Dolores River Core Science Report

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of which rise vertically over 1000 feet from the water's edge (CO DNR et a1.,1976). The river's gradient in this reach is 0.21% (11 ft/mile) (CO DNR et al., 1976). San Miguel River to Colorado River The San Miguel River is the largest tributary to the mainstem Dolores, and because it is relatively unregulated flow volume increases substantially here at certain times of the year. Downstream of the San Miguel confluence, the Dolores River flows through another narrow deep canyon for 4 miles before the valley again widens, after which the river is paralleled by Highway 141 and numerous gravel and dirt roads. The 31-mile reach from the San Miguel confluence to Gateway has a gradient of 0.19-0.28% (10-15 ft/mile). Downstream of Gateway, the river initially meanders through a broad (approximately 2-mi. wide) valley flanked by a steep, sandstone dominated mesas rising 800 to 2500 ft. above the valley floor (River mile 31- 17), before entering a narrow, steep-walled canyon (about 0.25-mile wide) past the state border (River mile 17-11) (USDI, 1979). Bed substrates consist of sand, gravel, and boulders derived from episodic debris flows out of tributaries, and from roclcfall from canyon walls. The lowermost portion of the Dolores River (River mile 11-0) flows through soft sediments and a wider valley before entering the Colorado River (USDI, 1979). Effects of flow on downstream geomorphic processes The effects of diversions and storage on baseflows and high flows downstream of McPhee Dam are described in detail in the Dolores River Dialogue Hydrology Report. From a geomorphic perspective, the magnitude, frequency, and duration of high flows are the most important elements of a river's flow regime. The following section describes the geomorphic importance of high flows, historic changes in the geomorphically important components of flow regimes in the Dolores River, channel-forming discharge estimates, and the expected effects of flow modifications on river morphology. The role of high flows in driving geomorphic processes k1 recent years, river scientists have identified flow as a master variable controlling river ecosystem function and have developed a strong conceptual understanding of how natural flow regimes contribute to the health of river ecosystems (Poff et al. 1997, Bunn and Arthington 2002). Understanding of the geomorphic importance of high-flow components of river flow regimes is especially well established. Peak flows are important for formation and maintenance of the shape and form of the river channel (Leopold et al., 1964; Wolman and Leopold, 1957; Wolman and Miller, 1960). Increases in river discharge are accommodated by increases in the width, depth and/or velocity of the flow, creating greater forces on the channel bed and resulting in increased ability of the river to mobilize larger and larger sediment sizes. Mobilization and transport of bed sediment at high flows is responsible for maintaining channel form and can result in flushing of fine sediments, reworking of gravels, formation and maintenance of pool and riffle sequences, and formation and/or erosion of river bars. High flows play an important role in mobilizing and transporting coarse and fine sediment inputs delivered from tributaries. hl addition, ligh discharges that flow out of the channel banks carry sediment that deposits on the floodplain, rejuvenating the floodplain surface and facilitating reproduction of certain riparian species. High flows also counteract the effects of vegetation encroachuilent and the growth of mid-chamlel bars by disturbing vegetation establislunent within the active channel, including on mid-chamlel bars, and on floodplain surfaces. The geomorphic effectiveness of high flows is a 22