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o c:-, ... "'<'f z:-...:' .Colonel John N Reese DRAFT 8 tv sand reservoir is again filled. Without cobble movement, sand will be scoured only to a depth of one-half to one median cobble diameter below the cobble bed surface." In the flume experiments, the sand level was observed approximately 0.50 to 1 cobble diameter below the surface of the cobble bed, which compared to field observations of sand depth at approximately 0.50 to 1 median cobble diameter. 0'8rien reported a cobble size range of 50-100 rom with a median size of 75 rom at the spawning site. Milhous (1982) proposes discharges of approximately one-half that required to initiate cobble movement will be capable of extracting sands and fines from the cobble substrate. Thus, after the supply of sand diminishes, flows of sufficient magnitude and duration are required to scour the cobble bed in preparation for spawning. and incubation. Colorado Sauawfish Activitv in the San Juan River As a result of the Service's 1991 biological opinion for the Animas-La Plata Project, the Bureau agreed to fund approximately seven years of research on the San Juan River and its tributaries. While these studies are not yet complete, annual reports indicate that a small reproducing population of Colorado squawfish exists on the San Juan River. Based on radio telemetry studies and visual observations, two potential spawning areas have been located at river mile 132.0 and 131.15 (Miller 1994, Ryden and Pfeifer 1995a). Both of these sites are located in an area of the river known as the "Mixer" (RM 133.4 to RM 129.8). The highest concentration of adult Colorado squawfish in the San Juan River occur between the Cudei Diversion (RM 142.0) and Four Corners (RM 119.2). Ryden and Pfeifer (1995a) report that a Colorado squawfish captured at river mile 74.8 (between Bluff and Mexican Hat) made a 50-60 mile migration to the Mixer during the suspected spawning season in 1994. The fish then returned to within 0.4 river miles of its original capture location. Successful reproduction was documented in the San Juan River in 1987, 1988, 1992, 1993, and 1994 by the collection of young-of-year Colorado squawfish. A majority of the young-of-year squawfish were collected in the San Juan River inflow to Lake Powell (Buntjer et. al. 1994, Lashmett 1994, Platania 1990). Some young-of-year squawfish have been collected from the vicinity of the Mancos River confluence in New Mexico and in the vicinity of the Montezuma Creek confluence near Bluff, Utah, and at a.drift station near Mexican Hat, Utah (Buntjer et. al. 1994, Platania 1990). In 1994, a young-of-year squawfish was collected at the confluence with the Mancos River, which is the first specimen collected at this site since 1987 (F. Pfeifer, pers. comm.). The San Juan River is one of only three remaining areas where a wild, reproducing population of Colorado squawfish still persists. The San JOan River subbasin, isolated from the Colorado River and Green River subbasins, provides a third population of wild fish, contributing an essential buffer against a catastrophic event (such as an oil spill) elsewhere in the basin. While the Colorado squawfish population may be small in the San Juan River, it may be important as unique genetic stock. Because .of this. the entire Colorado River Recovery Team (consisting of scientists from the entire Colorado River Basin, including representatives .from State wildlife agencies of California,