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<br />VII, DESERT FISHES
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<br />391
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<br />Fig, ld, Colorado River in Grand Canyon. While much of the native fish fauna
<br />persists here, introduced species predominate.
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<br />may change from essentially clear water to highly turbid in a matter of
<br />seconds. Water temperatures may drop more than 150C, and current and
<br />turbulence may change radically. Even stable bottoms may be scoured and
<br />mobilized (Ivcs, 1936; Jahns., 1949; Kcsseli and Beaty, 1959; Thomson
<br />and Schumann, 1968), only to be redeposited in about the same configura-
<br />tion as waters reccde (Leopold and Maddock, 1955; Leopold, 1962). On
<br />the other hand, pcriods of drought may be accompanied by water tempera-
<br />tures excceding 40oC, by ovcrgrowth of pools and channels by algae, by
<br />evaporative concentration of salts, and, finally, by total desiccation.
<br />Climatic oscillations over past millenia (Martin and Mehringer, 1965),
<br />and prior to that thc processes of mountain building and elevational adjust-
<br />ments, quite obviously influenccd thc sizes, positions, and permancnccs
<br />of desert waters. Great valley systems exist within the Sahara Desert
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