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Executive Summary Background The 15-Mi1e Reach (15-MR) of the Colorado River is located between the Gunnison River confluence at Grand Junction (River Kilometer (rkm) 275.1) and Palisade (rkm 298.1), Colorado (Figure ES-1). Populations of federally-listed threatened and endangered native fish species (Colorado pikeminnow, Ptychocheilus lucius, and razorback sucker, Xyrauchen texanus) occupy this reach of the river, and it has been suggested that their recovery is dependant, among other factors, on increasing depleted spring snowmelt runoff peaks (Osmundson, 1999). It has been hypothesized that increased peak flows are required for channel maintenance purposes and for removal (flushing) of fine sediment from gravels and cobbles that constitute the bed material of the river (Osmundson et al. 1995). It has also been suggested that removal offines from the bed of the river as a result of bed mobilization by increased peak flows would increase periphyton and macro invertebrate productivity, and thus increase the overall carrying capacity for the listed fishes of the Upper Colorado River (Lamarra 1999; Osmundson and Scheer 1998; Osmundson 2002). A number of previous studies on primarily salmon spawning streams and rivers have shown that fine sediment cannot be winnowed or flushed from appreciable depths without mobilization of framework gravels (Diplas 1994; Wilcock et al. 1995; Kondolf and Wilcock 1996; Milhous, 1999). However, suspended sediment loads in the Colorado River were higher prior to 1940, primarily as a result of widespread arroyo incision that commenced in the mid 1800' s (Thompsen, 1982, 1984; Gellis et aI, 1991). Even though the sediment concentrations have remained relatively constant in the last 50 years, total sediment loads in the 15-MR have been reduced because of the . reduced flows (Pitlick and Van Steeter, 1998). Since the 1950's there has been a 30 to 40 percent reduction in the magnitudes of the 2- and 5-year peak flows as a result of upstream water development projects (Pitlick et al. 1999; this study). Reductions of the peak flow magnitudes and reduced suspended sediment loads have caused a 10-15 percent reduction in average channel width, and about a 25 percent reduction in side channels and backwaters (Van Steeter and Pitlick, 1998; Pitlick and Van Steeter, 1994; Pitlick et aI, 1999). Regardless of the changes in hydrology and sediment supply, Pitlick et al. (1999) concluded that the current channel morphology is in equilibrium with the current peak flow regime and suspended sediment load, and'therefore, it can be concluded that there are unlikely to be further channel adjustments if the present peak flow regime is maintained. While the current morphology of the Colorado River within the 15-MR may be in equilibrium with the post-1950's peak flow regime, the question of the adequacy of the current peak flow regime forfine sediment flushing and biological productivity (Osmundson et aI, 2002) cannot be addressed by a generalized study of sediment mobilization. Pitlick et al (1999) identified discharges for incipient mobilization of the bed material (10,000 cfs) and for general mobilization of the bed material (22,000 cfs), but as pointed out by Downes et a1 (1997) flow- sediment-habitat relations occur at micro-and meso-scales levels that are not represented by macro-scale analyses. The initial focus of this study within the 15-MR was to address the hypothesis that the current peak flow regime is limiting to the recovery of the listed native fish species and the aquatic Draft Final Report - 15 Mile Reach Studies Miller Ecological Consultants, Inc. Page ES-l October 24, 2003