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<br />o <br />~ Reach 1 (RM 0-16) Lake Powell confluence to near Slickhorn Gulch. <br />"-I <br />(2 Reach 1 is located in a narrow, canyon-bound valley. Channel morphology in the reach has been <br />e,:, strongly influenced by Lake Powell, with influence increasing in a downstream direction and <br />variable depending on lake level. Reduced water velocities associated with the backwater effect of <br />the lake have resulted in a large deposit of sediment in this reach and a channel gradient that is <br />lowest of all reaches (0.00105 ftfft). Substrates are primarily sand and silt with cobble substrates <br />occurring rarely and in isolated areas. Sinuosity of the channel is low in this reach, and shallow <br />sandy runs and shoals are the most prevalent habitats. Except for some road access points, direct <br />effects of human activity adjacent to the river in the reach are minimal. <br /> <br />Reach 2 (RM 17-67) Near Slickhorn Gulch to near confluence with Chinle Creek. <br /> <br />This is also a canyon-bound reach with the narrowest valley width of all eight geomorphic reaches. <br />Sinuosity of the channel is low as in Reach 1, but the channel gradient is significantly higher than <br />both adjacent reaches (0.00178 ftfft). Riffle comprise a large proportion of the surface habitat and <br />most of the major rapids in the study area are found in this reach. ' High water velocities and a <br />relatively deep main channel are reflected in the low total water surface area observed in this reach <br />over a wide range of flows. Cobble substrate is more prevalent than in Reach 1 but less than Reach <br />3. A series of bedrock and talus control points increase channel complexity in localized areas within <br />this reach. Reach 2 includes the municipality of Mexican Hat, UT, and an increased amount of <br />human activity adjacent to the channel relative to Reach 1. <br /> <br />Reach 3 (RM 68-105) Near confluence of Chinle Creek to just upstream of Aneth, DT. <br /> <br />The river flows through a moderately broad valley in this reach, with a more sinuous channel than <br />in either Reach 1 or 2. Channel gradient is significantly lower than in both adjacent reaches and is <br />second lowest of all reaches (0.00143 ftfft). Cobble substrate is also more prevalent in Reach 3 than <br />in the lower reaches; however, it, is interspersed with a series of large, sand shoal complexes ' <br />associated with areas of channel instability. This area of sand shoal complexes, termed the "debris <br />field," appears to be associated with the effects of a serie_s of cobble areas acting as channel controls. <br />One of the largest of these cobble deposits is located at the entrance to the canyon at the beginning <br />of Reach 2. It is speculated that the abrupt narrowing of the valley as it entered the canyon acted as <br />a control point during large magnitude flow events during the last 5,000 to 10,000 years, resulting <br />in this large cobble deposit just upstream of the canyon mouth and creating the conditions partly <br />responsible for the channel morphology presently seen in this reach (V. Lamarra 1997, Ecosystems <br />Research Institute (ERI), pers. corom.). Secondary channels also become more prevalent in this <br />reach compared to Reach 1 or 2. Reach 3 includes the municipalities of Bluff and Aneth, UT, and <br />an corresponding increase in human activity adjacent to the channel relative to Reach I or 2, <br />including the presence of some irrigated areas within the valley bottom. This reach also includes <br />the largest number of oil wells adjacent to the river channel. Two perennial tributaries, Chinle and <br />McElmo creeks, and six major ephemeral channels flow into the San Juan River in this reach. <br /> <br />San Juan River Recovery Implementation Program <br />Summary Report ' <br /> <br />6 <br /> <br />17 January 1997 <br />PR-576-2 <br />