Dolores River Core Science Report

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(TSS) increases from an average of 182,000 tons/yr (0.34 tons/acre/yr) above McPhee Dam (USDI BLM 1990) to 2,524,000 tons/yr (0.86 tons/acre/yr) at the Dolores River at Cisco (TJSGS gage 0918000; 1949-1964). Estimated sediment yields in the watershed upstream of McPhee dam are very low (<02 ac- ft/mil/yr in most of the drainage area), which is in order of magnitude lower than downstream portions of the Dolores River basin) (USDA, 1972). Areas downstream of McPhee Dam have a more arid climate, sparser vegetation, and less resistant rock types than upstream areas, resulting in increased erosion potential Much of the Dolores River downstream of McPhee Dam is flanked by rock outcrops (mostly sandstone, with some shale outcrops) and/or areas with very shallow soils. Roclcfall from canyon walls is likely an important source of coarse sediment to the river, and the estimated sediment yield from these areas ranges from <.2-1 acft/ mil/yr (USDA, 1972). Substantially higher sediment yields (1-3 ao-ft/mil/yr) are found in the three large stn~ctural valleys that separate the canyon reaches of the Dolores River: Disappointment Valley, Big Gypsum Valley, and Paradox Valley (USDA, 1972). Among these, Disappointment Valley delivers the largest amount of sediment to the mainstem Dolores River due to the perennial flow in Disappointment Creek, which enters the mainstem at River mile 124 near Sliclcrock (USDI BLM, 1990). Much of Disappointment Creek's 350-mil drainage area is underlain by Marcos Shale, which is mantled by shallow, easily eroded soils that produce considerable sheet and gully erosion (USDA, 1972). The above description of sediment yield characteristics in different portions of the Dolores River basin provides a basis for analyzing the likely effects of McPhee Dam on sediment dynamics in the Dolores River. Dams can potentially alter downstream sediment dynamics by blocking sediment supply to downstream reaches and/or by reducing the frequency and magnitude of sediment-mobilizing flows (Ligon et al., 1995). In terms of the first of these potential impacts, available evidence suggests that the effect of McPhee Dam on downstream sediment supply to the Dolores River is small. As discussed above, the geology and climate of the watershed upstream of McPhee Dam result in a low rate of natural sediment delivery to channels. k1 addition, field observations of McPhee Reservoir suggest that miiumal sediment accumulation has occurred in the reservoir since dam closure in 1984, although no formal measurements of reservoir sedimentation have been completed. Moreover, nat<iral and anthropogenic sediment delivery to the Dolores River downstream of McPhee Dam, including from Disappointment Creek and other tributaries, is substantial, likely overcompensating for any potential reduction in upstream sediment supply. McPhee Dam has had substantial effects on the frequency and magnitude of sediment mobilizing flows, however, likely resulting in changes in bed sediment composition and in channel form. Data on bed-substrate sizes in the Dolores River are limited, but geomorphic principles and analysis of flow modification suggests that flow modification has likely produced an overall fining (reduction) of bed sediment sizes and a decreased frequency of sediment mobilization. Historically, sediment entering the main stem Dolores River from tributary basins and/or from canyon walls would be mobilized and reworked with a frequency proportional to the size of the sediment. For example, fine sediments delivered from tributaries to the mainstem would likely be transported downstream at low to moderate discharges under unregulated conditions, but such sediments now are deposited on the channel bed and result in siltation. Such siltation increases the embeddedness of bed materials, as fine sediments infiltrate into the 37