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<br />VII. DESERT FISHES
<br />
<br />317
<br />
<br />which they were introduced. We have attempted conservation of space by
<br />emphasizing reviews of literature when possible.
<br />
<br />D. Aquatic Environments of Deserts
<br />
<br />.
<br />
<br />A. DESERT RIvERS
<br />
<br />Stream habitats art~. open-ended systems characterized by net, unidirec-
<br />tional movement of water, and by great spatial and temporal variability.
<br />Lotic habitats in deserts are exemplified at one extreme by large, through-
<br />flowing rivers such as the Nile River of Africa, and the smaller Colorado
<br />River of western North America, and at the other extreme by small, almost
<br />dry creeks (Figs. 1 a-d and 4).
<br />Major streams that flow through deserts usually have their origins in
<br />distant places of higher elevation that receive comparatively high, per-
<br />sistent or seasonal, precipitation. Their regimes of discharge are therefore
<br />. dictated in large part by temporality of rains or melting snows. Large,
<br />through-flowing streams have major tributaries that also rise in uplands,
<br />but subside into the sands or evaporate, reaching to the mainstream on
<br />the surface only in times of flood. Lesser watercourses, lying for the most
<br />part on the desert floor, contribute water only during local storms, unless
<br />directly spring-fed. Large rivers are more predictable than smaller ones,
<br />since the latter are greatly influenced by cyclonic storms so characteristic
<br />of many arid regions. The Nile River, prior to its stabilization by Aswan
<br />High Dam, was doubtless the epitome of predictability. The spring floods
<br />occurred so regularly as to have provided a principal basis for development
<br />of the Egyptian civilization.
<br />However, gross variability of desert rivers seems the rule, with most
<br />of them subject to long periods of reduced flow, ~hen scoured by major
<br />flooding. For instance, the Colorado River at Yuma, Arizona, prior to up-
<br />stream dams, varied from a peak discharge of near 4.25 X 105 m3 of water
<br />per minutc in 1916, to a mere 20.5 m3/minute in 1934 (Dill, 1944). The
<br />Murray-Darling River system of eastern Australia has a drainag~ area
<br />comparable in size, but greater in aridity, than the Colorado, and is even
<br />more prone to drought, often becoming a series of isolated pools for hun-
<br />dreds of kilometers of its lower channel (C.S.I.R.O., Australia, 1960;
<br />Weatherley, 1967). This is not only a function of low precipitation, but
<br />also of high cvapotranspiration rates in deserts, which typically exceed pre-
<br />cipitation by a substantial pcrcentage (Logan, 1968; McGinnes et al.,
<br />1968) ,
<br />Under natural conditions, water temperatures in large desert rivers are
<br />higher in wintcr than in streams of temperate zones, For example, water
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