Correspondences Concerning Upper Colorado Biological Opinions 1993

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Elizabeth Estill, Regional Forester 10 (pond temperatures were as much as 10.5 °C warmer than the adjacent river). Some of the squawfish captured from one pond were well tuberculated by June 3, when nearby river temperatures were only 10 °C -13 °C (Kaeding, pers. comm.). It has been hypothesized by some investigators that thermal energy units above those provided in the mainstream are important to gonadal maturation. If this is true, then access to these sheltered off - channel pools may be very important to successful spawning in the upper reaches of the Colorado River. The 15 -mile reach has experienced major agricultural water depletions for many years. During late summer and early fall, this reach can be severely dewatered. Water depletions in the 15 -mile reach have been identified as a limiting factor for Colorado squawfish. Therefore, the Service has recommended a flow of 700 -1,200 cfs during July, August, and September (Osmundson and Kaeding 1989). The objectives of these flows are to provide near maximum aggregate amount of run, pool, and riffle habitat for adult Colorado squawfish and to enhance the first -year growth of age -0 Colorado squawfish in the Grand Valley area by increasing water temperatures. Flow recommendations for the rest of the year call for high spring flows that are critical for shaping the river channel, determining substrate composition, and influencing the abundance of various species for the remainder of the year. Flow recommendations for winter call for flows between 1,000 and 2,000 cfs (no change from current flows). Razorback Sucker The razorback sucker, an endemic species unique to the Colorado River Basin, was historically abundant and widely distributed within warmwater reaches throughout the Colorado River Basin. Historically, razorback suckers were found in the main stem Colorado River and major tributaries in Arizona, California, Colorado, Nevada, New Mexico, Utah, Wyoming, and in Mexico (Ellis 1914; Minckley 1983). Bestgen (1990) reported that this species was once so numerous that it was commonly used as food by early settlers and, further, that commercially marketable quantities were caught in Arizona as recently as 1949. In the Upper Basin, razorback suckers were reported in the Green River to be very abundant near Green River, Utah, in the late 1800's (Jordan 1891). An account in Osmundson and Kaeding (1989) reported that residents living along the Colorado River near Clifton, Colorado, observed several thousand razorback suckers during spring runoff in the 1930's and early 1940's. In the San Juan River drainage, Platania and Young (1989) relayed historical accounts of razorback suckers ascending the Animas River to Durango, Colorado, around the turn of the century. A marked decline in populations of razorback suckers can be attributed to construction of dams and reservoirs, introduction of nonnative fishes, and removal of large quantities of water from the Colorado River system. Dams on the main stem Colorado River and its major tributaries have segmented the river system and drastically altered flows, temperatures, and channel geomorphology. Major changes in species composition have occurred due to the introduction of numerous nonnative fishes, many of which have thrived due to man - induced changes to the natural riverine system.