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<br />j -- 500l ^ <br />I (;1 I <br />- l.\- <br />i . ~ <br />I 0 <br />I '- <br />I . I~ <br />I Grand Canyon <br />I l1.. 400 I --- To pock <br />, 0 I <br />i ........ Yuma <br /> CJ) 1 <br /> z <br /> 0 .. <br /> ...J. 1\ <br /> ...J <br /> - I <br /> ~ 300 <br /> "-" I <br /> 0:: <br /> W I <br /> r- I .o. <br /> l- I . . \ <br /> ~ . . <br /> ~ 200 I:' . <br /> 0 I: <br /> w t . <br /> 0 1 .. <br /> z . . <br /> .. . .' <br /> - . - <br /> 0.. <br /> CJ) . <br /> ::J 100 <br /> (J') <br /> <br /> <br />. '\ <br />.'. "'-" <br />.... -.......:...... <br />... ~~.... <br /> <br />..... <br /> <br />. .' <br /> <br />. <br />. <br /> <br />o <br /> <br />1926 <br /> <br />1928 <br /> <br /> <br />1930 1932 1934 1936 <br />YEARS <br /> <br />1938 <br /> <br />1940 <br /> <br />FIGURE 5. Annual sediment loads at three localities in the Colorado River. 1926-40 (Howard <br />1947). Grand Canyon is upstream from Lake Mead; Topock and Yuma are downstream. " <br /> <br />submer.gent vegetation was scarce and much of the river bottom was composed of shifting <br />sand. Many of the invertebrates that colonized the mainstream were probably immigrants <br />from upstream reservoirs and backwaters. " <br /> <br />The effects of dam construction on biota in the mainstream are most clearly illustrated by <br />changes in fish populations. Exotic species. some present in small numbers since their <br />initial introduction in the early 1900's (Gilbert and Scofield 1898; Grinnell 1914; Miller <br />1961). responded favorably to the altered conditions and became abundant. Concurrently. <br />most native fishes declined - some to extinction - in most areas of the lower river. The <br />major probable c~uses for the decline of native species of fish are competition with <br />exotic fishes (Dill 1944; Miller 1961). predation by exotic fishes (Dill 1944; Miller 1961) <br />and construction of upstream spawning migrations by dams (Miller 1961). Recent data on <br />Colorado squawfish. Colorado chub. and bo~ytail chub (Vanick and Kramer 1969) indicated <br />that competition may have been of special importance; the foods of squawfish and chubs <br />apparently are similar to those of many introduced .centrarchids. but the native species <br /> <br />CAt-NEVA WILDLIFE TRANSACTIONS 1980 <br /> <br />105 <br /> <br />. <br /> <br />-. ,- .~'". .-----:-- -.- ~ . -, - '. ,- _.....- . .--:------ <br />