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<br />,..., <br />,--, <br />r-- <br />-:-:- <br />C'l <br />(;( <br /> <br />Plutania, Dudley, and Maruca. 2000. Drift of Fishes in the San Juan River J99] -1997. <br /> <br />FINAL <br /> <br />Overall, flow regulation by operation of Navajo Dam has resulted in post-dam peak spring discharge <br />averaging about 54% of pre-dam values, Conversely. post-dam base tlow increased markedly over <br />pre-dam base flows, <br />Since 1992, Navajo Dam has been operated to mimic a "natural" San Juan River hydrograph <br />with the volume of release during spring linked to the amount ofprecipiIatioll recorded during the <br />preceding winter. Thus in years with high spring snowmelt, reservoir releases were "large" and <br />"small" in low runoff years , Base flows since 1992 were typically greater than during pre-dam years <br />but less than those between 1964-1991. <br />The primary study area for most investigations conducted under the auspices of the San Juan <br />River Seven Year Research Program, including that reported herein, were accomplished in the <br />mainstem San Juan River and its immediate vicinity between Navajo Dam and Lake Powell. There is <br />considerable human activity within the floodplain of the San Juan River between Shiprock and <br />Navajo Dam. Irrigated agriculture is pracIiced throughout this portion of the San Juan River Valley <br />and adjacent uplands. Much of the river valley not devoted to agriculture (crop production and <br />grazing) consists of small communiIies (e.g" Blanco and Kirtland) and several larger towns (e,g" <br />Bloomfield and Fannington). The Animas River Valley is similarly developed, Small portions of <br />Ihe river valley and uplands from Ship rock to Bluff are farmed with dispersed livestock grazing as <br />the primary land use, In the vicinity of Montezuma Creek and Aneth, petroleum extraction occurs in <br />the floodplain and adjacent uplands. There are few human-caused modifications of the system from <br />Bluff to Lake Powell. <br />A multivariate analysis of a suite of geomorphic features of the San Juan drainage was <br />performed to segregate the river into distinct geomorphic reaches, enhance comparison between <br />studies, and to provide a common reference for all research, This effort (Bliesner and Lamarra, <br />1999) resulted in Ihe identification of eight reaches of the San Juan River between Lake Powell and <br />Navajo Dam, A brief characterization of each reach (from downsIream 10 upstream) follows. <br /> <br />Reach J (RM 0 to 16, Lake Powell confluence to near Slickhorn Canyon) has been greatly <br />influenced by fluctuating reservoir levels of Lake Powell and iIs backwaIer effect. Fine sediment <br />(sand and silt) has been deposited to a depIh of about 12 m in the lowest end of this reach since the <br />reservoir first filled in 1980. This deposition of suspended sediment inIO the delta-like environmenI <br />of the river/reservoir transition makes it the lowest-gradient reach in the river. This portion of the <br />river is canyon bound wiIh an active sand bollom. Although an abundance oflow-velocity habitat is <br />present at certain flows, it is highly ephemeral, being intluenced by both river flow and Lake <br />powell's elevation. <br /> <br />Reach 2 (RM 171067, near Slickhorn Canyon to confluence with Chinle Creek) is also <br />canyon bound but is upstream of the influence of Lake Powell. The gradient in this reach is greater <br />than in either adjacent reach and the fourth highest in the system. The channel is primarily bedrock <br />confined and influenced by debris fans aI ephemeral tribUlaty mouths. Riffle-type habiIat dominates, <br />and the only major rapids in the San Juan River occur in this reach. Backwater abundance is low in <br />this reach, usually occuning in associaIion with debris fans <br /> <br />Reach 3 (RM 68 10 105, Chinle Creek to Aneth, Utah) is characterized by higher sinuosity <br />and lower gradient (second lowest) than the other reaches, a broad floodplain, multiple channels, high <br />island count, and high percentage of sand substrate. While this reach has the second greatest density <br />of backwater habitats after peak spring runoff, it is extremely vulnerable ro change during summer <br />and autumn storm events. After these storm events, this reach may have the second lowest density of <br />backwaters of the eight reaches. The active channel distributes debris piles throughout the reach <br />following spring runoff. leading to Ihe nickname "Debris Field". <br /> <br />5 <br />