VII. DESERT FISHES
387
which they were introduced. We have attempted conservation of space by
emphasizing reviews of literature when possible.
II. Aquatic Environments of Deserts
A. DESERT RIVERS
Stream habitats are open-ended systems characterized by net, unidirectional movement of water, and by great spatial and temporal variability.
Lotie habitats in deserts are exemplified at one extreme by large, throughflowing rivers such as the Nile River of Africa, and the smaller Colorado
River of western North America, and at the other extreme by small, almost
dry creeks (Figs, la-d and 4 ) .
Major streams that flow through deserts usually have their origins in
distant places of higher elevation that receive comparatively high, persistent or seasonal, precipitation. Their regimes of discharge are therefore
dictated in large part by temporality of rains or melting snows. Large,
through-flowing streams have major tributaries that also rise in uplands,
but subside into the sands or evaporate, reaching to the mainstream on
the surface only in times of flood. Lesser watercourses, lying for the most
part on the desert floor, contribute water only during local storms, unless
directly spring-fed. Large rivers are more predictable than smaller ones,
since the latter are greatly influenced by cyclonic storms so characteristic
of many arid regions. The Nile River, prior to its stabilization by Aswan
High Dam, was doubtless the epitome of predictability. The spring floods
occurred so regularly as to have provided a principal basis for development
of the Egyptian civilization.
However, gross variability of desert rivers seems the rule, with most
of them subject to long periods of reduced flow, then scoured by major
flooding. For instance, the Colorado River at Yuma, Arizona, prior to upstream dams, varied from a peak discharge of near 4.25 X 10
5
m
3
of water
per minute in 1916, to a mere 20.5 m
3
/minute in 1934 (Dill, 1944). The
Murray-Darling River system of eastern Australia has a drainage area
comparable in size, but greater in aridity, than the Colorado, and is even
more prone to drought, often becoming a series of isolated pools for hundreds of kilometers of its lower channel (C.S.I.R.O., Australia, 1960;
Weatherley, 1967). This is not only a function of low precipitation, but
also of high évapotranspiration rates in deserts, which typically exceed precipitation by a substantial percentage (Logan, 1968; McGinnes et al.,
1968).
Under natural conditions, water temperatures in large desert rivers are
higher in winter than in streams of temperate zones. For example, water
387
which they were introduced. We have attempted conservation of space by
emphasizing reviews of literature when possible.
II. Aquatic Environments of Deserts
A. DESERT RIVERS
Stream habitats are open-ended systems characterized by net, unidirectional movement of water, and by great spatial and temporal variability.
Lotie habitats in deserts are exemplified at one extreme by large, throughflowing rivers such as the Nile River of Africa, and the smaller Colorado
River of western North America, and at the other extreme by small, almost
dry creeks (Figs, la-d and 4 ) .
Major streams that flow through deserts usually have their origins in
distant places of higher elevation that receive comparatively high, persistent or seasonal, precipitation. Their regimes of discharge are therefore
dictated in large part by temporality of rains or melting snows. Large,
through-flowing streams have major tributaries that also rise in uplands,
but subside into the sands or evaporate, reaching to the mainstream on
the surface only in times of flood. Lesser watercourses, lying for the most
part on the desert floor, contribute water only during local storms, unless
directly spring-fed. Large rivers are more predictable than smaller ones,
since the latter are greatly influenced by cyclonic storms so characteristic
of many arid regions. The Nile River, prior to its stabilization by Aswan
High Dam, was doubtless the epitome of predictability. The spring floods
occurred so regularly as to have provided a principal basis for development
of the Egyptian civilization.
However, gross variability of desert rivers seems the rule, with most
of them subject to long periods of reduced flow, then scoured by major
flooding. For instance, the Colorado River at Yuma, Arizona, prior to upstream dams, varied from a peak discharge of near 4.25 X 10
5
m
3
of water
per minute in 1916, to a mere 20.5 m
3
/minute in 1934 (Dill, 1944). The
Murray-Darling River system of eastern Australia has a drainage area
comparable in size, but greater in aridity, than the Colorado, and is even
more prone to drought, often becoming a series of isolated pools for hundreds of kilometers of its lower channel (C.S.I.R.O., Australia, 1960;
Weatherley, 1967). This is not only a function of low precipitation, but
also of high évapotranspiration rates in deserts, which typically exceed precipitation by a substantial percentage (Logan, 1968; McGinnes et al.,
1968).
Under natural conditions, water temperatures in large desert rivers are
higher in winter than in streams of temperate zones. For example, water