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>,;'~ '-.,;.. . Lt. Colonel Dorothy F. Klasse 7 1969). The Colorado squawfish is the largest cyprinid fish (minnow family) ~ native to North America and. during predevelopment times. may have grown as large as 6 feet in length and weighed nearly 100 pounds (Behnke and Benson 19B3). These large fish may have been 25-50 years of age. The Colorado squawfish currently occupies about 1.030 river miles in the Colorado River system (20 percent of its original range) and is presently found only in the San Juan and other subbasins above Glen Canyon Dam (Tyus 1990). Based on early fish collection records. archaeological finds. and other observations. the Colorado squawfish was once found throughout warmwater reaches of the entire Colorado River Basin. including reaches of the upper San Juan River and possibly its major tributaries. Colorado squawfish were apparently never found in colder headwater areas. Seethaler (1978) indicated that the species was abundant in suitable habitats throughout the entire Colorado River Basin prior to the 1850's. Platania and Young (1989) summarized historic fish collections in the San Juan River drainage which indicate that Colorado squawfish once inhabited reaches above what is now the Navajo Dam and Reservoir near Rosa. New Mexico. Since closure of the dam in 1962 and the accompanying fish eradication program. physical changes (flow and temperature) associated with operation of the Navajo Project have eliminated Colorado squawfish in the upper San Juan River, both from the reservoir basin as well as from several miles of river downstream of the dam. ~ The San Juan River currently flows approximately 225 river miles from the Navajo Dam downstream to Lake Powell. Th~ reach of currently known occupied Colorado squawfish habitat extends from Lake Powell upstream to river mile 158.4 and could be significant]Y_imRacted 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 weir at river mile 166.2), span the entire river channel and are believed to be effective blocks to upstream fish migrations (Platania 1990). The Hogback is often breached during high water which may allow passage at certain water levels, however. after breaching. the structure is rebuilt and is a block to upstream fish movement. Extreme fluctuations occurring within the framework of a natural annual hydrograph may enhance spawning success of native species and inhibit exotic species. Haynes et al~. (1984) reported that fish species. such as Colorado squawfish. that evolved under highly fluctuating flow conditions. were better able to survive and successfully recruit under those conditions than the introduced species. The decline of endemic Colorado River fishes seems to be partially related to competition or other behavioral interactions with ~ . ,j