Regional and local controls on the spatial distribution of bedrock reaches in the Upper
Guadalupe River, Texas
Amanda Keen-Zebert a,
, Joanna C. Curran b
a
Department of Geography, Texas State University--San Marcos, San Marcos, TX 78666, United States
b
Department of Civil and Environmental Engineering, University of Virginia, Charlottesville, VA 22904, United States
a b s t r a c ta r t i c l e i n f o
Article history:
Received 25 June 2008
Received in revised form 22 June 2009
Accepted 23 June 2009
Available online 8 July 2009
Keywords:
Fluvial geomorphology
Guadalupe River Texas
Channel type
Bedrock channels
Balcones Escarpment
Fluvial adjustment to base level
While studies on gravel mantled and mixed alluvial bedrock rivers have increased in recent decades, few field
studies have focused on spatial distributions of bedrock and alluvial reaches and differences between reach
types. The objective of this work is to identify the spatial distribution of alluvial and bedrock reaches in the
Upper Guadalupe River. We compare reach length, channel and floodplain width, sinuosity, bar length and
spacing, bar surface grain size, and slope in alluvial and bedrock reaches to identify whether major
differences exist between channel reach types. We find that local disturbances, interaction of the channel and
valley sides, variation in lithology, and regional structural control contribute to the distribution of bedrock
reaches in the largely alluvial channel. Alluvial and bedrock channel reaches in the Upper Guadalupe River
are similar, particularly with respect to the distribution of gravel bars, surface grain size distributions of bars,
and channel slope and width. Our observations suggest that the fluvial system has adjusted to changes in
base level associated with the Balcones Escarpment Fault Zone by phased incision into alluvial sediment and
the underlying bedrock, essentially shifting from a fully alluvial river to a mixed alluvial bedrock river.
 2009 Elsevier B.V. All rights reserved.
1. Introduction
Because of the recognition of the importance of fluvial bedrock
incision on continental landscape evolution, many studies of bedrock
incision have been undertaken in recent decades (e.g. Howard, 1980,
1998; Seidl and Dietrich, 1992; Howard et al., 1994; Sklar and Dietrich,
1998, 2001; Whipple and Tucker, 2002; Sklar and Dietrich, 2004; Bishop
et al., 2005; Jansen, 2006; Montgomery and Stolar, 2006; Sklar and
Dietrich, 2006; Turowski et al., 2007; Chatanantavet and Parker, 2008).
Understanding the role of coarse bed material in bedrock incision
processes is essential to understanding bedrock incision mechanics
(Sklar and Dietrich, 1998, 2004, 2006) and has led to an increased
interest in gravel mantled and mixed alluvial bedrock streams. Studies of
mixed alluvial bedrock channels are generally focused on long profile
development and are based on a model that attributes the occurrence of
bedrock or alluvial channel type to spatial variations in the balance of
sediment transport capacity and sediment supply that dates back to
Gilbert's (1877) work on the Henry Mountains. While some field studies
approach mixed alluvial bedrock streams as unusual fluvial forms and
focus on geomorphologic descriptions (Kale et al., 1996; Gupta et al.,
1999; Heritage et al.,1999; Tooth and McCarthy, 2004), few field studies
have focused on spatial distributions of bedrock and alluvial reaches and
differences between reach types (Brakenridge,1985; Ashley et al.,1988;
Montgomery et al., 1996; Montgomery and Buffington, 1997; Heritage
et al., 2001; Massong and Montgomery, 2000).
In North America, field studies of mixed alluvial bedrock streams and
reference reaches used for testing bedrock incision models have been
conducted in humid regions in active tectonic settings (Brakenridge,
1985; Ashley et al., 1988; Seidl and Dietrich, 1992; Montgomery et al.,
1996; Montgomery and Buffington, 1997; Howard, 1998; Sklar and
Dietrich, 2004; Lancaster and Grant, 2006; Sklar and Dietrich, 2006) and
focus on local and reach scale controls on the distribution of bedrock
reaches (Seidland Dietrich,1992; Montgomeryet al.,1996; Montgomery
and Buffington,1997; Howard,1998; Sklar and Dietrich, 2004; Lancaster
and Grant, 2006; Sklar and Dietrich, 2006). In order to improve the
understanding of mixed bedrock­alluvial rivers, field investigations
across a range of climates and tectonic settings are necessary. The Upper
Guadalupe River in central Texas is a mixed alluvial bedrock river in a
sub-humid, post-orgenic setting that provides an opportunity to
investigate controls on the spatial distribution of alluvial and bedrock
reaches in a setting that is often neglected in scientific research in fluvial
geomorphology.
The objective of this work is to identify the spatial distribution of
alluvial and bedrock reaches in the Upper Guadalupe River. We compare
reach length, channel and floodplain width, sinuosity, bar length and
spacing, bar surface grain size, and slope in alluvial and bedrock reaches
to identify whether major differences exist between channel reach
types. To determine whether local or regional processes dominate the
Geomorphology 112 (2009) 295­305
 Corresponding author. Current address: Aberystwyth University, Llandinam Building,
Penglais Campus, Aberystwyth, SY23 3DB, Wales, UK. Tel.: +44 19 7062 5980; fax: +44
1970 622 659.
E-mail address: keenzebert@gmail.com (A. Keen-Zebert).
0169-555X/$ ­ see front matter  2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2009.06.017
Contents lists available at ScienceDirect
Geomorphology
journal homepage: www.elsevier.com/locate/geomorph
distribution of channel reach type in the Upper Guadalupe River, we
examine local conditions at channel transitions from alluvial to bedrock
(or vice versa) as well as regional scale patterns of structure and
lithology that may influence the spatial pattern of channel type.
2. Regional setting
In central Texas, the Upper Guadalupe River flows from headwaters
in the Edwards Plateau region of the Great Plains province at
~600 m above mean sea level. The study reach begins at Flat Rock Dam
near the city of Kerrville and extends downstream for 140 km, ending
just upstream of Canyon Lake Reservoir (Fig. 1). The drainage area of
the study reach is 3405 km2
. The catchment geology is predominately
Cretaceous limestone with some thinly bedded layers of shale and
sandstone cropping out in the channel banks and valley sides (Brown
et al., 1974; Ashworth, 1983). Average rainfall on the Edwards Plateau
is 66 cm/year (Carr, 1967) and potential evapotranspiration is 114 cm/
year (Clark, 1983). Precipitation is very seasonal and the region is
prone to periods of drought and flood. The combination of low
infiltration capacity of upland soils, a steep highly-dissected
watershed, and dryland vegetation result in conditions for rapid
runoff. Flooding and its effects on geomorphology in central Texas are
well documented (Baker, 1975, 1977; Patton and Baker, 1977).
The Upper Guadalupe River channel type alternates from fully
alluvial to fully bedrock reaches. Alluvial reaches have fully alluvial
beds and banks. Bedrock reaches vary from bedrock floored reaches
with alluvial banks to reaches with the cross-section composed
entirely of bedrock. Many incised reaches appear incised into bedrock
and overlying alluvium such that the bed and lower channel walls are
composed of bedrock and the upper channel walls and banks are
composed of alluvium (Fig. 2). Ground water sapping is evident in
bedrock channel walls and on contacts between bedrock and
overlying alluvium. Erosional features such as longitudinal grooves,
knickpoints, and abrasion or quarrying scars are present in planar
bedrock floored reaches (Fig. 3).
In a small nearby Texas watershed, Tinkler (1971) noted a channel
morphology with two channels in cross-section, a small channel on
the outside of the bend at a lower elevation and a larger one on the
inside of the bend at a higher elevation. In cross-section, both of these
channels are inset within a larger flood channel. Although the pattern
is discontinuous, this morphology is common on the Upper Guadalupe
River. This channel-within-channel morphology has been observed in
rivers that have two dominant channel-forming flow regimes or are
subject to high-magnitude floods (Tinkler, 1971; Rhodes, 1990; Gupta,
1995; Gupta et al., 1999) and has been observed in bedrock channels
(Shepherd and Schumm, 1974; Baker, 1988; Wohl, 1992; Tinkler and
Wohl, 1998; Richardson and Carling, 2005). In many cases on the
Upper Guadalupe River, the small narrow channel on the outer bend is
incised into bedrock and the wider, elevated inner channel is cut
through coarse alluvial sediments. Between these channels, on the
inside bends of the outer channels, are large gravel deposits stabilized
by vegetation which contribute to the persistence of the form through
high flows (e.g. Baker, 1977; Abernethy and Rutherfurd, 1998; Tooth
and Nanson, 2000; Gurnell et al., 2001; Brooks and Brierley, 2002;
Brooks et al., 2003; Pollen et al., 2004).
Coarse grained fluvial deposits are exposed in channel cut banks,
suggesting a complex history of aggradation and incision (Fig. 4).
Baker (1977), in an tributary catchment of the Guadalupe River
downstream of our study reach, observed a similar sedimentary
stratigraphy which he identifies as a series of ancient flood deposits
and buried torrifluvents. Mid-channel, lateral, and point bars up to
2 km long are present in both alluvial and bedrock reaches. Bars are
predominantly composed of rounded pebbles to cobbles although
some point bars also have thin sand lenses along the inside of the bend
Fig.1. The location of the Balcones Escarpment Fault Zone and major streams and watersheds in central Texas. The upstream end of the study reach is indicated by  within the Upper
Guadalupe River watershed. The study reach extends 140 km downstream to the Canyon Lake Reservoir. Adapted from Caran and Baker (1986, Fig. 1 pg. 3).
296 A. Keen-Zebert, J.C. Curran / Geomorphology 112 (2009) 295­305
and a few small tributary mouth bars are composed entirely of sand.
The coarse alluvial bars may be depositional in the current environment,
but may also be the remaining eroded surfaces of older deposits
of which the soil surface horizons have been washed away by floods
(Dittemore and Hensell, 1981; Dittemore and Coburn, 1986).
Regional structural control is dominated by the Balcones Escarpment
Fault Zone (Fig. 1), a NE­SW trending surface expression of a
crustal discontinuity resulting from the Ouachita orogen that divides
the Great Plains province from the Coastal Plain (Woodruff and
Abbott, 1986). The Balcones Escarpment is a series of "en echelon"
normal faults with downthrown sides to the east that create a series of
ramps and steps across the fault belt (Jordan, 1977; Grimshaw and
Woodruff, 1986). The fault was highly active during the Miocene with
subsequent adjustments during the Cretaceous but is considered
inactive today (Woodruff and Abbott, 1986).
3. Materials and methods
In order to identify the spatial distribution of channel type and
differences between alluvial and bedrock reaches, we have used field
techniques, historical aerial photography, and map interpretation to
investigate the pattern and characteristics of alluvial and bedrock
reaches. Field mapping of the 140 km study reach was undertaken
because significant differences between bedrock and the flat surfaces
of large gravel bars were not distinguishable using remote sensing or
aerial photography techniques. The channel was classified as alluvial
or bedrock based on the 50% wetted perimeter criteria (Howard,1987;
Tinkler and Wohl, 1998; Carling, 2006). Herein, "reach(es)" refers to
the classified alluvial and/or bedrock reaches. In the few instances
where the water in the channel was not clear enough to observe the
bed visually, the bed was felt by hand or with a stake at regular
intervals to determine if it was bedrock or covered with gravel. In
addition to channel type, gravel bar perimeters, boulder berms,
knickpoints, tributaries, individual boulders N1 m or groups of
boulders, and built structures such as dams, road crossings, and
bridges were mapped in the field using differentially corrected GPS
and verified on aerial photographs. The surface grain size distribution
of bars was determined using the Wolman pebble count method
(Wolman, 1954). The same person performed all the pebble counts in
order to reduce error attributable to the user.
Digital historical aerial imagery was obtained from Texas Natural
Resources Inventory Services (TNRIS) for years 1996 and 2004 and the
Fig. 2. Examples of incision through bedrock and overlying alluvium and ground water sapping in the Upper Guadalupe River. Dashed lines indicate the contact between bedrock and
alluvium. In these locations, the channel bed is bedrock.
297A. Keen-Zebert, J.C. Curran / Geomorphology 112 (2009) 295­305
channel was digitized using ArcMap GIS software for both years. The
locations of bars mapped in the field in 2005­06 were used to aid
identification of bars in current and historical aerial photography.
Digitized floodplain zones for 1 year and 500 year floodplains
designated by the US Federal Emergency Management Agency
(FEMA) were also obtained from TNRIS. Valley sides were digitized
using the historical aerial imagery with reference to 1:24,000
topographic maps available from US Geologic Survey (USGS). Widths
of the wetted channel,1 year floodplain, 500 year floodplain, and river
valley were measured perpendicular to the downstream flow
direction at intervals of 160 m (the largest unit that captured all
reach transitions between types). Reach sinuosity was calculated by
dividing the channel reach length by the valley length for each alluvial
or bedrock reach. The surficial geologic unit for each reach was
determined from geologic maps at 1:62,500 published by the Texas
Bureau of Economic Geology (Barnes, 1977). Channel slope was
determined from 1:24,000 USGS topographic maps. The text of this
paper makes frequent reference to "river kilometers," abbreviated:
Rkm. This is analogous to river miles and refers to longitudinal
distance from the upstream end of the study reach.
4. Results
Bedrock and alluvial reach type varies longitudinally downstream
and most of the study reach is alluvial in character with short reaches
of bedrock interspersed throughout (Fig. 5). In the Upper Guadalupe
River, bedrock and alluvial channel reaches have similar physical
characteristics (Table 1). While there are statistically significant
differences between alluvial and bedrock reaches with respect to
reach length; sinuosity; and channel, floodplain, and valley widths,
actual differences are small. For example, channel widths were found
to be statistically different but the average channel width of alluvial
reaches is 26 m and the average of bedrock reaches is 28 m. Surface
grain size distributions and bar size vary little between alluvial and
bedrock reaches (Table 2). We observed negligible change in the
location and size of gravel bars between 1996 and 2004­05,
suggesting overall stability despite a 250 year flow event in 2002.
All variables were analyzed for apparent spatial trends. No
downstream trends in the surface grain size of bars are evident
when examined over the entirety of the study reach. When separated
into reaches between tributary inputs, only 11 between tributary
Fig. 3. Examples of bedrock channel features. Longitudinal grooves and fluting are common in bedrock reaches in the study area. Both photos show downstream view.
298 A. Keen-Zebert, J.C. Curran / Geomorphology 112 (2009) 295­305
reaches contained more than three bars. These 11 reaches were used
to examine trends in surface grain size between tributaries. Of these,
six have downstream trends: four fining, and two coarsening. There
was no correlation between these results and channel reach type.
Channel width measured from 1996 and 2000 aerial photography
revealed negligible change in channel position or width. There is no
evident downstream trend in channel width (Fig. 6). Channel
meander wavelength measured from topographic maps and both
1 year and 500 year floodplain width measured from FEMA floodplain
data decrease downstream.
Surficial geology varies over the study area. At Rkm 58, the channel
crosses a fault and there is a distinct change in the surficial geology
(Fig. 5). Upstream of Rkm 58, the channel meanders across broad
fluviatile terrace deposits bounded by the Upper Member of the Glen
Rose Limestone. Downstream of Rkm 58, the channel cuts into the
lower member of the Glen Rose Limestone, Hensell Sand, and Cow
Fig. 4. Gravel deposits are exposed in many cut banks in the study area. The Orif Soil/Fluviatile Terrace deposits are extensive and consist of mostly water worn pebbles to cobbles.
Fig. 5. Alluvial and bedrock channel type, valley width, geologic units, and the location of faults and knickpoints in the study reach.
299A. Keen-Zebert, J.C. Curran / Geomorphology 112 (2009) 295­305
Creek Limestone (Fig. 5). The floodplain is wider in the upstream
section than it is downstream of Rkm 58 where the channel migrates
across a much narrower valley width (Fig. 5).
Half of the observed bedrock reaches are downstream of Rkm 58
and occur where the outsides of channel meanders are in contact with
the valley side. These outer bends are characterized by steep bluffs and
gentler sloped inner bends where large point bars have been
deposited. The one exception is a 2.5 km straight reach beginning at
Rkm 95 where the valley is locally constricted by steep bluffs on both
sides of the channel and the channel is incised into the narrow valley
~20 m, greater than the rest of the study reach (~5­10 m).
The bedrock reaches observed upstream of Rkm 58 have a different
spatial pattern than the downstream section. Bedrock reaches
upstream of Rkm 58 occur on the outside of a few bends, but also
occur mid valley, and in straight reaches that are not constricted. In
this section, bedrock reaches tend to be only slightly incised (1­2 m)
into both bedrock and the overlying alluvium, or are bedrock floored
and lined with gravel terraces or banks composed of vegetated
alluvium (Figs. 2, 3, 7).
The bedrock reaches upstream of Rkm 58 occur in two main
groups: just upstream of Rkm 58 and just upstream of Rkm 21 where
the channel flows across faults (Fig. 5). Both groups of bedrock reaches
have low stepped translational knickpoints at their upstream-most
ends near the fault crossing. These faults are at the most western
extent of the Balcones Escarpment Fault Zone at this latitude. In
addition to the grouping of bedrock reaches just upstream of these
faults, spatially discontinuous strath and alluvial terraces, and the
presence of knickpoints at the upstream end of the bedrock reach
group suggest a history of phased incision into the valley (Fig. 8). The
faults cause 0.5­1 m displacements and lower the local base level
enough to initiate knickpoint retreat which translates the adjustment
to base level change upstream. There is no evidence of scarps in the
channel bed or banks, which suggests that erosion has equaled the
amount of vertical displacement at the fault and is further geomorphological
evidence for upstream translation of changes in base level.
The fault pattern is more complex in the downstream end of the
study reach. There are two faults with a drag block between at Rkm
127 where the river crosses the fault (Fig. 5). Downstream of Rkm 127,
the valley and floodplain widen entering the backwaters of Canyon
Lake Reservoir. We observe some knickpoints between Rkm 58 and
127, and it is likely that phased incision has occurred in this reach,
although it is not as geomorphically evident as in the section upstream
because of differences in lithology. The fluvial response to base level
changes is likely more complex closer to the Balcones Escarpment
because there are more faults and the complexity in the fault pattern
increases.
Table 1
Comparison of physical properties of alluvial and bedrock reaches in the upper Guadalupe
River.
Alluvial reaches Bedrock reaches Total study reach
Channel coverage (km) 118 22 140
Percent of total channel
coverage
84 16 100
Average reach slope 0.0023 0.0090 0.0045
Average reach length
(channel widths)
124 28 72
Standard deviation 179 42 134
Median 71 20 29
Average transition spacing
(channel widths)
148 140 73
Standard deviation 180 167 134
Median 92 91 29
Average channel width (m) 26 28 26
Standard deviation 9.0 8.9 9.0
Median 25 27 25
Average 1 year floodplain
width (m)
528 476 520
Standard deviation 417 349 407
Median 419 380 419
Average 500 year floodplain
width (m)
668 707 674
Standard deviation 472 429 466
Median 487 680 524
Average reach sinuosity 1.25 1.09 1.16
Standard Deviation 0.32 0.09 0.30
Median 1.12 1.06 1.07
Measurements for the entirety of the study reach are recorded in the far right column.
Statistically significant differences in feature measurements between alluvial and
bedrock channel types are noted by  for 0.01 confidence level.
Table 2
Comparison of bar properties in alluvial and bedrock reaches in the upper Guadalupe
River.
Alluvial reaches Bedrock reaches Total study reach
Number of bars 203 40 243
Bars per kilometer 1.72 1.81 1.74
Percent coverage by bars 22 15 17
Average bar length (m) 96 109 102.5
Standard deviation 105 98 103
Median 66 67 67
Percent of total bars 84 16 100
Bar spacing (channel widths) 23 117 38
D50 (mm) 32 30 32
Standard deviation 13 11 13
D84 (mm) 67 73 68
Standard deviation 28 25 28
Measurements for the entirety of the study reach are recorded in the far right column.
Statistically significant differences in feature measurements between alluvial and
bedrock channel types are noted by  for 0.01 confidence level.
Fig. 6. Channel width remains constant in the study reach while meander wavelength
decreases downstream.
300 A. Keen-Zebert, J.C. Curran / Geomorphology 112 (2009) 295­305
Dams, roads, tributary confluences, large woody debris, and
individual (or groups of) boulders N1 m in diameter mapped in the
field were carefully checked for evidence of local control on channel
reach type. The large wood observed consisted of either single felled
trees parallel to and against the channel banks or riparian flood debris
that does not obstruct flow or sediment transport enough to affect the
channel boundary. The single exception is where woody debris is
trapped behind a low bridge. In one location, the channel has avulsed
around a bridge that has trapped enough large wood to obstruct flow
through the opening under the bridge (Fig. 9). Individual boulders or
groups of individual boulders did not affect channel boundary type
because the boulders were not clustered close enough together to trap
sediment and did not extend across the channel width. Of the 41
tributaries in the study reach, only one tributary at Rkm 97 had a
concurrent channel boundary change from alluvial to bedrock directly
downstream of its confluence with the main channel. No stream gage
exists on this tributary and, therefore, no quantitative evidence to
establish that the channel transition results from increased flow at the
location, but circumstantial and morphological evidence indicate that
the channel type is forced at this location.
Many of the road crossings in the study area are elevated bridges
well above the elevation of the water surface even at high flows and
have no observed effect on the geomorphology of the river. Of the 29
road crossings mapped in the study reach, six are low water crossings
where the elevation of the road is the same, or built on top of, the
channel bed and water flows over the top of the road. Four road
crossings trap enough sediment to cause scour downstream, locally
forcing the boundary type from alluvial upstream to bedrock downstream.
Two small weir type dams also trap sediment resulting in
scour down to a flat plane of bedrock downstream. The longitudinal
extent of scour to bedrock downstream of roads and dams in the study
reach ranges from 0.5­2 km. As mentioned in the previous paragraph,
at one location, wood is trapped beneath a low bridge and the channel
has avulsed around the entire structure.
Fig. 7. Examples of knickpoints in the study reach.
301A. Keen-Zebert, J.C. Curran / Geomorphology 112 (2009) 295­305
5. Discussion
The spatial distribution of bedrock reaches in the Upper Guadalupe
River is controlled regionally by structural geology and locally by
channel intersection with the valley margins and obstruction
disturbances such as dams and low water road crossings. Bedrock
reaches occur where lithology controls the lateral extent of channel
migration resulting in a narrow valley, upstream of small regional
faults that represent a change in local base level, and downstream of
dams and road crossings.
Surprisingly, unlike other studies that compare alluvial and
bedrock characteristics, (Montgomery et al., 1996; Montgomery and
Buffington, 1997; Massong and Montgomery, 2009) we find little
difference in slope between alluvial and bedrock reaches and no
correlation between slope breaks and channel type transitions. The
channel slope threshold relationship has been used to successfully
predict the occurrence of alluvial or bedrock channel type in other
studies on mixed alluvial bedrock rivers (e.g. Howard, 1980;
Montgomery et al., 1996; Montgomery and Buffington, 1997; Sklar
and Dietrich, 1998; Massong and Montgomery, 2000). A likely
explanation for our study's divergence from others is that slope
threshold relationships do not account for variations in discharge,
peak discharges, or lithology (Pazzaglia et al., 1998), all of which are
important in forming the morphology of central Texas Rivers. It is also
possible that small differences in slope between alluvial and bedrock
reaches are undetectable with the method used here. As we interpret
the results of the techniques used in this stage of the research, with
respect to comparisons between alluvial and bedrock reaches, our
results are more comparable to Ashley et al. (1988) than to other
studies even though many of our reaches are incised into small
bedrock canyons while their study reach was bedrock floored with
alluvial banks.
In agreement with previous studies (Montgomery et al., 1996;
Montgomery and Buffington, 1997; Massong and Montgomery, 2000;
Lancaster and Grant, 2006), we find that local disturbances affect the
distribution of bedrock reaches. The weir type dams and low watercrossing
roads in the Upper Guadalupe River are analogous to large
wood and debris in other studies in that they obstruct the channel and
influence hydraulic processes and sediment transport, trapping
sediment upstream and resulting in scour downstream. In two
instances, we observed a dam or low water crossing in aerial
photography from 1996 that had been removed or partially removed
by the time of field work in 2005­06. In both instances, sediment
deposits that were observed behind the structure in aerial photography
were not observed in the field and there was no evidence of a
channel type transition. These results are analogous to observations
by Massong and Montgomery (2000) in that there is a relatively rapid
response following obstruction removal.
Regional lithology and structure create two distinct patterns of
bedrock reach distribution and characteristics in the Upper Guadalupe
River. Variation in lithology result in variation in valley characteristics
that largely control the spatial pattern and characteristics of bedrock
reaches such that bedrock reaches generally occur on the outside of
channel meanders in the narrow valley downstream of Rkm 58.
Differences in lithology and erodibility have been observed by others
to result in variations in channel type and bedrock exposures in mixed
alluvial bedrock channels (e.g. Howard and Kerby, 1983; Howard,
1994; Stock and Montgomery,1999; Massong and Montgomery, 2000;
Fig. 8. Examples of strath terraces in the study reach.
302 A. Keen-Zebert, J.C. Curran / Geomorphology 112 (2009) 295­305
Whipple et al., 2000; Whipple and Tucker, 2002; Tooth et al., 2004;
Jansen, 2006; Sklar and Dietrich, 2006).
The characteristics and pattern of bedrock reaches resulting from
regional structure is different upstream of Rkm 58 than in the
downstream narrow valley section in that extensive sections of
bedrock reaches occur upstream of currently inactive faults. The
groups of bedrock reaches are characterized by translational knickpoints
at their upstream extents and by spatially discontinuous strath
and alluvial terraces downstream. These groups of bedrock reaches
create two large knickzones, each composed of a series of short
bedrock reaches that represent propagating base level fall upstream.
Knickpoints have been described as a response to base level
disturbance that migrate upstream (Howard et al., 1994; Whipple
and Tucker,1999; Whipple et al., 2000; Bishop et al., 2005; Crosby and
Whipple, 2006). The two bedrock dominated knickzones represent
the channel adjustment to base level disturbance that occurred in
association with the Balcones Escarpment Fault Zone. However, the
timing of response to changes in base level on the Upper Guadalupe
River is complex as the system adjusts to small local faults, catchment
wide shifts as the Guadalupe River incised the Balcones Escarpment,
and changes in sea level in the Gulf of Mexico.
It is important to note the role of ground water interaction and
chemical weathering on the geomorphology of the Upper Guadalupe
River. The substrate is almost entirely limestone and is highly
susceptible to ground water­surface water interaction and chemical
weathering, which may account for more erosion than the mechanical
means alone. Sapping is evident and decreases the stability of canyon
walls, increasing the propensity for blockfall and eventual valley
widening. The ground water is supercharged with calcium carbonate
in many alluvial sections, and cut banks appear to be stabilized by the
formation of calcrete from calcium deposition by ground water as
ground water levels fluctuate. Although the geochemical denudation
rate of the Glen Rose Limestone at the Guadalupe River is only 25 mm/
Ka (Veni, 1997), the effect of geochemical weathering increases the
propensity towards detachment by mechanical means. Abrasion scars
were observed in areas where geochemical weathering at low flows
increases the friability of the channel bed. In a few alluvial reaches,
gravel and cobbles on the channel bed were found to be so weathered
that they easily broke apart. The combination of geochemical and
mechanical processes in karst systems is likely to account for the
removal of more material and landscape denudation in fluvial systems
than mechanical means alone.
6. Conclusions
In the Upper Guadalupe River, the spatial distribution of bedrock
channel reaches is the result of a combination of local disturbances,
interaction of the channel and valley sides, variation in lithology, and
regional structural control. In comparing bedrock and alluvial channel
reaches, we find a large degree of parallelism, particularly with respect
to the distribution of gravel bars, surface grain size distributions of
bars, and channel slope and width. Observations made on the Upper
Guadalupe River in this study and observations of other central Texas
rivers that flow across the Balcones Escarpment indicate that the
fluvial systems have adjusted to changes in base level associated with
the Balcones Escarpment Fault Zone by phased incision into alluvial
sediment and the underlying bedrock, essentially shifting from a fully
alluvial river to a mixed alluvial bedrock river. However, the rate and
timing of incision is unknown.
While the understanding of bedrock incision and mixed alluvial
bedrock channels has increased in recent decades, little work has been
done on long term timing of incision or on the role of geochemical
processes and mechanical­chemical interaction. More field based
studies of gravel mantled streams and mixed alluvial bedrock streams
can elucidate bedrock incision models by providing natural analogues to
experimental studies and improve understanding across a range of
environmental settings. Studies of local conditions as well as regional
patterns of structure and lithology that may influence channel type are
important to understanding fluvial processes and controls operating at
different scales. Work in geochronology can add understanding of the
temporal aspects to bedrock incision, especially where incision rates are
slow and the possibility of episodic incision exists.
Our future work will employ geochronologic techniques to
compare rates of incision upstream and downstream of the Balcones
Fig. 9. The pre-existing low elevation road crossing was left in place after the new bridge was built. Wood and flood debris is trapped in the bridge openings and the channel is
reworking avulsing around the old bridge. The new channel is shown in the photos on the left of the aerial photo.
303A. Keen-Zebert, J.C. Curran / Geomorphology 112 (2009) 295­305
Escarpment in order to establish differences in timing and between
fluvial response to sea level and to the Balcones Escarpment Fault
Zone. This work will allow comparisons of base level connectedness
across the escarpment which ultimately controls landscape evolution
between the Great Plains and the Coastal Plain.
Acknowledgements
We thank the three anonymous reviewers whose comments have
helped greatly to improve this manuscript. This project was funded by
a National Science Doctoral Dissertation Research Improvement grant
and the Texas Advanced Research Program.
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