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o o (:-) 8 ( ~-\ comprising the Upper Colorado River Basin. The San Juan River drains about 38,000 square miles of southwestern Colorado, northeastern Arizona, northwestern New Mexico, and southeastern Utah. From its origins in the San Juan Mountains of Colorado, the San Juan River flows some 31 miles to the New Mexico border, 190 miles westward to the Four Corners area, and thence another 136 miles to Lake Powell. In its upper reaches, the river traverses rugged terrain and has a relativelY high gradient. The river emerges from canyon- bound reaches shortly after entering New Mexico and flows through a broad floodplain for much of its course in New Mexico and Utah. About 70 miles upstream of Lake Powell, the river again enters canyon reaches for the remainder of its course. The river is generally restricted to a single channel in canyon portions, but is often divided into several channels in floodplain reaches. c..'c) Q The San Juan River has comparatively few perennial tributaries, most of which are in upper reaches. Historically, the Pinos, Piedra, Navajo, Animas, La Plata, and Mancos rivers, Rio Blanco, and McElmo Creek were the only perennially flowing tributaries. Other streams such as Montezuma and Chinle creeks seasonally contributed flows. Numerous washes and arroyos also entered the river, but none provided regular flow. Among tributaries, the Animas River contributed the greatest flow. Eight distinct geomorphic reaches within the San Juan River have been identified by Bliesner and Lamarra (1995) in the San Juan between the two major features that essentially define the river for the purposes of conservation of the Colorado squawfish: Lake Powell at the downstream terminus of flowing water, and Navajo Dam at the upstream control of flows in the river. The eight reaches were screened through several data sets within the categories of: river valley width, channel contact geology (material in cut-banks & bedrock contact). riparian vegetation, channel gradient, channel pattern (braiding and sinuosity). tributary influence, anthropogenic influences (diversion dams, irrigation, levees, etc.) and aquatic habitat (six categories at three flow rates). Utilizing these categories, the following reaches of the San Juan have been identified (by River Mile [RM]): 1. Lake Powell influence (RM 0 to RM 14) 2. Canyon (RM 15 to RM 67) 3. Chinle to Aneth (RM 68 to 105) 4. Aneth to Mixer (RM 106 to RM 130) 5. Mixer to Hogback (RM 131 to RM 154) 6. Hogback to Animas (RM 155 to RM 180) 7. Animas to Blanco (RM 181 to RM 213) 8. Blanco to Navajo Dam (RM 214 to RM 224) The reach of currently known occupied Colorado squawfish habitat extends from Lake Powell upstream to river mile 158.4 (Hogback Diversion) and could be significantly impacted due to upstream water withdrawals. Of the 225 miles, about 159 of those are potentially available to the Colorado squawfish. Two diversion structures near Fruitland, New Mexico (the Hogback at river mile 158.6 and the Public Service Company of New Mexico (PNM) diversion weir at river mile 166.1). span the entire river channel and are believed to be effective blocks to upstream fish migrations (Platania 1990). At certain times, these structures may present barriers or impediments to fish passage and, thus, preclude habitat use of areas upstream.