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<br />o <br />c., <br />N <br /> <br />. ... <br />\.' , <br /> <br />tv <br />W <br /> <br />The primary study area for most studies conducted under the auspices of the San Juan River <br />Seven Year Research Program, including that reported herein. was the mainstem San Juan River <br />and its immediate vicinity between Navajo Darn and Lake Powell. Between Navajo Dam and <br />Shiprock there is considerable human activity within the floodplain of the San Juan River. <br />Irrigated agriculture is practiced throughout this portion of the valley and much of the immediate <br />uplands. Much of the river valley not devoted to agriculture (crop production and grazing) <br />consists of smal1 communities (e.g. Blanco and Kirtland) and several larger towns (e.g, <br />Bloomfield and Farmington). The val1ey of the Animas River, the San Juan's largest tributary in <br />the study area, is similarly developed. Downstream of Ship rock to Bluff, small portions of the <br />river valley (and uplands) are farmed; dispersed livestock grazing is the primary land use. In the <br />vicinity of Montezuma Creek and Aneth, petroleum extraction occurs within the floodplain and <br />the adjacent uplands. Between Bluff and the confluence with Lake Powel1, there are few human- <br />caused modifications ofthe system. <br /> <br />To enhance comparisons among studies and to provide a common reference for all research. a <br />multivariate analysis of a variety of geomorphic features of the drainage was performed to <br />segregate the river into distinct geomorphic reaches, This effort (Bliesner and Lamarra, 1999) <br />identified eight reaches between Navajo Dam and Lake Powell. The following provides a brief <br />characterization of each reach (Table 2), <br /> <br />Reach 1 (RM 0 to 16, Lake Powell confluence to near Slickhorn Canyon) has been heavily <br />influenced by the fluctuating reservoir levels of Lake Powell and its backwater effect Fine <br />sediment (sand and silt) has been deposited to a depth of about 12 m in the lowest end of the <br />reach since the reservoir first filled in 1980, This deposition of suspended sediment into the <br />delta-like environment of the river/reservoir transition has created the lowest-gradient reach in <br />the river. This reach is canyon-bound with an active sand bottom. Although there is an <br />abundance of low velocity habitat at certain flows, it is highly ephemeral, being influenced by <br />both river flow and the elevation of Lake Powell. <br /> <br />Reach 2 (RM 17 to 67, near Slickhorn Canyon to confluence with Chinle Creek) is also canyon- <br />bound but is located above the influence of Lake Powell. The gradient in this reach is higher <br />than in either adjacent reach and the fourth highest in the system. The channel is primarily <br />bedrock confined and is influenced by debris fans at ephemeral tributary mouths. Riffle-type <br />habitat dominates, and the major rapids in the San Juan River occur in this reach. Backwater <br />abundance is low in this reach, occurring most in association with the debris fans. <br /> <br />Reach 3 (RM 68 to 105, Chinle Creek to Aneth, Utah) is characterized by higher sinuosity and <br />lower gradient (second lowest) than the other reaches, a broad floodplain, multiple channels, high <br />island count, and high percentage of sand substrate. This reach has the second highest density of <br />backwater habitats after spring peak flows, but is extremely vulnerable to change during summer <br />and autumn storm events, after which this reach may have the second lowest density of <br />backwaters, The active channel leaves debris piles deposited throughout following spring runoff, <br />leading to the nickname "Debris Field." <br /> <br />San Juan River Non-nalive Species Interactions, Final Report, 1 February 2000 <br /> <br />8 <br />