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I I I I I I I I I I I I I I I I I I I 2 These photographs were digitized and measurements of channel change were calculated using a Geographic Information System (GIS). In addition to this analysis, three backwater sites were monitored before, during, and after the 1993 and 1994 spring runoff to determine the effects of these flows on the morphology of the sites. Finally, a series of measurements were taken at 6 sites in the Grand Valley near Grand Junction, Colorado and downstream in Ruby-Horsethief Canyon to evaluate conditions under which the coarse bed material of the Colorado River becomes mobile. Setting The study area covers approximately 91 kilometers (57 miles) of the Colorado River (Fig. 1). This area includes the 15 and 18 mile reach in the Grand Valley near Grand Junction, and Ruby-Horsethief Canyon. These areas are important habitat for both the Colorado squawfish (Ptychocheilus lucius) and razorback sucker (Xyrauchen texanus) (USFWS,1987). In the Grand Valley, the Colorado River flows within a broad (-1 km) floodplain, although bank revetments and dikes limit channel change in many places. In the Ruby- Horsethief Canyon reach, the river is more confined by sandstone bedrock, but a small discontinuous floodplain borders much of the river. The median grain size (Dso) in both reaches is near 50mm. The flow of the Colorado River in this area is dominated by snowmelt, but upstream water development also affects flows. For example, the average peak discharge of the Colorado River at Glenwood Springs has decreased by 28% when comparing separate time periods from 1931-1961 and 1962-1993, and the Colorado River at Cameo has decreased by 19% for roughly the same time period (Pitlick and Van Steeter, 1994). These data clearly indicate that reservoirs constructed in the last 30 years have had a significant impact on the natural flow regime.