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METHODS Study Area The Grand Valley is the uppermost portion of the range of the Colorado squawfish in the main- stream Colorado River. The upper end is demarcated by the Price Stubb Diversion structure (RM 188.3) that blocks upstream movement of fishes. However, most rare fish use extends only to the Grand Valley Diversion (RM 185.4), a seasonal barrier three miles downstream of the Price Stubb Dam. The Grand Valley consists of two major reaches: one above the mouth of the Gunnison River (15-mile reach) and one below (18-mile reach). These segments contains more adult Colorado squawfish per mile than any other portion of the Colorado River and also contain what may be the only remnant population of nverine razorback sucker (USFWS unpublished data). Because of inflow from the Gunnison River, the 18-mile reach has a greater discharge on any given day than does the 15-mile reach (Fig. 1). Also the two reaches differ in average gradient: river elevation falls 9.0 ft/mile in the 15-mile reach, and 6.7 ft/mile in the 18-mile reach. Within the 15-mile reach the Colorado River alternates between single-thread and multi-thread channels. Pitlick and Van Steeter (1994) suggest that it is very close to a threshold between braiding and meandering. They describe the riverbed as formed by cobble- and gravel-sized 30,000 25,000 U W Q 20,000 U U) 0 15,000 J p 10,000 2 Z 2 5,000 0 Figure 1. Contribution of flows to the Colorado River from the Colorado and Gunnison rivers, using 1991-1993 as an example. Returned irrigation flows account for the difference between discharge at the State Line gage and the combined Colorado-near-Palisade and the Gunnison-near- Grand Junction gages. Flows at the top of the 18-mile reach would be somewhere between the combined flow and the State Line flow because 15-mile reach return flows would be added but not the 18-mile reach return flows. 6 OCT DEC FEB APR JUN AUG OCT DEC FEB APR JUN AUG OCT DEC FEB APR JUN AUG