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

difference between the observed gauge height and the mean flow depth (Jacobson, 1995), with
adjustments for changes in the location and datum of the gauge as necessary.
Aerial Photograph-GIS Analysis
Long-term changes in channel morphology of the Colorado River and Gunnison River were
determined from black and white aerial photographs taken in 1937, 1954, 1968, 1993 and 1995.
These photographs are of similar scale (1:20,000); however, they cover different parts of the study
area, their quality varies, and they were flown with the rivers at different flow levels. The 1954
and 1968 photographs of the Colorado River were taken during periods of relatively low flow;
discharges in the 15-mile reach on the days that the photographs were taken were 54 and 60 m3/s,
respectively. The 1937 and 1993 photographs of the Colorado River were taken during periods of
moderate flow; discharges in the 15-mile reach on the days that the photographs were taken were
209 and 186 m3/s, respectively. The 1937 and 1995 photographs covering the Gunnison River
were taken at low to moderate discharges of 18 m3/s and 84 m3/s, respectively. The differences in
photograph quality and flow level introduce several problems which we discuss below.
The steps involved in measuring features on the aerial photographs were to (1) register the photo-
graphs to a common scale, (2) digitize the outlines of specific features, and (3) export these images
to a Geographic Information System (GIS). The photographs were registered to coordinates by
defining four or five common points on the photographs and on 1:24,000 scale topographic maps.
The registration points were usually road intersections and bridge crossings. The outlines of
banks, islands, bars, side channels, and backwaters were digitized with a computer aided design
system (AutoCAD). Figure 5 shows an example of how these features were differen-tiated. Side
channels formed where the river splits around an island were distinguished from the main channel
on the basis of their smaller size. Backwaters were often associated with side channels, thus, we
grouped them as one feature. The digitized images were then exported into ARC INFO, a vector-
based GIS, for further analysis. Measurements of instream water area, island area, and side
channel-backwater area were made on a mile by mile basis throughout the study reaches.
0 500 m
1 1 1 ! H
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side channels and backwaters
Figure 5. Digitized maps of the Colorado River in 1937 and 1993 showing how specific
geomorphic features within the river channel and floodplain were differentiated.
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