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Last modified
7/14/2009 5:02:33 PM
Creation date
5/20/2009 10:24:41 AM
Metadata
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UCREFRP
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
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The accuracy of photogrammetric measurements is affected to varying degrees by the clarity of the <br />photographs, differences in flow level, and distortion. Differences in clarity lead to problems in <br />the interpretation of features and the accuracy with which they can be digitized; differences in flow <br />level affect the planform area of the river and associated features; and distortion near the edges of <br />photographs can make objects appear larger or smaller than they really are. For practical reasons, <br />we did not rectify the photographs to correct for distortion. We did, however, evaluate the <br />potential error from these various sources. Errors due to interpretation and tracing of objects on a <br />set of photographs were evaluated by re-digitizing reaches of the river and comparing the results to <br />the original measurements. Errors due to differences in flow level were evaluated from field <br />measurements of channel cross sections at different flows. Finally, errors due to distortion were <br />estimated by measuring the area of 20 islands near the center of the photographs, and comparing <br />this to the area of the same islands when they were near the edges of the adjacent photographs. <br />The results of these tests indicate that the error associated with interpreting and tracing the main <br />channel boundary is negligible (2%). The error associated with tracing side channels and <br />backwaters is more sizable (10%), because these features are harder to interpret. Differences in <br />discharge have a negligible (-3%) effect on measurements of planform area, as long as the <br />difference in discharge is less than about 30%. Thus, for the Colorado River, we feel confident <br />comparing the photographs from 1954 with those from 1968, and those from 1937 with those <br />from 1993, but not in comparing them all together. For the Gunnison River, the difference in <br />discharges between 1937 and 1995 is relatively large (-80%), but the higher discharge occurs in <br />the more recent set of photographs, which would tend to make the channel appear larger, even if it <br />had not changed. With respect to other sources of error, the average error due to distortion at the <br />edge of photographs is approximately 3%, but since we tried to avoid measuring features near the <br />edge, the error introduced by distortion is certainly much less. Even so, if we assume a worst case <br />scenario where the individual errors are additive, then it is possible that the photogrammetric <br />measurements of main channel area are off by as much as 8%, and that the measurements of side <br />channel and backwater area are off by as much as 16%. If we further assume that every feature <br />was overestimated in one set of photographs, and underestimated in another set, then the maximum <br />potential error could be twice as large. Although it is highly unlikely that the errors are all additive <br />and always in the same direction, we use these values as a basis for saying whether or not the <br />observed changes in channel morphology are significant. <br />Field Studies <br />Field studies were conducted from 1993 through 1996 to (1) monitor geomorphic changes; (2) <br />determine the average characteristics of the main channel (width, depth, slope and grain size); and <br />(3) develop flow and sediment transport models. Geomorphic changes in side channels and <br />backwaters were monitored by repeated surveys. Prior to the start of the 1993 snowmelt runoff <br />period, three side channel-backwater sites along the Colorado River were selected for detailed <br />study. USFWS biologists recommended these sites to us because they were typical of habitats <br />used by adult Colorado squawfish. Figure 3 (presented earlier) shows a site that we monitored in <br />the 15-mile reach; the mouth of this side channel is a backwater at most flows. Another site, <br />located near river mile (RM) 162 in the 18-mile reach, is formed by an alternate bar and chute <br />channel. The chute channel conveys water at moderate to high flow but becomes a backwater at <br />low flow. The third site, located near RM 160, is formed by a permanent island and side channel. <br />This side channel conveys water at moderate to high flow but likewise becomes a backwater at low <br />flow. At each site, a series of cross sections were surveyed around the head and mouth of the side <br />channel. These areas were of interest to us because they control the amount of flow into and out of <br />the side channel, which determines whether or not fish can access the site. <br />10
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