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Snowmelt runoff in the Colorado River basin follows a predictable annual pattern, with flows typically rising in April and May, reaching a peak in early- to mid-June, and receding through July and August (Fig. 2). Localized thunderstorms and moderately intense rainfalls associated with the southwest monsoon (a regional weather pattern produced by the flow of moist air from the Gulf of California and Pacific Ocean) occur sporadically through late summer and early fall. Fleener's (1997) analysis of precipitation and streamflow records from various stations in the region shows that the hydrologic influence of the monsoon clearly diminishes from southwest to northeast, with the most noticeable effects occurring in San Juan and Delores River basins near the Four Corners area. Several such storms in the 1940s and 1950s produced moderately large floods with high suspended sediment concentrations (Ioms et al., 1964). Our analysis of streamflow and sediment records from USGS gauges further upstream indicates that these events are not nearly as common in the Grand Junction area as they are further south. Colorado River near Cameo, CO 1000 800 m E 600 m rn t 400 D 200 0 --------- 1995 1996 -----1997 r' --o-1998 \ APR MAY JUN JUL AUG SEP Figure 2. Hydrographs of the Colorado River for 4 recent periods of snowmelt runoff. Natural streamflows of the Colorado River, and its two major tributaries, the Gunnison River and the Dolores River, are affected by many dams and water diversions upstream. Liebermann et al. (1989) report that there are 24 reservoirs with a storage capacity of more than 6.2 x106 m3 (5000 acre-feet) upstream of the Colorado-Utah state line. Collectively, these reservoirs store the equivalent of about half the annual streamflow of the Colorado River (Pitlick et al., 1999). Most of the dams and reservoirs in the upper basin were constructed between 1950 and 1980, making this a potentially important period with respect to changes in streamflow, sediment loads and habitat characteristics. Diversion structures and canals are likewise located throughout the upper basin. The amount of water diverted out of the Colorado River basin varies from 10 to 20% of the annual natural flow (Pitlick et al., 1999). Large diversions, including several near Grand Junction, have the added effect of blocking upstream fish migrations. 4