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EXECUTIVE SUMMARY The 15-mile and 18-mile reaches of the Colorado River in western Colorado provide important habitat for three endangered fishes: the Colorado pikeminnow (Ptychocheilus-lucius), the razorback sucker (Xyrauchen-texanus), and the humpback chub (Gila Cypha). Success in recovering these fishes will depend in large part on the maintenance and improvement of existing habitats which have been lost or altered as a result of water management activities in the upper Colorado River basin. Under certain conditions reservoir operations can be adjusted to allow a portion of the runoff to bypass reservoirs, boosting peak spring flows in the 15- and 18-mile reaches. The purpose of this study, therefore, was to assess the effects of coordinated reservoir releases and normal snowmelt flows on geomorphic processes in the 15- and 18-mile reaches. Conditions in specific reaches were monitored from 1998-2004 to verify thresholds for sediment transport and to provide additional information on discharges that perform important geomorphic functions. Field measurements focused on geomorphic effects of late spring-early summer flows and seasonal variations in the movement of fine sediment. The study period included several years of intense drought with very low runoff produced throughout the upper Colorado River basin. U.S. Geological Survey records of streamflow suggest that water years 2002-2004 were perhaps the lowest in the upper Colorado River basin in the last 100 years. Nonetheless, coordinated reservoir operations were implemented for 7 days in 1998 and 10 days in 1999, resulting in increases in discharge of 40-56 m3/s (1500-2000 ft3/s), in the 15-mile reach. The bypass flows were successful in boosting background flow levels by 10- 15%, which was sufficient to mobilize the bed material in a number of places in the 15- and 18- mile reaches. The limited availability of water in subsequent years prevented further tests of the geomorphic effects of bypass flows. The flows that did occur were well below average, thus the