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. . <br />k ~ '* <br />~, , , ~. <br />., ~~ <br />INTRODUCTION <br />The Colorado River system of the western United States-has been <br />greatly modified during the last 100 years., The primary method of <br />alteration has been the construction of dams for hydro-e]ectric and <br />irrigation purposes. The resulting impoundments and alteration of <br />the flow regime below these impoundments are felt to have been the <br />major factor in the dec]ine of four, endemic Colorado River fish species: <br />the Colorado squawfish (Ptychocheilus Lucius), humpback chub (Gila <br />c ha), bonytail chub (Gila elegans), and humpback sucker (Xyrauchen <br />texanus) (Vanicek, Kramer, and Franklin 1970,; Minckley 1973; Holden <br />and Stalnaker 1975b). The first two fish have been officially designated <br />"Endangered" by the U.S. Department of the Interior (1973) and the <br />latter two are also considered rare (Miller, 197.2; Holden and Stalnaker <br />1975b). <br />Previous investigators (Holden and Stalnaker 1975x) have determined <br />areas they felt were important to the continued survival of the basin's <br />endangered fish fauna. This investigation was initiated to ~~~~:,;,i;,~ <br />the importance of one of these areas, the Yampa River in northwestern <br />Colorado, to the Colorado squawfish and humpback sucker. The study <br />was later expanded to include a section of the Colorado River near <br />Grand Junction, Colorado to facilitate the accumulation of life history <br />information. Life history information is also being collected on two <br />other indigenous suckers whose abundance-seems unaltered by the modi- <br />fications of the system (Holden and Stalnaker ]975b): The flannel mouth <br />sucker (Catostomus latipinnis) and bluehead sucker (C. discobolus). <br />