial extent
and thus are not necessarily traceable or correlative for long distances
up- or downstream.
In more general terms, our ability to recognize and correctly interpret
the evidence of climate change in the ﬂuvial sedimentary record
is confounded by the complexity of river response to external forcings.
Elements of this complexity include (i) spatial and temporal scale, in
that deposition could be occurring in one reach of a river while another
reach is experiencing erosion; (ii) convergence, in which different
causes can produce similar effects; (iii) divergence, in which similar
causes or processes can produce different responses; (iv) variable sensitivity
of systems to external forcings because of differing thresholds for
change (Schumm and Brakenridge, 1987); and (v) complex response to
external forcings due to internal system dynamics (Schumm, 1973;
Blum and Tornqvist, 2000). Because of these, ﬂuvial system responses
to climate change may appear to have quite localized extents, or responses
may seem inconsistent between neighboring or nearby drainages.
Determining a regionally coherent sequence of events, therefore,
requires a systematic approach to geologic investigations in a representative
number of rivers or drainage areas.
Santa Rosa Island, in Channel Islands National Park, California, USA,
is ideal for this study because it is relatively small in area, which allows
examination of its ﬂuvial systems in their entirety from headwaters to
mouths. It has multiple drainages, which enables collection of a rich
data set and facilitates distinguishing local from islandwide processes,
forcings, and effects. The island has an essentially uniform climate, and
all of its major drainages terminate at the ocean. Thus, we can assume
that major external forcings, i.e., climate conditions (primarily precipitation
amount, intensity, and seasonality) and sea-level changes acted
uniformly and simultaneously (at least in a geologic sense) on all of
the drainage systems on the island.
2. Physical setting
Santa Rosa Island (SRI) is the second largest of the California Channel
Islands (Fig. 1), measuring ~25 km long and 16 km wide, with an area of
215 km2
, and elevations ranging from sea level to 480 m. It is part of the
group of four east-west aligned islands that make up the northern Channel
Islands chain, located roughly 50 km southwest of Santa Barbara and
Fig. 1. Shaded relief map of Santa Rosa Island (SRI) showing names and locations of major drainages and place names. Yellow square indicates location of Black Mountain. Red line follows
the island's drainage divides. Dashed black line depicts the Santa Rosa Island fault. Points labeled 2A–D indicate location and direction of photos in Fig. 2. Inset: Location of SRI with respect
to the other Channel Islands and southern California mainland.
323
70 km west of Oxnard, CA. Santa Rosa Island is one of the ﬁve islands in
Channel Islands National Park. The island has a maritime Mediterranean
climate, with cool, rainy winters and warm, dry summers. Average annual
precipitation is ~280 mm, with roughly 75% of the total falling in
winter months (Dec–Mar) (Western Regional Climate Center, 2015).
Air temperatures on the island rarely exceed 30 °C or fall below 10 °C.
Modern plant communities include coastal sage scrub, island chaparral,
grassland, and very sparse oak and pine woodlands (Junak et al., 2007).
During and following the last glacial period, the climate of the region
was cooler and wetter, supporting an extensive coastal conifer forest of
pine, ﬁr, and cypress on adjacent Santa Cruz Island (Anderson et al.,
2008) and probably also in the uplands of SRI. By about 11.8 ka, patchy
pine woodlands, coastal sage scrub, and grasslands had replaced the forests
as the climate warmed (Anderson et al., 2010). Further drying occurred
during the early Holocene (~9–7 ka), after which a modest
recovery was reﬂected in the reestablishment of grassland vegetation.
Winter precipitation increased again slightly after ca. 4.5 ka, essentially
establishing the modern climate conditions that persist today (Cole and
Liu, 1994; Anderson et al., 2010).
Santa Rosa Island and the other three northern Channel Islands are
the emergent parts of an ~125-km-long, uplifted anticlinal platform
that, along with the Santa Monica Mountains to the east, form the
southern part of the western Transverse Ranges crustal block (Crouch,
1979; Vedder and Howell, 1980). Approximately 2000 m of mostly marine
shale, siltstone, sandstone, conglomerate, and volcaniclastic rocks
of Eocene to Miocene age, with local volcanic ﬂows and shallow intrusions,
are exposed on Santa Rosa Island (Avila and Weaver, 1969;
Weaver and Doerner, 1969; Dibblee and Eherenspeck, 1998). Most of
the pre-Pleistocene rocks were deposited in shallow- to deep-water
ocean environments that predated the island's emergence. Pleistocene
and Holocene surﬁcial deposits include colluvium and alluvium on the
slopes and ﬂoors of valleys, eolian sand sheets and dunes, marineterrace
deposits (some of which form narrow coastal plains) and modern
beach sands (Dibblee and Eherenspeck, 1998; Woolley, 1998).
Landslides are common in many areas, particularly on steeper slopes
in the southern and central parts of the island.
The island is bisected by the roughly east-west striking, subvertical
to steeply north-dipping, Santa Rosa Island fault (SRIF; Fig. 1). Leftlateral
strike-slip movement along the fault has caused the northern
part of the island to move west, or left, relative to the southern part, giving
Santa Rosa Island its distinctive, crudely parallelogram shape. Drainages
that cross the fault are deﬂected as much as 1 km to the west on the
north side of the fault, reﬂecting relatively recent (late Pleistocene and/
or Holocene) lateral movement (Colson et al., 1995). Displacement of
bedrock units suggests that the SRIF has also experienced vertical displacement,
with rock units on the south side of the fault being displaced
upward relative to the north side by as much as 400 m (e.g., Dibblee and
Eherenspeck, 1998). North of the SRIF, the landscape is dominated by
uplifted, early to middle Pleistocene marine terraces (Muhs et al.,
2014) that form a relatively ﬂat, gently seaward-sloping plain
interrupted by the valleys of northwest-ﬂowing streams. South of the
fault, the topography is steeper, more rugged, and highly dissected. Because
of the higher relative uplift on the south side of the fault, the
island's drainage divide has also shifted southward (Fig. 1). Geomorphic
evidence and orientations of striae along fault-slip surfaces suggest that
more recent fault displacement caused relative uplift of the north side of
the SRIF, particularly in the area around Black Mountain (Minor et al.,
2012; Schumann et al., 2014). The SRIF is the only fault that crosses
the entire island, although a number of secondary faults strike subparallel
to the SRIF.
Some of the more striking landscape features on Santa Rosa Island
are the incised alluvial valleys that characterize nearly all of the
northward-draining streams, as well as a few large streams elsewhere
on the island. These wide, trough-shaped valleys were ﬁlled with
Fig. 2. Photos illustrating the contrast in morphology between drainages ﬂowing north (A,B) and south (C,D) of the island's main drainage divide. (A) Cañada Verde. (B) Cañada Tecelote.
(C) Wreck Canyon. (D) Cañada La Jolla Vieja.
324
alluvium and later incised to form steep- to vertical-walled arroyos, or
barrancas, leaving a relict ﬂoodplain as much as 125 m wide and 10 or
more meters above the active modern channel (Woolley, 1998). In
many of the canyons, the alluvial ﬁll exhibits a distinctive sequence of
alternating lighter and darker beds. The physical character of these
drainage systems and their sediments, what they may indicate regarding
the response of the island's ﬂuvial systems to the extreme lowering
of sea level at the Last Glacial Maximum (LGM), and how they
responded to the subsequent return of sea level to its present elevation
during the late Pleistocene and Holocene are the focal points of this
study.
3. Field and laboratory methods
Exposures of alluvial ﬁll in several canyons on SRI were investigated
during multiple visits to the island between 2012 and 2015. The location
of each station was accurately determined by Global Positioning System
(GPS); and the section was measured, photographed, sketched, and described.
Samples of bulk sediment were collected from selected horizons
for radiocarbon dating or determination of physical properties. In
many locations, discrete fragments of charcoal or shells of the San
Miguel Shoulderband snail (Helminthoglypta ayresiana) were collected
for radiocarbon age determination.
Sediment, charcoal, and snail shell samples were submitted to the
U.S. Geological Survey's radiocarbon laboratory in Reston, Virginia, for
radiocarbon (14
C) dating, except samples Cv52-2 and CV52-4, which
were analyzed by Aeon Laboratories, LLC. Charcoal samples were subjected
to a standard acid-base-acid (ABA) treatment and were
combusted to CO2 under vacuum in the presence of CuO and Ag.
Humic acids were collected as the base-soluble fraction of the sample
material, ﬁltered through a 0.45-μm Metricel membrane ﬁlter, and
then acidiﬁed in 1 M HCl. The precipitated humic acids were then neutralized
in ultrapure water, isolated by centrifuge, dried in an oven overnight
at 70 °C, and combusted as above. Snail shells were separated
from the host sediment, placed in a beaker of ASTM Type 1, 18.2 MW
(ultrapure) water, and subjected to an ultrasonic bath for a few seconds.
The shells were then repeatedly immersed in a second beaker of ultrapure
water to remove material adhering to the shell surface or lodged
within the shell itself. This process was repeated until the shells were
visibly clean. The shells were then treated in 3% H2O2 for 24 h at room
temperature to remove all remnants of organic matter. Finally, the
shells were washed repeatedly in ultrapure water and dried in an
oven overnight at 70 °C. After cleaning, the shells were converted to
Fig. 3. Channel longitudinal proﬁles of major drainages on SRI. North-draining streams
plotted in red, south-draining streams are in blue. Lobo Canyon, which drains north, is
plotted in green. SA – San Augustin; L – Lobo; W – Wreck; JV – Jolla Vieja; G – Garañon;
T – Tecelote; S – Soledad; A – Arlington; V – Verde.
Fig. 4. Map showing locations of alluvial sections measured, described, and sampled in this study. Numbers correspond to 14
C-dated sections (Table 2) or to sections otherwise mentioned
in text.
325
CO2 using ACS reagent grade, 85% H3PO4 under vacuum at 50 °C, for
~1 h, until the reaction was visibly complete.
Sample CO2 was converted to graphite using an Fe catalyst and a
standard hydrogen reduction process. Graphite samples were pressed
into targets and submitted to the Center for Accelerator Mass Spectrometry
(CAMS) at Lawrence Livermore National Laboratory for 14
C analysis.
For most samples, a second aliquot of CO2 was isolated and
submitted to the Stable Isotope Laboratory, University of California,
Davis, for δ13
C analysis in order to correct the measured 14
C activity of
the samples for isotopic fractionation. All 14
C ages were calibrated
using the IntCal13 data set and CALIB 7.1 (Stuiver and Reimer, 1993;
Reimer et al., 2013). Ages from this study are presented in thousands
of calibrated 14
C YBP (Years Before Present; 0 YBP = 1950 CE; ka =
thousands of calibrated 14
C YBP), and uncertainties are given at the
95% (2σ) conﬁdence level. Ages from other sources are reported as published.
Bacon statistical age-depth modeling software (Blaauw and
Christen, 2011) was used to model age-depth relationships of the alluvial
sediments and to extrapolate an estimated age of alluvium at the
base (modern stream level, assumed to be at or very near the bedrock
valley ﬂoor) of the alluvial deposit at each section.
Samples of alluvial sediments from Cañada Verde were also analyzed
for particle-size distribution and organic content. The sediment
samples were passed through a 2-mm sieve to remove gravel and larger
particles, then prepared by digesting organic matter and CaCO3 using
30% H2O2 and 10% HCl, respectively. When required, (NaPO3)6 was
added to each sample as a clay dispersant. The samples were then analyzed
using a Malvern Masterizer 2000 laser particle size analyzer in the
USGS soils laboratory in Denver, CO.
Total organic matter content of sediment samples was determined
from loss on ignition (LOI) by the Colorado State University Soil,
Water, and Plant Testing Laboratory in Fort Collins, CO. The samples
were dried in an oven at 90–100 °C for 1 h, then weighed. The samples
were then heated to 550 °C for 1 h and weighed again. The difference in
the two masses, expressed in percent, represents the amount of organic
carbon lost from the samples during combustion (Dean, 1974).
Digital representations of the topography and river long proﬁles
were extracted from a 1-m resolution, LiDAR-based digital elevation
model (DEM) of the Channel Islands (USGS Channel Islands ARRA
LiDAR Project). Bathymetry data for the northern Channel Islands platform
are from the National Oceanic and Atmospheric Administration's
(NOAA) Coastal Inundation Project (Carignan et al., 2009). Data from
both sources were processed with Environmental Systems Research
Institute's ArcGIS software to develop shaded relief images of topography.
ArcHydro tools were used within ArcGIS to infer drainages from
the DEM data and extract distance-elevation data for stream proﬁles.
4. Results
The shapes and conﬁgurations of the river valleys on Santa Rosa Island
are largely a result of ﬂuvial dissection of landscapes created by tectonic
and eustatic processes. North of the Santa Rosa Island fault, the
upland surfaces of the island are the remnants of uplifted marine terraces,
and the interﬂuves are broad, relatively ﬂat, and gently seaward
sloping. The major drainages on this side of the island trend to the
northwest, and the valleys are relatively broad, with a trough-like appearance,
containing remnants of alluvial ﬁll in the form of narrow to
broad alluvial terraces that typically stand 2–14 m above the present
stream channels (Fig. 2A, B). South of the SRIF, the relief is greater, the
valleys are generally deeper and more V-shaped, and the interﬂuves
are typically narrow ridges (Fig. 2C, D). Most of the bedrock valleys in
the southern part of the island also contain alluvial ﬁll, but the terraces,
where present, are discontinuous and are only found in wider sections
of the valley. The north- and south-draining streams also exhibit different
channel longitudinal proﬁles (Fig. 3). The south-draining streams
are shorter in overall length because of the southward offset of the
island's drainage divide, and have steeper gradients than the northTable
1
Station location information.
Station # Latitude (°N) Longitude (°W) Elevation (m) Dated by 14
C?
Cherry Canyon
CC-56 34.0010 120.0584 33 N
Cañada Garañon
GC-51 33.9945 120.1927 36 N
Cañada La Jolla Vieja
JV-17 33.9143 120.0736 41 N
JV-18 33.9153 120.0744 28 Y
JV-19 33.9271 120.0944 34 N
JV-20 33.9278 120.0961 124 N
JV-21 33.9338 120.0986 121 N
Old Ranch Canyon
ORC-53 33.9643 119.9876 4 N
ORC-54 33.9668 119.9917 9 N
ORC-55 33.9702 119.9999 15 N
Quemada Canyon
QC-11 33.9625 120.0131 52 Y
QC-12 33.9673 120.0239 72 Y
Cañada Soledad
CS-08 33.9904 120.1414 70 N
CS-09 34.0093 120.1532 39 N
CS-10 34.0062 120.1527 40 N
CS-34 33.9859 120.1371 79 Y
Cañada Tecelote
TC-37 33.9688 120.1538 111 N
TC-38 33.9721 120.1564 96 Y
TC-39 33.9710 120.1601 98 N
TC-40/41a
33.9847 120.1696 56 Y
TC-42 33.9886 120.1708 52 N
TC-43 33.9928 120.1731 38 Y
TC-44 33.9922 120.1720 39 N
TC-45 33.9972 120.1775 31 Y
TC-46 33.9978 120.1779 32 Y
Cañada Verde
CV-01 34.0147 120.1195 26 Y
CV-02 34.0107 120.1130 50 Y
CV-03 34.0034 120.1099 60 Y
CV-04 33.9776 120.0952 136 N
CV-05 33.9825 120.1003 121 Y
CV-06 33.9890 120.1078 106 N
CV-07 33.9975 120.1134 78 Y
CV-13 33.9735 120.0871 183 N
CV-14 33.9736 120.0896 170 N
CV-15 33.9737 120.0911 159 Y
CV-16 33.9759 120.0950 151 N
CV-35 34.0136 120.1163 119 Y
CV-52 34.0042 120.1091 49 Y
Water Canyon
WC-22 33.9636 120.0568 125 N
WC-23 33.9655 120.0530 116 N
WC-24 33.9673 120.0507 125 N
WC-25 33.9706 120.0465 99 N
WC-26 33.9771 120.0487 76 N
WC-31 33.9849 120.0511 45 N
WC-32 33.9769 120.0460 63 N
WC-33 33.9857 120.0515 37 N
Windmill Canyon
WMC-57 34.0041 120.0683 57 N
WMC-58 34.0046 120.0714 64 Y
Wreck Canyon
RC-27 33.9390 120.0761 104 Y
RC-28 33.9376 120.0717 120 Y
RC-29 33.9313 120.0642 70 Y
RC-30 33.9258 120.0620 61 N
RC-36 33.9342 120.0667 82 N
a
Stations TC-40 and TC-41 were located ~10 m apart on opposite sides of an active
wash.
326
draining streams. A standout exception to this pattern is Lobo Canyon,
which is in the north, but plots in the grouping with the southwarddraining
streams (L, Fig. 3). Lobo Canyon heads at Black Mountain in
an area thought to be experiencing recent tectonic uplift associated
with the SRIF (Minor et al., 2012; Schumann et al., 2014). Also unlike
the other northward-draining streams, Lobo Canyon is mostly devoid
of alluvial ﬁll, except in its lowest ~0.5 km, in which only sparse, eroded
remnants of alluvial terraces remain. Thus, Lobo Canyon resembles the
south-draining stream systems more than the adjacent north-draining
streams.
We examined 54 exposures of alluvial ﬁll in 10 canyons on SRI between
2012 and 2015 (Fig. 4, Table 1). These drainages contain alluvial
deposits ranging from ~2 to ~14 m thick, into which arroyos have been
incised and through which the modern streams ﬂow (Fig. 2). In most
places, the modern streams appear to be ﬂowing on the bedrock valley
ﬂoor surface or on a thin (typically b0.5 m) veneer of modern alluvium.
The alluvial terrace deposits typically consist of horizontal to
subhorizontal, alternating lighter and darker beds of sediment of varying
thickness, with occasional beds or lenses of gravel or cobbles
(Fig. 5). The darker beds generally contain silty clay or clay and organic
Fig. 5. Photos of alluvial sections in Cañada Verde, Cañada Tecelote, and Wreck Canyon. Photos for each canyon are presented in order, proceeding upstream. Except for photo 0, numbers
on photos indicate the station number, keyed to locations shown on Fig. 4. Photo 0 taken ~1.5 km downstream of station 1, near the mouth of Cañada Verde at the coast. At this location, the
channel has incised into sandstone bedrock. The incision likely happened during a period of low sea level. At stations 5 and 15, the dashed yellow line follows erosional base of channel ﬁll.
At station 16, note coarse-grained colluvium interbedded with ﬂoodplain alluvium (yellow arrows).
327
Fig. 6. Sedimentary sections, calibrated ages, particle-size data, and organic content of sediments for three sections in Cañada Verde. Colors approximate actual color of sediments. Length
of bars indicate dominant grain size of each unit ranging from clay (c) to gravel (g).
328
Table 2
Summary of AMS sample information, carbon-14 ages, and calibrated ages.a
Depth δ13
C 14
C age Intercept 1 Intercept 2 Intercept 3 Median ageSample # Lab #b
AMS # Station Material datedc
(m)d
(‰ vpdb)e
(14
C ka BP) Age (cal ka BP)f
Pg
Age (cal ka BP)f
Pg
Age (cal ka BP)f
Pg
(cal ka BP)h
Cañada La Jolla Vieja
JV18-1a WW-9874 CAMS-166416 18 shell 0.95 −4.3 1.30±0.04 1.24±0.06 1.00 1.24
Quemada Canyon
QC11-8 WW-9425 CAMS-161484 11 humic acids 1.75 −25.6 0.95±0.03 0.86±0.06 1.00 0.85
QC11-7 WW-9194 CAMS-159705 11 charcoal 2.60 −25.4 1.14±0.03 1.03±0.06 0.88 1.13±0.02 0.08 1.03
QC11-7 WW-9273 CAMS-160270 11 humic acids 2.60 −25.3 1.08±0.03 0.97±0.04 0.73 1.04±0.02 0.27 0.98
QC11-3 WW-9271 CAMS-160280 11 charcoal 3.15 −25 1.06±0.04 0.97±0.04 0.84 1.04±0.02 0.16 0.96
QC11-4 WW-9193 CAMS-159703 11 charcoal 4.20 −22.3 1.31±0.03 1.20±0.01 0.28 1.26±0.04 0.72 1.25
QC11-4 WW-9308 CAMS-160421 11 humic acids 4.20 −25 1.59±0.04 1.48±0.08 1.00 1.47
QC11-1 WW-9192 CAMS-159702 11 charcoal 4.30 −27.0 1.19±0.03 1.12±0.06 0.98 1.12
QC11-1 WW-9272 CAMS-160281 11 humic acids 4.30 −26.3 1.45±0.03 1.34±0.04 1.00 1.34
QC12-2 WW-9885 CAMS-166429 12 charcoal 4.75 −26.3 9.57±0.03 10.81±0.07 0.45 11.00±0.08 0.55 10.95
Cañada Soledad
SC34-B170 WW-9922 CAMS-167390 34 humic acids 1.50 −25.5 1.19±0.03 1.12±0.06 0.98 1.12
SC34-1 WW-9884 CAMS-166428 34 charcoal 4.50 −25.5 8.60±0.03 9.57±0.05 1.00 9.55
Cañada Tecelote
TC38-1 WW-9883 CAMS-166427 38 charcoal 9.00 −25.1 12.19±0.03 14.08±0.11 1.00 14.08
TC40-1 WW-9881 CAMS-166425 40 shell 9.20 −8.3 10.57±0.03 12.6±0.04 0.25 12.58±0.06 0.75 12.55
TC41-10 WW-10167 CAMS-168755 41 charcoal 8.85 −25 11.15±0.04 13.02±0.09 1.00 13.04
TC41-9 WW-10101 CAMS-168388 41 charcoal 9.60 −23.5 12.50±0.05 14.69±0.36 1.00 14.72
TC41-5 WW-10179 CAMS-168767 41 charcoal 10.10 −25 9.65±0.04 10.88±0.09 0.47 11.13±0.06 0.51 11.04
TC41-6 WW-10100 CAMS-168387 41 charcoal 10.74 −24.0 10.92±0.03 12.77±0.06 1.00 12.77
TC41-7 WW-10102 CAMS-168389 41 charcoal 11.04 −22.7 10.77±0.03 12.70±0.04 1.00 12.70
TC41-1 WW-9882 CAMS-166426 41 charcoal 11.25 −25.2 10.94±0.03 12.78±0.06 1.00 12.77
TC43-1 WW-9760 CAMS-165479 43 Physa shell 0.70 −1.7 0.36±0.03 0.36±0.04 0.48 0.46±0.04 0.52 0.43
TC43-4 WW-9761 CAMS-165480 43 shell 4.05 −7.9 0.77±0.03 0.70±0.03 1.00 0.69
TC45-4 WW-10085 CAMS-168374 45 shell 3.80 −10.8 8.84±0.03 9.86±0.10 0.59 10.01±0.03 0.09 10.10±0.05 0.32 9.92
TC45-3 WW-9763 CAMS-165482 45 shell 5.30 −9.8 9.96±0.03 11.33±0.08 0.84 11.46±0.03 0.08 11.57±0.03 0.08 11.35
TC45-2 WW-9762 CAMS-165481 45 shell 5.80 −9.1 10.03±0.03 11.49±0.16 0.93 11.69±0.02 0.07 11.51
TC46-1 WW-10170 CAMS-168758 46 charcoal 4.25 −25 13.33±0.04 16.02±0.18 1.00 16.03
TC46-2 WW-10172 CAMS-168760 46 humic acid 4.80 −24.1 13.57±0.03 16.36±0.17 1.00 16.34
Cañada Verde
CV1-B6 WW-9779 CAMS-165497 1 bulk organics 0.80 −19.3 4.50±0.03 5.12±0.08 0.65 5.25±0.04 0.35 5.17
CV1-10 WW-9875 CAMS-166417 1 shell 2.60 −10.6 7.11±0.03 7.88±0.02 0.18 7.95±0.03 0.82 7.95
CV1-B4 WW-9778 CAMS-165496 1 bulk organics 4.60 −25.0 9.80±0.04 11.22±0.03 1.00 11.22
CV1-4 WW-9889 CAMS-166433 1 charcoal 5.55 −23.8 10.15±0.04 11.86±0.16 0.95 11.83
CV1-9 WW-9890 CAMS-166434 1 charcoal 7.05 −25.7 9.83±0.03 11.23±0.02 1.00 11.23
CV1-2 WW-9964 CAMS-167494 1 charcoal 7.78 −25 9.77±0.03 11.20±0.03 1.00 11.21
CV1-8 WW-9965 CAMS-167495 1 charcoal 8.16 −25 9.84±0.03 11.23±0.03 1.00 11.24
CV2-1 WW-9423 CAMS-161482 2 humic acids 1.45 −24.9 9.22±0.04 10.35±0.09 0.86 10.47±0.02 0.14 10.37
CV52-1i
WW-10491 CAMS-171266 52 charcoal 9.10 −25 8.32±0.04 9.35±0.11 0.95 9.35
CV52-2i,j
P-1519A Aeon-2111 52 conc organics 10.05 −25 9.80±0.05 11.22±0.06 1.00 11.22
CV52-4i,j
P-1519C Aeon-2113 52 conc organics 11.80 −25 11.27±0.05 13.15±0.10 1.00 13.13
CV3-B38 WW-9395 CAMS-161188 3 humic acids 1.05 −25.5 1.74±0.03 1.58±0.01 0.06 1.65±0.06 0.94 1.65
CV3-12 WW-9382 CAMS-161173 3 shell 1.06 −8 1.10±0.04 1.00±0.07 1.00 1.00
(continued on next page)
329R.R.Schumannetal./Geomorphology268(2016)322–340
Depth δ13
C 14
C age Intercept 1 Intercept 2 Intercept 3 Median ageSample # Lab #b
AMS # Station Material datedc
(m)d
(‰ vpdb)e
(14
C ka BP) Age (cal ka BP)f
Pg
Age (cal ka BP)f
Pg
Age (cal ka BP)f
Pg
(cal ka BP)h
CV3-9 WW-9381 CAMS-161172 3 shell 2.30 −8.6 2.50±0.03 2.61±0.12 1.00 2.59
CV3-B34 WW-9394 CAMS-161187 3 humic acids 2.30 −25.7 3.23±0.03 3.43±0.05 0.97 3.44
CV3-B29 WW-9393 CAMS-161186 3 humic acids 3.75 −25.6 3.76±0.03 4.02±0.02 0.09 4.12±0.04 0.76 4.21±0.02 0.11 4.12
CV3-7 WW-9380 CAMS-161171 3 shell 3.80 −7.1 3.52±0.03 3.79±0.09 1.00 3.78
CV3-4 WW-9379 CAMS-161170 3 shell 6.10 −9.4 5.60±0.03 6.36±0.05 0.99 6.36
CV3-B23 WW-9392 CAMS-161185 3 humic acids 6.40 −25.7 6.11±0.03 6.96±0.07 0.90 7.13±0.02 0.10 6.97
CV3 -B19 WW-9391 CAMS-161184 3 humic acids 7.64 −26.1 7.51±0.03 8.24±0.02 0.11 8.35±0.05 0.89 8.35
CV3-3 WW-9378 CAMS-161169 3 shell 7.67 −9.5 7.62±0.03 8.41±0.03 1.00 8.41
CV3-16 WW-9764 CAMS-165483 3 shell 8.26 −8.9 7.98±0.03 8.86±0.14 1.00 8.87
CV3-19 WW-9765 CAMS-165484 3 shell 8.93 −9.1 8.45±0.04 9.48±0.05 1.00 9.48
CV3-21 WW-9766 CAMS-165485 3 shell 9.44 −13.6 8.71±0.03 9.64±0.09 1.00 9.64
CV5-B44 WW-9887 CAMS-166431 5 bulk organics 1.80 −25.2 0.93±0.03 0.85±0.06 1.00 0.85
CV5-B174 WW-9891 CAMS-166435 5 bulk organics 2.20 −24.3 5.17±0.03 5.92±0.03 0.85 5.98±0.01 0.15 5.92
CV5-B174 WW-9915 CAMS-167383 5 humic acids 2.20 −25.0 5.31±0.03 6.09±0.09 1.00 6.09
CV5-1 WW-9424 CAMS-161483 5 humic acids 5.20 −24.6 6.67±0.03 7.54±0.05 1.00 7.54
CV5-B41 WW-9923 CAMS-167391 5 humic acids 6.73 −25.7 7.62±0.03 8.41±0.03 1.00 8.41
CV7-2 WW-9196 CAMS-159707 7 charcoal 5.95 −25.9 4.49±0.03 5.16±0.12 1.00 5.18
CV7-2 WW-9270 CAMS-160279 7 humic acids 5.95 −25 4.41±0.03 4.96±0.09 0.95 4.98
CV15-B119 WW-10003 CAMS-167751 15 bulk organics 1.00 −25.3 1.47±0.03 1.36±0.05 1.00 1.36
CV15-1 WW-9886 CAMS-166430 15 discrete organics 4.60 −23.5 1.74±0.03 1.58±0.01 0.06 1.65±0.06 0.94 1.65
CV15-B120 WW-9780 CAMS-165498 15 bulk organics 10.90 −25 7.79±0.04 8.48±0.02 0.10 8.57±0.06 0.90 8.57
CV35-B178 WW-9920 CAMS-167388 35 humic acids 0.90 −26.6 0.48±0.03 0.52±0.02 1.00 0.52
CV35-B178 WW-9921 CAMS-167389 35 bulk organics 0.90 −26.7 0.54±0.03 0.54±0.02 0.81 0.62±0.01 0.19 0.54
CV35-1 WW-9873 CAMS-166415 35 shell 3.35 −9.9 6.99±0.04 7.80±0.08 0.81 7.91±0.02 0.19 7.83
Windmill Canyon
WMC58-1 WW-10487 CAMS-171262 58 charcoal 2.00 −23.9 2.26±0.04 2.21±0.06 0.65 2.32±0.03 0.35 2.24
Wreck Canyon
RC27-B153b WW-9892 CAMS-166436 27 humic acids 0.30 −25.7 0.51±0.03 0.53±0.02 1.00 0.53
RC27-B153a WW-9888 CAMS-166432 27 bulk organics 0.30 −25.7 0.68±0.03 0.58±0.01 0.33 0.66±0.02 0.67 0.66
RC27-B152 WW-9917 CAMS-167385 27 humic acids 2.20 −25.7 2.24±0.03 2.21±0.06 0.75 2.32±0.02 0.25 2.23
RC27-1 WW-9876 CAMS-166420 27 shell 4.20 −7.0 4.69±0.03 5.37±0.05 0.66 5.45±0.02 0.23 5.56±0.02 0.10 5.40
RC28-B157 WW-9919 CAMS-167387 28 humic acids 0.30 −25.5 1.63±0.03 1.44±0.02 0.23 1.54±0.03 0.69 1.54
RC28-2b WW-9878 CAMS-166422 28 shell 2.70 2.7 3.41±0.03 3.64±0.06 1.00 3.65
RC28-1b WW-9877 CAMS-166421 28 shell 3.10 1.2 3.39±0.04 3.64±0.08 0.99 3.63
RC29-B180 WW-9918 CAMS-167386 29 humic acids 0.90 −24.9 1.53±0.03 1.39±0.04 0.64 1.49±0.03 0.34 1.41
RC29-1 WW-9916 CAMS-167384 29 humic acids 4.50 −24.8 2.85±0.03 2.95±0.08 0.99 2.95
RC29-3 WW-9879 CAMS-166423 29 shell 8.40 −6.9 4.78±0.03 5.52±0.04 0.85 5.58±0.01 0.15 5.52
RC29-4 WW-9880 CAMS-166424 29 shell 9.70 −8.0 4.72±0.03 5.37±0.04 0.49 5.46±0.02 0.22 5.55±0.03 0.29 5.45
RC29-2 WW-9781 CAMS-165499 29 charcoal 11.95 −26.7 7.52±0.04 8.24±0.02 0.10 8.35±0.05 0.90 8.35
a
Uncertainties for the calibrated ages are given at the 2σ (95%) conﬁdence level. All other uncertainties are presented at 1σ (68%).
b
WW = USGS Radiocarbon Laboratory, Reston VA; P = USGS Desert Research Laboratory, Denver CO.
c
shell = Helminthoglypta ayersiana; bulk/discrete organics refers to the base-insoluble fraction of the organic matter; humic acids refers to the base-soluble fraction.
d
Depth from top of exposure.
e
Data reported for δ13C with a single decimal place represent measured values. Those without decimal places are assumed values.
f
Calibrated ages were calculated using CALIB v.7.1 in conjunction with the IntCal13.14C dataset; limit 50 ka cal BP. Calibrated ages are reported as the midpoint of the calibrated range. Uncertainties are reported as the difference between the
midpoint and either the upper or lower limit of the calibrated age range, whichever is greater. Ages are reported when the probability of a calibrated age range exceeds 0.05.
g
P = probability of the calibrated age falling within the reported range as calculated by CALIB.
h
Median age as calculated by CALIB.
i
Same stratigraphic section as Pinter et al. (2011). Sample CV52-1 was taken from approximately the same depth as their sample #SRI-07-P3.
j
Following the initial acid step, these samples were treated at the USGS Desert Research Laboratory with HF and HCl to remove silicates and concentrate organic material. The samples were then submitted to Aeon Laboratories, LLC and treated in an
identical manner to the others with the exception that combustion of the concentrated organic material was done online.
Table 2(continued)
330R.R.Schumannetal./Geomorphology268(2016)322–340
matter, whereas the lighter-colored beds are composed primarily of silt
and sand and do not contain as much organic matter. The darker sediments
appear to represent ﬂoodplain paleosols that developed during
relatively stable and (or) wetter periods, when lush grasses or wetlands
covered the ﬂoodplains, or perhaps organic-rich, low-energy channel
sediments. The lighter sediments are interpreted as channel-margin or
ﬂoodplain aggradational deposits. Channel-ﬁll sequences containing
sand and gravel are locally found cutting into the ﬂoodplain deposits
(Fig. 5, stations 5, 15).
Some discrete beds can be traced up- or downstream for hundreds of
meters, whereas in other locations the sedimentary sequences appear
to be quite localized in lateral extent. Sediments with more continuity
in the up/downstream direction tend to be horizontally bedded, suggesting
longer downstream transport distances. Beds that are dipping
toward the center of the valley suggest more localized contribution of
sediment from valley side slopes or from tributary drainages, probably
with shorter downstream transport distances. Proximal to valley walls
(or where the valleys are relatively narrow) gravel, cobbles, and boulders
of bedrock from slope wash and mass movements form lenses or
beds of colluvium that may be mixed with the axial alluvial sediments
(Fig. 5, station 16).
Plots of particle size and organic content of sediments for three sections
in Cañada Verde illustrate the episodic deposition of the alluvial
deposits and display an inverse relationship between sand and silt content
in sediments (Fig. 6). The proportion of clay generally varies between
0 and 20%, and clay content roughly follows silt content in
sediments. The alluvium also contains 5–15% organic matter, with
higher concentrations of organics in the darker-colored, ﬁner-grained
sediments (Fig. 6), strongly suggesting that the darker shades of the
sediments are related to the presence and abundance of organic material,
in contrast to the ﬁndings of Pinter et al. (2011). There appears to be a
very slight overall coarsening of sediments in the downstream direction,
with a corresponding slight decrease in organic content (Fig. 6).
This is contrary to the expected downstream ﬁning of sediments and
may be caused by input from local landslides and slopewash that are
relatively common along the valley walls (Fig. 5, station 16). Such activities
would contribute coarse sediment to the alluvium along the length
of the canyon, partially negating the stream's downstream sorting processes.
We did not analyze sediments from other drainages for particle
size and organic content, so we were unable to determine if this trend
applies to other drainages on the island, or if the observed trend is related
solely to ﬂuvial processes, contributions of coarser sediment from
valley side slopes and tributaries, differences in the bedrock source of
sediment, or other factors.
Calibrated 14
C ages obtained from charcoals, humic acids, plant material,
and terrestrial gastropod shells are given in Table 2. Except for
several late Pleistocene samples from Cañada Tecelote, the alluvium
ranges from early to late Holocene in age. In sections lacking obvious
unconformities such as channel ﬁlls, the 14
C ages of samples generally
increase with increasing depth, within the ranges of analytical
uncertainties.
We analyzed a subset of the total suite of samples collected, focusing
primarily on three canyons with well-expressed sedimentary records:
Verde and Tecelote Canyons, which drain to the north, and Wreck Canyon,
which drains to the south coast of the island (Figs. 4 and 5). The estimated
age of the base of each alluvial deposit (Fig. 7) should indicate
the time at which aggradation began at that location. At station TC41,
the samples were collected in a relatively narrow range of depths and
the ages do not linearly increase with depth, so the oldest of the samples
(14.72 ka) was chosen to represent the estimated age of the base of the
deposit. Likewise, at station TC38, only one sample was dated, and it is
within 25 cm of the base of the deposit; its age was used as the estimated
basal age. The age-depth curve for station CV5 is interrupted by a cutand-ﬁll
channel sequence at ~200 cm depth (Fig. 5), but below the erosional
unconformity, the rate of sediment accumulation appears to have
been relatively constant (Fig. 7). Sample CV15-1 (Table 2) was collected
within a channel ﬁll deposit (Fig. 5) and did not form part of a continuous
depositional sequence, so it was excluded from the age-depth plot
(Fig. 7).
Vertical aggradation rates can be inferred from the sediment data at
each exposure. An average sediment accumulation rate was calculated
by dividing the vertical distance between the youngest and oldest samples
at each location by the difference in age of the samples. Observed
vertical sediment accumulation rates range from 60 to nearly
200 cm ka−1
. Where the age-depth curves presented in Fig. 7 are relatively
linear, we assume that the aggradation rate was, on average, relatively
constant, and that rapid, short-duration depositional or erosional
events are not discernible within the resolution of sampling. However,
the multiple depositional sequences at each section suggest that episodic,
not continuous, aggradation built the ﬂoodplains. The varying thickness
of aggradational sequences implies that the ﬂood events were of
varying magnitudes. The varying degree of paleosol development at
the top of each sequence further suggests that the time period between
overbank ﬂoods was also variable. Thus over the short term (tens to
hundreds of years), ﬂoodplain aggradation rates would be expected to
vary greatly. Over longer time intervals (thousands of years), however,
the vertical aggradation rates appear to have been surprisingly constant
(Fig. 7). For example, stations CV3, RC29, and TC45/46 have long sedimentary
records interpreted from multiple samples. At each of these
stations, a straight (or nearly straight) line can be ﬁtted to the agedepth
data (Fig. 7), indicating long-term consistency of vertical aggradation
rates. No obvious erosion surfaces or unconformities were
noted within these sections, although station 45/46 is a composite of
two nearby sections.
At station CV1, the rate was calculated only for the upper ~400 cm,
below which there appears to be an unconformity that is visible in the
ﬁeld and also indicated by a change in slope of the age-depth plot
(Fig. 7). Below about 400 cm, all of the ages are very similar regardless
of depth, suggesting that a rapid depositional event took place between
about 12 and 11 ka at that location. At stations TC38 and TC41, the aggradation
rate calculation is based on the depth and age of the oldest
(or only) sample and assumes that the upper surface of the alluvium
is modern, with negligible erosion. At these two locations, the aggradation
rates should be considered minima.
Plots of the estimated maximum age of each alluvial section against
distance of each section upstream from the river's mouth at the coastline
show trends of decreasing age with distance upstream (Fig. 8). Linear
regressions were ﬁtted to the data in order to identify and quantify
trends; however, the regressions are based on only 3–4 points each, insufﬁcient
to conﬁrm that the relationships are indeed linear. In fact, the
age-distance relationships may be considerably more complex. Nevertheless,
the relatively good ﬁt of simple regression lines to the data facilitates
comparing trends between drainages and allows more informed
speculation about the interaction and timing of sea-level rise and alluvial
backﬁlling. The slope of the lines indicates the rate at which
backﬁlling of sediment progressed upstream—the steeper the slope,
the more slowly the wave of aggradation progressed upstream. It
would be logical to expect that the rate of backﬁlling is related to physical
characteristics of the drainages, one of the most obvious being channel
gradient. Indeed, Cañada Verde, which has the lowest overall
channel gradient of the three drainages plotted in Fig. 8, backﬁlled
more rapidly than Tecelote Canyon, which has a slightly steeper gradient
(Fig. 3). Wreck Canyon, with the steepest channel gradient of the
three (Fig. 3), backﬁlled most slowly (Fig. 8). Local variations in gradient
and valley morphology, or obstructions in the channel such as bedrock
outcrops, would also affect the progression of sediment backﬁlling upstream.
Assuming that all sediment was transported downstream until
it reached the advancing edge of the aggradational wedge, the location
and age of sediment at the base of each alluvial section may document
the progression of backﬁlling in each canyon (Fig. 8). Extending the regression
lines in Fig. 8 to the modern shoreline (i.e., the X intercepts of
each regression) suggests that backﬁlling alluvium would have reached
331
the positions of the future modern shoreline, i.e., the canyon mouths, at
~19.5 ka in Tecelote Canyon, at ~12.1 ka in Verde Canyon, and at
~11.3 ka in Wreck Canyon.
5. Discussion
5.1. Reconstructing paleoshorelines and paleochannels
Because sea level is the local base level for the island's river systems,
ﬂuctuations of sea level exert a strong control on the timing and
processes of erosion and aggradation. During the late Pleistocene and
Holocene, eustatic sea level varied from as much as 8 m above present
during the last interglacial (LIG) period about 120 ka (Muhs et al.,
2011), to between −120 and −135 m below present sea level at the
last glacial maximum (LGM) about 20 ka (Yokoyama et al., 2000;
Clark and Mix, 2002; Peltier and Fairbanks, 2006). However, sea level
at a speciﬁc location is also inﬂuenced by local or regional isostatic, tectonic,
and gravitational movements of the Earth's crust. For example,
relative sea level is affected by deﬂections of the ocean ﬂoor caused by
isostatic movement of the solid earth in response to ice-ocean mass
Fig. 7. Age-depth plots for alluvial sections in Cañada Verde, Cañada Tecelote, and Wreck Canyon. Solid lines were determined using the Bacon computer program (Blaauw and Christen,
2011). Dashed lines are hand-drawn estimates.
332
transfer during Quaternary glacial-interglacial cycles. The ocean surface
elevation is also changed by gravitational attraction between glacial ice
and ocean water, ocean water with itself, and between ocean water and
land masses, which themselves are being deformed by ice or ocean
loading or unloading (Farrell and Clark, 1976; Mitrovica and Milne,
2003; Milne and Mitrovica, 2008). Expansion and contraction of glacial
ice sheets also affects the earth's rotation, which in turn, affects tides
worldwide (Milne and Mitrovica, 1996). These local or regional departures
from eustacy have been termed glacial isostatic adjustment
(GIA) effects.
A relative sea level (RSL) curve that incorporates adjustments for
GIA effects has been modeled for the Channel Islands of California, and
speciﬁcally for Santa Rosa Island (Fig. 9; Clark et al., 2014). The GIAadjusted
sea level curve differs signiﬁcantly from the eustatic sea level
curve at several points. For example, whereas the eustatic curve indicates
that sea level at LGM was ~ − 130 m below present, the LGM
sea level at SRI reached a minimum of ~ − 106 m. After 13 ka, the
modeled RSL at SRI is as much as ~15 m lower than the eustatic curve,
and RSL approaches modern sea level 2–3 ka later than the eustatic
curve (Fig. 9).
The GIA-adjusted RSL curve was used with a DEM of bathymetry
data to determine the locations of paleoshorelines around SRI at various
times as sea level rose from its local LGM minimum to its present level
(Fig. 10). The accuracy of the paleoshoreline reconstruction is highly dependent
on the accuracy of the sea-level curve used. Because of the gentle
slope of the shelf surrounding SRI, elevation differences of 15–30 m
between the eustatic sea-level curve and a localized, GIA-adjusted
curve could result in errors of hundreds to thousands of meters in the
inferred paleoshoreline positions.
When sea level was lower than present, large areas of the shelf surrounding
SRI were exposed, extending the channel lengths and drainage
basin areas of streams on the island. ArcHydro Tools were used
within ArcGIS to generate drainage lines on SRI and on the shelf surrounding
the island by routing ﬂow from DEM grid cells of higher elevation
into lower elevation grid cells. Predictably, the modeled ﬂow lines
follow the existing drainages on the island. On the shelf, however, the
ﬂowpaths are very straight. The paleochannels on the shelf would probably
be more sinuous than indicated by the modeled ﬂowpaths, because
as sea level rose following the LGM, the streams would likely have initially
reduced their gradients by increasing sinuosity via lateral channel
migration, which would also have increased valley width. Fluvial valleys
on continental shelves are typically as much as 1–2 orders of magnitude
wider than they are deep (Zaitlin et al., 1994). Modeling studies as well
as direct observations suggest that deep valleys are generally not incised
into a shelf unless sea level drops below the outer edge of the shelf (Koss
et al., 1994; Woolfe et al., 1998; Ethridge et al., 2005), which did not
occur at LGM around the northern Channel Islands. Possible topographic
remnants of these valleys that are 1–2 km wide and 5–20 m deep can
be seen in a cross section of the shelf surface north of Santa Rosa Island
(Fig. 11). The drainage lines modeled by ArcGIS follow these shallow topographic
lows.
Longitudinal proﬁles of three streams modeled using the GIS tools
(Fig. 12) exhibit typical concave-up proﬁles for reaches above present
sea level (0 m), but the now-submerged portions of the proﬁles follow
the gradient of the marine shelf. This probably reﬂects partial to nearly
complete ﬁlling of the stream channels with alluvial, eolian, and/or marine
sediment. Such sediment-ﬁlled paleochannels might be located by
seismic surveys or offshore coring, but in the absence of such data, we
cannot determine if the modeled ﬂowpaths on the shelf correspond to
the actual locations and paths of the paleochannels. The ﬂowpaths are,
however, acceptable for the purposes of this study. Because the
paleochannels would likely have been more sinuous than the modeled
ﬂowpaths, the ﬂowpaths suggest a best-case scenario, i.e., the steepest
possible channel gradients and shortest channel lengths of the offshore
portions of the streams. Longer, less steep channels would further enhance
the magnitude of aggradation and backﬁlling, but otherwise,
the behavior of the system would be the same or extremely similar, so
the modeled ﬂowpaths are sufﬁcient to illustrate the physical conditions
on the shelf that dictated the ﬂuvial systems' responses to sealevel
changes.
5.2. Late Pleistocene aggradation on the shelf
The marine platform surrounding Santa Rosa Island has a considerably
shallower gradient than those of the streams that drain onto the
shelf (Fig. 12). This difference reﬂects the dominance of marine, rather
Fig. 8. Plots of estimated maximum age of alluvial stack versus distance upstream from
modern shoreline for three drainages on SRI.
Fig. 9. Modeled relative sea-level curve for Santa Rosa Island (34°N, 120°W) incorporating
GIA effects (in blue). For comparison, red curve is modeled global eustatic sea-level curve,
based on glacial ice volume and mass-transfer models developed at Australian National
University (ANU; Fleming and Lambeck, 2004). Redrawn from Clark et al. (2014).
333
than terrestrial, landscape-forming processes. The gentle seaward slope
and relatively ﬂat topography of marine shelves resulted from the longterm
erosional and depositional forces of waves, tidal ﬂuctuations,
storms, and nearshore ocean currents (e.g., longshore drift). These processes
can either erode the marine platform surface or redistribute sediment
across the platform in response to localized excesses or
deﬁciencies of sediment relative to the energy of the ocean system.
Other processes, such as tectonics and ﬂuvial erosion/deposition, can
modify the submerged marine platform surface, but subaerial processes
become more important when the shelf is exposed during sea-level
lowstands.
The proximal, or nearshore, portion of the marine platform also appears
to have more topographic relief than the distal, or seaward, part
(Figs. 10 and 11). Lobate features on the platform may be wedges of deltaic
sediment deposited during the period of rapidly rising sea level
(Fig. 10; Posamentier and Vail, 1988; Wright and Marriott, 1993). However,
some of the topographic features on the platform may be remnants
of eolian duneﬁelds. Modeling by J.X. Mitrovica in Muhs et al.
(2012) indicates that sea level was −55 to −75 m during marine oxygen
isotope stage (MIS) 3 (~60–30 ka), after which sea level dropped to
its LGM minimum, then rose again, reaching the break in slope between
the nearshore platform and distal shelf at about 9–10 ka (Figs. 9, 10 and
12). Thus the proximal shelf was exposed from ~60 ka until ~10 ka. Eolian
sand on SRI and the other Channel Islands is derived largely from
eroded fragments of calcium-carbonate-rich, marine-invertebrate skeletal
material from the sea ﬂoor. Prevailing winds from the northwest
deﬂated these sands from the exposed shelves and blew them onto
what are presently the land areas (Muhs et al., 2009). However, remnants
of duneﬁelds may have survived deﬂation and subsequent marine
incursion on the platform and may form some of the topography offshore
of Sandy Point and Carrington Point (Fig. 11). These dunes may
have also ﬁlled and obscured some of the ﬂuvial channels on the proximal
shelf.
An alternative, and perhaps more likely, interpretation of the features
on the shelf is that they are lobes of delta sediments. Fluvial aggradation
on the shelf would be expected to occur in a considerably
different manner than aggradation within the island's valleys because
the channels are relatively unconﬁned on the shelf, whereas they are
constrained within their valleys on land. As sea level rose and the shoreline
progressed landward (Figs. 10 and 12), the streams would most
likely have built relatively extensive deltas, spreading sediment laterally
across the shelf rather than backﬁlling a single channel (Posamentier
and Vail, 1988; Wright and Marriott, 1993; Zaitlin et al., 1994). The extent
of delta building by each drainage would inﬂuence the rate at
Fig. 10. Bathymetric/topographic map of Santa Rosa Island and the surrounding marine shelf. Elevation data from NOAA (Carignan et al., 2009). Paleoshorelines shown in yellow; dates and
elevations from Fig. 9 (Clark et al., 2014). Light blue lines denote major drainages (onshore) and modeled locations of paleodrainages (shelf).
334
which aggrading sediment reached each canyon mouth, in that building
more or larger delta lobes would slow the upstream progression of an
accretionary wedge. An additional consideration is the extent to
which channels may have merged on the shelf, as well as the locations
of such merging (Fig. 10). If sediment from multiple drainages were
funneled into a single channel, channel inﬁlling and/or delta deposition
would presumably occur at a faster rate than from a drainage that
retained a single channel for a long distance on the shelf, thereby
inﬂuencing the rate of upstream sediment wedge migration. Conversely,
a network of relatively evenly-spaced single channels would likely be
more effective in distributing alluvium broadly across the shelf surface.
If the drainages are roughly of equal size and length and the shelf gradient
is relatively consistent in a shore-parallel direction, one might expect
to see a roughly even landward progression of aggradation across
the shelf, reaching the canyon mouths at approximately the same
time. This is not indicated by the data in this study.
The gradient of the shelf proﬁle (Fig. 12) exerts signiﬁcant controls
on river system behavior. For the same vertical change, the shoreline
moves horizontally much farther along a gently sloping shelf surface
than across a steeply sloping shelf. Off the north (Verde and Tecelote
Canyons) and south (Wreck Canyon) coasts of SRI, the proximal
(inner) shelf, immediately adjacent to the modern shoreline, has a
much gentler slope than the onshore channel gradient. The gradient of
the distal (outer) shelf steepens sharply below a break in slope at
about −30 to −35 m water depth (Fig. 12). This gently sloping inner
shelf surface is simply the modern marine platform, composed of a
short (a few hundred meters) steep section closest to shore, then a
more gently sloping section that can extend up to several kilometers offshore
(Bradley and Griggs, 1976). The distal portion of the shelf is probably
a submerged marine terrace bench.
Although the position of the advancing shoreline is a primary control,
a number of other factors also inﬂuence the timing and rate of aggradation.
Following the LGM, the climate, which exerts a major
control on water and sediment supply, gradually became warmer and
drier (Anderson et al., 2010). Discharge therefore probably decreased,
but erosion of valley side slopes may have increased owing to loss of
vegetative cover. Intrinsic processes and complex response of the system,
causing out-of-phase erosion and deposition in different reaches,
would have caused sediment to move downstream in irregular pulses
(Schumm, 1993; Wright and Marriott, 1993).
The rate of sea-level change also varied during this period (Fig. 9). If
base-level change is gradual, the channel is more likely to adjust its gradient
by changing its sinuosity, whereas if base-level change is rapid, incision
or aggradation is more likely (Schumm, 1993). Local channel size
and shape, as well as shelf slope, would deﬁne the accommodation
space (Posamentier and Vail, 1988) for sediment arriving at each
point from upstream and thus inﬂuence the rate at which the alluvial
wedge progressed upstream.
5.3. Aggradation in the canyons of Santa Rosa Island
The period between LIG and LGM, ~80-20 ka, was characterized primarily
by erosion in upstream areas (i.e., landward of present sea level)
and deposition on the distal regions of the shelf. This 60 ky span of falling
sea level appears to have been sufﬁcient time to have ﬂushed most
of the late Pleistocene alluvium from the valleys, in most places exposing
the bedrock valley ﬂoor. This is indicated by the lack of alluvium
samples dated older than ~16 ka in this study (Table 2), as well as in
most previous studies that have dated alluvium from SRI (e.g., Orr,
1968; Colson, 1996; Kennett et al., 2008). One exception to this is
Fig. 11. Oblique three-dimensional aerial view looking south across marine shelf and SRI, and topographic cross section across portion of shelf. Image generated from NOAA topographic/
bathymetric data (Carignan et al., 2009). White line denotes location of topographic section across marine platform surface. Vertical exaggeration of section is approximately 180:1.
335
charcoal collected near the base of the alluvial sequence in Cañada
Verde at our station 52. At that locality, Pinter et al. (2011) reported calibrated
14
C ages of 25 and 29 ka at depths of 1105 cm and 1130 cm, respectively,
whereas we obtained ages of 11.2 and 13.1 ka from charcoal
collected at depths of 1005 and 1180 cm (Table 2). When the samples
from this study are plotted with those from Pinter et al. (2011) on an
age-depth plot (as in Fig. 7), our results form an essentially straight
line, suggesting relatively continuous and steady deposition throughout
the sedimentary proﬁle. Using data from this study, the plot also closely
resembles the age-depth plot for station 3 (Fig. 7), located 130 m upstream
from station 52. In contrast, Pinter et al.’s older ages would require
the presence of an unconformity corresponding to a depositional
hiatus, which is not supported by our observations and data.
As sea-level began to rise following the LGM lowstand, backﬁlling of
the island's canyons appears to have begun almost immediately, at least
in Cañada Tecelote, which may have begun backﬁlling as early as
~19.5 ka (Fig. 8). Considering that Tecelote and Verde Canyons have
similar relief, gradients, and channel lengths, the nearly 8000-year difference
between the times when the aggradational wedge is estimated
to have reached the modern positions of each canyon mouth appears to
be anomalous. However, the interaction of the downstream-ﬂowing
river systems with the upstream-approaching shoreline is complex.
Possible reasons for the different timings of alluvial wedge progression
were discussed in the previous section.
The extensive areas of the shelf that were exposed during lowstands
(Fig. 10) increased overall channel lengths to 2–4 times their highstand
lengths (Fig. 12). The expanded drainage systems provided ample
routing and, ultimately, storage on the shelf for older alluvial sediments
as upstream reaches eroded and transported alluvium downstream. As
sea level rose, stream lengths shortened. The rate of channel shortening
on the shelf is controlled largely by the shelf gradient and the rate of sea
level rise. North of SRI, the distal shelf is considerably steeper than the
proximal shelf, whereas to the south, although divided by a similar
break in slope, the outer shelf is only slightly steeper than the inner
shelf (Fig 12). Between 20 and ~13 ka, sea-level change was relatively
gradual, then sea level rose rapidly until ~8 ka (Fig. 9). Between ~20
and 10 ka, the channels of Verde and Tecelote Canyons shortened by
~7 km, but Wreck Canyon shortened by almost 10 km as a result of
the more gentle slope of the outer shelf on the south side of the island
(Fig. 12). After 10 ka, rapidly rising sea level across the gently sloping
inner shelf shortened the channels of Verde, Tecelote, and Wreck canyons
by 5, 8, and ~4.5 km, respectively, in only ~2000 years (Figs. 10
and 12). These values are useful for comparison because all three drainages
had begun aggrading in their canyons by 11 ka (Fig. 8). After that
time, because migration of the aggrading sediment wedge was conﬁned
between the valley walls, more aggressive channel shortening would
likely drive more rapid advancement of backﬁlling sediment and,
Fig. 12. Channel longitudinal proﬁles of three major drainages on SRI, extended to the
postulated LGM (~21 ka) shoreline (data from Figs. 9 and 10). Postulated shoreline
positions at 60, 21, 15, 10, 8, and 5 ka are indicated above proﬁles. Proximal shelf was
exposed above sea level from ~60 to 10 ka.
Fig. 13. Photo of station 11 (see Fig. 4 for location) in Cañada Quemada, looking southwest, showing stratigraphic positions and calibrated radiocarbon dates of charcoals and humic acid
from alluvial fan sediments (Table 2). Fan originated from the tributary in background.
336
indeed, that appears to have been the case. Cañada Verde, which shortened
the most after 10 ka, backﬁlled more rapidly than the other two
streams (Fig. 8). This may also be in part because of the lower gradient
of Cañada Verde relative to the other two drainages, assuming that
other factors such as water and sediment discharge varied similarly in
all drainages.
By 12–11 ka, most, if not all, of the island's valleys were aggrading,
even though the paleoshoreline was still several kilometers from the
canyon mouths (Figs. 10 and 12). The channels would have maintained
and probably increased their sinuosities in response to the rising base
level (Schumm, 1993). However, channel pattern adjustment was insufﬁcient
to compensate for the rapid rate and large magnitude of
base-level change. At the leading edge of the upstream-advancing aggradational
wedge, channel gradient was locally reduced, resulting in
sediment deposition through frequent overbank ﬂooding (Wright and
Marriott, 1993), further reducing local channel and ﬂoodplain gradient
and increasing ﬂood frequency. This can be inferred from the multiple
episodic depositional sequences in the alluvial ﬁlls of each drainage.
Each sequence consists of sandy and clayey silts of overbank ﬂoods,
capped by poorly to well-developed, organic-rich paleosols of ﬂoodplains.
These probably remained wet for extended periods (Fig. 5), as indicated
by presence of the valves of the nonmarine ostracodes Candona,
Cypridopsis, and Ilyocypris in the sediments. Eventually, a minimum gradient
threshold was reached, after which a shallow channel would be
incised into the alluvium, indicated by pebble and gravel layers, but
rapid accretion kept the channels relatively shallow (see, for example,
the channel ﬁll at station CV5 in Fig. 5). As the channels ﬁlled with sediment,
overbank ﬂooding would occur more frequently and the sequence
would repeat.
The number, thickness, and frequency of these overbank ﬂooding
sequences vary between drainages, and between reaches of each
drainage, although some depositional surfaces can be traced laterally
for tens to hundreds of meters. Floodplain accretion was controlled
by a number of factors. Because some proportion of sediment probably
continued to be routed onto the exposed shelf, rates of delta
building, channel ﬁlling, and avulsions, i.e., ﬁlling of the accommodation
space (Posamentier and Vail, 1988) on the shelf, probably
exerted a downstream control on the frequency and timing of
overbank ﬂooding in the inland valleys. This downstream control
likely interacted in a complex manner with upstream controls, including
water and sediment availability from source areas, resulting
in complex responses causing phases of erosion, deposition, or rejuvenation
in various reaches, despite the overall aggradational mode
of the system.
Fig. 14. Generalized time series illustrating postulated ﬂuvial system response to post-glacial rising sea level. (A) At LGM (ca. 25–21 ka) the larger upland valleys were mostly free of
accumulated sediment, and channels on the shelf were relatively straight, broad, and shallow. (B) As sea level rose, channels on the shelf reduced gradient by meandering, and
alluvium began backﬁlling the channels. (C) By early Holocene, channels on the shelf were wide and ﬁlled with sediment, and deltas distributed sediment broadly across the shelf.
Aggradational sediment wedges reached into most upland valleys. (D) Aggradation in upland valleys ceased at ~0.5–0.1 ka, then deep arroyos incised through as much as 14 m of
valley alluvium.
337
An illustrative example of a complex response in the form of tributary
rejuvenation was observed at station 11 in Quemada Canyon
(Fig. 4). At this location, a large tributary enters the main channel
and nearly 6 m of coarse-grained alluvial fan sediment, dating to
~1 ka, displaced the main channel to the opposite side of the canyon
(Fig. 13, Table 2). The alluvial fan deposits persist for a few hundred
meters downstream of the tributary junction, ultimately grading into
the older and ﬁner-grained ﬂoodplain sediments. We investigated
tributary junctions in other drainages on SRI and did not ﬁnd
evidence of widespread tributary rejuvenation at 1 ka, therefore
concluding that rejuvenation of this tributary was likely triggered
by internal forcings rather than being part of an islandwide or
regional event.
Dateable material was sought as close as possible to the top of each
alluvial section to estimate the timing of cessation of aggradation. Surface
soil samples were not collected to avoid the risk of modern disturbance
and contamination. The youngest samples, ranging in age from
~0.5 to 1.5 ka, were collected at depths averaging ~1 m (Table 2), indicating
that ﬂoodplain aggradation generally continued until at least
0.5 ka, probably even later. Extrapolating the ﬁtted age-depth curves
to the ﬂoodplain surface (i.e., depth of 0 cm) may provide estimates of
the cessation of aggradation at each section (Fig. 7). For example, at station
CV3, the age-depth line passes through the origin, suggesting that
ﬂoodplain aggradation continued until the present, whereas at station
RC29, the line meets the surface at ~0.8–0.9 ka (Fig. 7). For any ﬂoodplain
surface with an extrapolated pre-modern age (i.e., the X intercept
of the age-depth curve is N0), two possible interpretations are (i) the X
intercept is a reasonable estimate of when ﬂoodplain accretion ceased
at that location, or (ii) the ﬂoodplain is an erosional surface from
which an indeterminate amount of sediment has been removed, so
the X intercept represents a maximum estimate of the cessation of
ﬂoodplain aggradation (i.e., aggradation ceased at that time or later). Excluding
station CV1, which likely experienced some surface erosion because
of its close proximity to a tributary drainage, estimates from all
the sections with dated samples collected within ~1 m of the surface
suggest that ﬂoodplain aggradation continued until at least 500 years
ago (Fig. 7).
5.4. Shift from aggradational to erosional sedimentary regime
The long period of late Pleistocene-Holocene aggradation ended
with the modern episode of major incision. Arroyos have incised the alluvium
ﬁlling all of the canyons on SRI, cutting through as much as 12–
14 m of accumulated ﬂoodplain sediment. It is this deep incision that
provides access to the alluvial record. In most areas, the modern stream
ﬂows over bedrock or a very thin (typically b0.5 m) veneer of sediment,
having abandoned the former Holocene ﬂoodplain which now forms
terraces on one or both sides of the valleys (Fig. 2A, 2B). Arroyos have
bisected Chumash archeological sites at several locations on the island
(Kennett, 2005; Rick, 2007). For example, shell middens, dwellings,
and burial sites dated to as late as 1820 CE in the Chumash village of
Niaqla (Rick, 2007), on the northwest coast of the island, are cut by
the arroyo of Skull Gulch, indicating that incision took place sometime
after this date. Another shell midden, at a site designated CA-SRI-77, located
at the eastern end of Bechers Bay, has been dated to as recently as
~1000 YBP (Rick et al., 2005) and is bisected by a deep arroyo. An early
photograph of Santa Rosa Island, taken by Philip Mills Jones in 1901 during
an archaeological survey (Jones, 1956, reproduced in Orr, 1968, and
Woolley, 1998), shows a well-formed arroyo in Cañada La Jolla Vieja,
suggesting that arroyo cutting took place prior to the beginning of the
twentieth century, at least at this location.
Sedimentation rates have been estimated from cores collected from
an estuarine marsh in Old Ranch Canyon (Cole and Liu, 1994; Anderson
et al., 2010). For the 5000 years prior to 1800 CE, the sedimentation rate
in the marsh was ~0.7 mm y−1
. During the early 1800s, the sedimentation
rate in the marsh increased to an average of 13.4 mm y−1
and
peaked at 23.0 mm y−1
between 1874 CE and 1920 CE (Cole and Liu,
1994), indicating that runoff increased signiﬁcantly. Ranching began
on Santa Rosa Island in 1844, and in the 1860s and early 1870s, as
many as 100,000 sheep grazed on the island (Allen, 1996). Several periods
of intense drought also occurred in the 1860s and 1870s. Extensive
overgrazing and livestock loss due to starvation were documented on
San Miguel, San Nicolas, and Santa Cruz Islands following the severe
drought of 1863–1864 and subsequent droughts (Johnson, 1980). Catastrophic
ﬂooding caused by the storm of 1861–1862 that dropped as
much as 1650 mm of rain on southern California in a 45-day period is
reported to have initiated arroyo cutting in a number of locations
(Engstrom, 1996), and it would have caused severe erosion of the denuded
land surfaces on the island. Major ﬂoods also occurred in 1867
and 1891 (Engstrom, 1996). This combination of reduced vegetative
cover caused by drought and/or overgrazing by sheep and episodes of
severe ﬂooding probably initiated arroyo cutting.
On neighboring Santa Cruz Island (SCI), valleys cut into bedrock contain
incised alluvial ﬁlls (Brumbaugh, 1980; Perroy et al., 2012) similar
to those observed on SRI. In contrast, however, the alluvium on SCI typically
occurs in two distinct facies: a lower, ﬁne-grained alluvium containing
multiple organic-rich beds similar to the alluvial sequences on
SRI, overlain by a coarser alluvial, colluvial, and debris-ﬂow unit lacking
substantial organic matter, which is not found on SRI. Charcoal and organics
near the top of the ﬁne-grained facies were dated to 1550 ±
80 14
C YBP and 1555 ± 80 14
C YBP (Brumbaugh, 1980), and it follows
that the coarser-grained facies was deposited after this time. Some authors
have suggested that this upper unit is the result of vegetation
stripping by grazing animals in the late 1800s (Brumbaugh, 1980;
Perroy et al., 2012). Drought and overgrazing in the late 1800s and
early 1900s were also postulated to be the primary causes of severe erosion
and sediment mobilization on San Miguel Island (Johnson, 1980).
By analyzing sedimentary data and historic topographic maps,
Perroy et al. (2012) determined that arroyo formation in Pozo Canyon,
on Santa Cruz Island, took place between 1875 and 1886, following a period
of intense sheep overgrazing, severe drought in 1877, and a large
rainstorm in 1878. They further used the thickness of post-grazing sediment
and late Holocene sedimentation rates to suggest that a natural
threshold for arroyo incision might have been reached within 100–
300 years, had conditions favorable for incision not been accelerated
by the untimely combination of intense livestock stress and unusual
weather events (Perroy et al., 2012).
By 5 ka BP, sea level was only 6 m below present, and the rate of sealevel
rise in the last 5000 years was extremely slow and gradual, in contrast
to the rapid rise during the preceding 8000 years (Figs. 9, 10). As
sea level stabilized, the aggradation rate would be expected to decrease,
though likely with a time lag as the effects of the changes were transmitted
upstream. No longer under the driving inﬂuence of continuing
base-level rise, the stream systems would have soon reached a natural
intrinsic threshold at which aggradation would slow or cease, followed
by incision of arroyos as the systems adjusted to the relative stability of
modern sea level (Schumm, 1973). Thus, arroyo formation would have
eventually begun as an intrinsic system response, even in the absence of
the vegetation stripping, drought stresses, and massive ﬂoods of the
mid- to late nineteenth century.
6. Summary
The island setting of Santa Rosa Island provides an unparalleled environment
in which to record and preserve the response of ﬂuvial systems
to major changes of sea level. The period of falling sea level
between the end of the last interglacial highstand at ~80 ka and the
last glacial lowstand at ~25–21 ka was marked by erosion and incision
in the uplands and deposition of alluvial sediment on the distal shelf.
Sea-level lowering was relatively gradual, falling ~100 m in
60,000 years. During this period, most of the alluvium in the main drainages
was transported out of the island's valleys and onto the shelf via
338
relatively straight, shallow channels (Fig. 14A). Streams throughout the
island probably incised into bedrock during this time.
Sea level rose relatively rapidly following the LGM lowstand of
−106 m, attaining its present elevation after ca. 20,000 years, triggering
a shift from an erosional to a dominantly depositional sedimentary regime.
The system began adjusting to rising sea level by increasing channel
sinuosity, thereby widening the channels by lateral migration.
Accumulation of sediment occurred ﬁrst through vertical and lateral accretion
in broad, shallow, channels on the shelf. Aggradation was probably
initially conﬁned to the channels (Fig. 14B). As sea level continued
to rise, however, channel avulsion and delta deposition distributed sediment
broadly across larger areas of the shelf (Fig. 14C). The rate of sealevel
rise did not correlate directly with alluvial aggradation rates. The
gradient of the shelf determined the extent of ﬂuvial channel shortening
relative to shoreline advance, but the volume of sediment delivered by
each drainage, and the extent to which the sediment was distributed
across the shelf surface, also controlled the rate at which aggradational
wedges progressed upstream across the shelf.
During the late Pleistocene and Holocene, backﬁlling of canyons
(landward of present sea level) appears to have progressed in a more
orderly and predictable fashion largely because the streams were conﬁned
within their valleys (Fig. 14C). Vertical aggradation locally reduced
stream gradients, causing frequent overbank ﬂooding, lateral
channel shifting (meandering across the width of the valley), and after
some minimum gradient threshold was reached, shallow incision into
the alluvium. Channel accretion occurred fairly rapidly, again leading
to frequent overbank ﬂooding and deposition of ﬁning-upward sequences
of ﬂoodplain sediment. Local channel gradient and morphology,
short-term climate variations, and internal controls also affected
the timing and magnitudes of these cut, ﬁll, and ﬂood episodes and
are reﬂected in the number and thickness of aggradational sequences.
Floodplain aggradation within the valleys continued until at least
~500 YBP, followed by intensive arroyo cutting that left the relict ﬂoodplains
as alluvial terraces standing as much as 12–14 m above the present
streams (Fig. 14D). Sedimentary and historical evidence points to
overgrazing, drought, and ﬂoods in the mid- to late nineteenth century
as factors that facilitated arroyo incision.
Acknowledgements
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