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<br />VII. DESERT FISHES
<br />
<br />319
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<br />Fig. Ib, Same locality as in (a), 2 days before, at a discharge of about 35 m'/min-
<br />ute. This was a rapid, high-intensity discharge, derived from a lateral wash which was
<br />a normally dry tributary; no rainfall was recorded at weather stations in the vicinity.
<br />Photograph by Robert L. Smith.
<br />
<br />side vegetation (Macan, 1958) or canyon walls. Orientation of a channel
<br />may effect completely different regimes of insolation, and therefore heating.
<br />On a single day, the time that maximum temperature is achieved may be
<br />successively later at each station downstream, and minimum temperatures
<br />that are achieved may be elcvated in streams of mesic zones, as volume
<br />of water increases and variation in summer air temperatures decreases at
<br />lower elevations (Schmitz, 1954). This is generally reversed at the lower
<br />ends of desert streams. In those with diminutions of discharge at both
<br />up- and downstrcam ends, the least variation may occur in middle reaches.
<br />Streams that receive melt water from montane snows have a later summer
<br />maximum in tempcrature than those receiving water from other sources.
<br />Interesting reversals of almost all factors may occur in streams fed by large
<br />thermal springs, and all thesc variations amount to a formidable array of
<br />environmental problems for the low desert fish.
<br />Most fishes of dcscrt strcams live in main rivers or in the high-elcvation
<br />parts of tributary networks. Some smaller washes and "dry creeks" support
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