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12 I I I I I I I I I I I I I I I I I I I for Colorado squawfish. Although flows of the Yampa River were nearly an order of magnitude larger than those of the Dolores River, the pattern of the hydrograph for both systems was similar. RM 64.4 to 177.0. Flows in the Dolores River above the confluence of the San Miguel River were dominated by releases from McPhee Dam. Although seasonal inputs from ephemeral and perennial tributaries contnbuted significant flows, the operation of McPhee Dam had the greatest influence on the hydrograph. Flows encountered in the Dolores River above the confluence of the San Miguel River ranged from 21 to 730 cfs (Table 34). A comparison of mean monthly flows from two USGS gages (the Dolores gage immediately above McPhee Reservoir, and the Bedrock gage 120 miles below McPhee dam) indicates that the operation of McPhee affects both the timing and magnitude of flows below the dam to some degree (Figure 9). Similarities between the two hydrographs, particularly associated with peak flows, was the result of several high flow years since the closure of the dam (1984, 1985 and 1987). Differences between mean montWy flows for the study period (1990 and 1991) and overall mean monthly flows indicate that in low to medium water years McPhee Dam drastically affects the hydrograph. Before 1990, releases from McPhee Dam were based on runoff predictions from snowpack and reservoir level on March 1 and April 24, respectively. Annual releases were based on criteria for dry, normal or wet years with base flows of 20,50 and 78 cfs, respectively. A series of wet years (1985-87) following closure of McPhee Dam in 1984 continued to affect reservoir levels and base flows were maintained at about 78 cfs. Persistent drought from 1988 through 1991 greatly reduced water availability in the Dolores River drainage and the operation of McPhee Dam was modified in 1990. On March 5, 1990 (first day of Trip 1), releases from McPhee Dam were reduced to 20 cfs because of low reservoir level and predicted of low runoff from high elevation snow-pack. Low flows from McPhee Dam continued in 1990, ranging from 20 to 50 cfs. Flows increased to 50 cfs during July 1990, as a result of an informal agreement between Reclamation, water users and CDOW (personal communication with Tom Beck, CDOW). Increased summer flows were intended to maintain cooler temperatures for the tailwater trout fishery. Low flows, ranging between 20 and 50 cfs were continued through 1991. One short release in 1991 provided flows for rafters for approximately 10 days during the Memorial Day weekend. 4.3.1.2 Temoerature. To evaluate main channel water temperatures in the Dolores River and factors that influence temperatures, the study area was divided into two regions, including 1) the Dolores River below the confluence of the San Miguel River (RM 0.0-64.4), and 2) the Dolores River above the confluence of the San Miguel River to Bradfield Bridge (RM 64.5-177). RM 0.0 to 64.4. Temperature in this region was influenced primarily by the San Miguel River. Main channel temperatures recorded in the Dolores River during the study ranged from a low of 3.50C (March 1-3, 1990) to a high of 28.50C (August 12, 1991) (Table 35). USGS temperature data at Bedrock (RM 75) ranged from a high of 30 oC in July to OOC during many days in winter. Maximum temperature recorded at the Bedrock gage was 33.50C on July 10, 1981. Temperatures in the lower Dolores River, below the confluence of the San Miguel, were moderated by larger volumes of water and ranged from 2~C in July and August to OOC during the winter. Maximum temperature recorded at the USGS gage near Cisco (approximately 9.5 miles above the Colorado/Dolores River confluence) was 2~C on August 14, 1958. Analysis of mean montWy temperatures from the USGS gages near Cisco for both the Colorado and Dolores Rivers indicate that lower volume and early runoff in the Dolores River resulted in