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spawning season (Hobbs and Miller 1953, Minckley 1973, Minckley <br />and Carothers 1979). <br />Habitat degradation and changes in streamflow patterns began <br />in the Gila River drainage in 1905 (Hastings 1959, Miller 1961), <br />and likely reduced razorback sucker habitat. The last <br />substantiated collection of young razorback suckers in the Gila <br />River drainage was in 1926 (Hobbs and Miller 1953, Minckley <br />1983). Razorback suckers disappeared from Roosevelt Reservoir, <br />Salt River drainage, before the 1930's, but the species persisted <br />in large numbers in other reservoirs in that drainage until the <br />mid-1950's. Since that time, the only known razorback sucker <br />captures from the Gila River drainage were hatchery-produced fish <br />stocked after 1981 (Minckley et al. in press). <br />IIpper Colorado River Basin <br />Major drainage units of the UCRB are the San Juan-Colorado, <br />upper mainstem Colorado, and Green river sub-basins (Iorns et al. <br />1965). All of these drainage units historically supported <br />razorback suckers. <br />San Juan-Colorado River Sub-basin--The San Juan River is the <br />southernmost major drainage of the UCRB and has only one major <br />tributary, the Animas River (Colorado and New Mexico). Other <br />major Colorado River tributaries of the San Juan River sub-basin <br />include Dirty Devil and Escalante rivers. The San Juan-Colorado <br />sub-basin has received the least survey and research effort among <br />the UCRB sub-basins, and the historic status of rare species, <br />14 <br />