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<br />station was wetter than the long-term average, which
<br />has probably overestimated the flows for McPhee
<br />shown in Table 3. With these considerations in mind,
<br />and by comparing the data in Table 3 to field data
<br />collected in 1989, the pre-dam bank-full discharge for
<br />the upper reaches of the study area is estimated to be
<br />2,000-2,500 ft3/s (Table 4). Since the pre-dam bank-
<br />full flow usually occurred from snowmelt flooding, a
<br />progressive downstream increase in the bank-full
<br />discharge would be expected as a result of tributary
<br />inflow, At Bedrock this expected increase is on the
<br />order of 300-500 ft3/s,
<br />The available sediment yield data for the Dolores
<br />River (Irons et aI., 1985, USDI, 1974) indicates that
<br />there is a substantial progressive downstream
<br />increase in Total Suspended Sediment (TSS). Data
<br />collected just above the study reach showed a TSS
<br />yield of 182,000 tons annually (0.34 tons/acre/year),
<br />whereas the Dolores River at Cisco yielded 2,254,000
<br />tons annually, (0.80 tons/acre/year). Sediment yield
<br />per acre characteristically decreases with increasing
<br />drainage area However, due to the high runoff and
<br />erosion potential of the semiarid Lower Dolores
<br />Basin, the situation here is reversed.
<br />
<br />Table 3, Bank-full Flow Estimates at USGS Gauging Sites
<br />
<br />Within the study area the primary sediment
<br />contributor is Disappointment Creek. Being one of
<br />the few perennial tributaries in the study area,
<br />Disappointment Creek's confluence with the Dolores
<br />River is about 5.5 miles upstream from the town of
<br />Slick Rock at river mile 124. The Disappointment
<br />drainage is about 350 square miles and is largely
<br />comprised of highly erodible saline soils. Data
<br />collected at USGS gauging station 9168100, roughly
<br />19 miles upstream from the confluence, shows the
<br />mean annual flow of Disappointment Creek to be
<br />about 17 ft3/s (annual water yield of 12,300 acre-
<br />feet). Specific data on the.sediment yield of Disap-
<br />pointment Creek is not available, but from regional
<br />data (USDA, Soil Conservation Service, 1974) it is
<br />estimated to be 225,000 to 500,000 tons per year (1-2
<br />tons/acre/year), Disappointment Creek also degrades
<br />the water quality of the Dolores River with salinity.
<br />During the 1984 water year, an estimated 10,000-
<br />11,000 tons of salt were discharged in the flows at the
<br />above-mentioned gauging station (USDI, 1986).
<br />This does not include the 200 square miles of
<br />drainage below the gauge.
<br />
<br /> 1,S-Year Recurrence Interval
<br />Station Period of Record Peak Flow 7-day High Flow
<br /> (ft3/5) (ft3/5)
<br />At Dolores 1895-1903,1910-12,1921-83 2,589 2,045
<br />Near McPhee 1939-1952 2,964 2,110
<br />At Bedrock 1918-22, 1972-83 3,068 1,808
<br />
<br />Table 4, Bank-full Flow Estimated at Selected Locations Within the Study Area
<br />
<br />Location Bank-Full Flow
<br />Bradfield Bridge 2,000
<br />Dove Creek Pump Station 2,000
<br />Below Disappointment Creek 2,200
<br />Below Gypsum Valley 2,300
<br />Above La Sal Creek 2,400
<br />At Bedrock 2,500
<br />
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