8157

• UCREFRP Catalog Number: 8157
• Author: Pitlick, J., M. V. Steeter, B. Barkett, R. Cress and M. Franseen.
• Title: Geomorphology and Hydrology of the Colorado and Gunnison Rivers and Implications for Habitats Used by Endangered Fishes.
• USFW Year: 1999.
• USFW - Doc Type: \
• Copyright Material: NO

The general morphologic characteristics of the Colorado River and the Gunnison River were
determined by surveying cross sections of the main channel at evenly spaced, 1-mile intervals. A
total of 57 cross sections were surveyed on the Colorado River between Palisade, CO and
Westwater, UT, and 53 main cross sections were surveyed on the Gunnison River between Delta,
CO and the Redlands diversion dam. These main channel cross sections provide a quasi-
continuous view of the downstream hydraulic geometry of each river, which is important if we are
to specify the discharges and flow conditions under which the channel is formed and maintained.
The main channel cross sections were surveyed with an electronic theodolite (total station) and a
motorized rubber raft equipped with a reflecting prism and depth sounder. To measure a section,
the total station was set-up over one of the end points, distance readings were taken along the line
of the cross section by targeting the prism on the rubber raft, and depth soundings were taken by
the person on the raft, who relayed the information by radio to the person on shore. The bankfull
width by and depth by at each cross section were determined in the field by what was usually a clear
break in slope between the channel and the floodplain. In September 1995, we re-surveyed 12 of
the cross sections (every fifth one) on the Colorado River to determine whether the high flows of
that year had caused significant geomorphic changes.
Average slopes of the study reaches were determined two different ways, depending on the
contour interval of the available topographic maps. The most recent USGS 1:24,000 topographic
maps of the Grand Valley show 10-ft contours along the Colorado River and Gunnison River; we
assumed that these data were adequate for determining reach-average slopes. In contrast, the
1:24,000 USGS maps of the Ruby-Horsethief Canyon reach use a 40-ft contour interval, which
we did not think was adequate for determining reach-average slopes. To obtain better estimates of
the slope through Ruby and Horsethief Canyons we measured water surface elevations at 0.5-mile
intervals using a global positioning system (GPS) capable of resolving elevations to less than 1.0
m with differential post-processing. We found that elevations determined with the GPS did not
differ appreciably from elevations on the existing topographic maps, but we nonetheless used the
GPS data to calculate slopes for this reach.
Sediment samples were taken at many different locations to determine the particle size distribution
of the surface and subsurface bed material (gravel-bed rivers often possess a surface layer of
sediment- called an "armor" layer or pavement- that is much coarser than the sediment underneath;
the surface layer characteristics influence the roughness and mobility of the bed, whereas the
subsurface sediment is the primary source of the bed load transported by the river). In this study,
bed surface particle size distributions were determined from point counts of 100 or 200 particles on
exposed gravel bars. Ideally, we would have sampled according to facies (riffles, runs or pools),
but flow depths exceeding 1 meter usually precluded us from taking samples far out into the main
channel. Subsurface particle size distributions were determined by taking bulk samples of about
100 kg of sediment, of which the coarse fraction (> 32 mm) was sieved in the field and the fine (<
32 mm) fraction was sieved in the laboratory., Tables A-1 through A-4 give a breakdown of the
individual size-fractions for each surface and subsurface sediment sample.
We did not find large variations in the size of the bed material of the Colorado River; most of the
surface samples have a median grain size D50 between 40 and 60 mm (Fig. 6). The bed material of
the Gunnison River is more variable, and slightly finer, than the bed material of the Colorado River
(Fig. 7). The subsurface sediment of both rivers is much more variable than the surface sediment;
the D50 of the subsurface sediment ranges from less than 10 mm to 50 mm (Figs. 6 and 7). On
average, the DSO of the surface sediment is approximately twice that of the subsurface sediment.
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