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Effects of Recent High Flows on Selected Reaches of the Upper Colorado River John Pitlick, Mark Van Steeter and Margaret Franseen Department of Geography University of Colorado Boulder, CO 80309 ABSTRACT This report summarizes recent observations and field measurements of channel change in selected reaches of the Colorado River near Grand Junction, Colorado. This work was undertaken to assess effects of recent high flows on sites that have been monitored as part of on-going fish habitat studies. Peak discharges and total runoff volumes of the upper Colorado River during the spring and summer of 1995 were the highest in over a decade. Field surveys of 24 cross sections at three separate backwater study sites and 12 cross sections of the main channel indicate that while high flows in 1995 produced clear changes in channel morphology at a few localities, most sites experienced relatively minor changes. Of the three backwaters, two were enlarged somewhat by the recent high flows, but the third was essentially unaffected. Changes observed at most of the main channel cross were restricted to localized scour and fill, usually much less than 0.5 m, and minor bank erosion. However, there was one instance where 30 m of bank was eroded at a cross section in the alluvial reach downstream of Ruby-Horsethief Canyon. The changes observed here are notable because they occurred immediately downstream of a suspected spawning site. INTRODUCTION In 1993 we began studies of historic and on-going changes in the geomorphology of the Colorado River near Grand Junction, Colorado. This segment of the river provides important habitat for the Colorado squawfish and razorback sucker, both Federally listed endangered species. Our study covers approximately 60 miles of the river and includes 3 contiguous reaches known separately as the 15-mile, the 18-mile, and the Ruby-Horsethief Canyon reaches (Fig. 1). The capture of adult and larval squawfish in the 15 and 18- mile reaches suggests that these reaches provide important spawning and rearing habitat (Osmundson and Kaeding, 1989). Habitats that might be used for similar purposes are not as common in the Ruby-Horsethief Canyon reaches, primarily because the river is more constrained here by bedrock, but larval fish pass through these reaches as they drift downstream as do adult fish when they return upstream to spawn.