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Marshes and riparian plants were left high and dry, and disappeared. The erosive power was concentrated downward by high channel walls. Fishes adapted to erosive habitats, those which could live in the hard-rock canyon reaches of streams, were scarcely affected by this change. Flow almost certainly became less reliable as upstream groundwater reservoirs were drained, but the habitat otherwise remained similar to before. On the other hand, fishes depending on constant, cool inflow of groundwater, amelioration of floods, or the presence of quiet habitats along meandering channels, were suddenly and severely impacted. Ecology of Present Systems Physical conditions in intermediate-elevation streams are highly variable. Summer water temperatures approach 300 C in afternoon (Fig. 24), and drop to 21.10 C or lower at daybreak. Relatively cool water, despite air temperatures of 43.30 C or more, is due to the high evaporation rates in dry desert air. Only on rare sunny days with high humidity do stream temperatures rise above 32.20 C; thermal death of fishes has been recorded under such conditions. Shading has a profound effect, and a creek with alternating flow through wide, unshaded channels then narrow, dark canyons, may vary along its course at the same time of day. In winter, stream margins often rise to 25 0 C on sunny days. Channel temperatures rise to 12.10 C, or perhaps more, and rarely drop to less than 50 C at night. lee nonetheless forms on isolated backwaters on especially cold nights. 35 95 30 Aug. 16, 1977 90 25 80 20 70 () LL 0 60 0 15 10 50 5 40 0 32 6 Noon 6 Figure 24. Summer and winter water temperatures over 24- hour periods in Bonita Creek, Arizona. Figure 25. Above: Boulder substrate in a high-gradient tributary to the Rio de Bavispe, Sonora. Photograph by D. A. Hendrickson. Below: Sand/gravel substrates in a wide segment of Aravaipa Creek, Arizona. These streams carry remarkable loads of material eroded from the mountains through which they pass Substrates are typically of boulder and cobble at higher elevations, grading to gravels and sand lower down (Fig. 25). Turbidities are high during floods, but particles tend to be large in size and most streams clear quickly. Because many watersheds are rocky, barren of vegetation, and impervious, sheetflow quickly concentrates into channels to produce abrupt and violent discharges. Unlike streams in more temperate zones, most of the annm water yield of these streams is during flash floods (Fig. 26), rather than as slow, constant discharge throughout the year. Chemically, most of these streams remain well within the limits for healthy fish populations. They have more total dissolved solids than mountain 13