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F _. ~ _ , ,. ,. w - - ~ - 8 Many smaller rivers and creeks of Arizona now are dry except in periods of snow-melt or heavy precipitation. This has resulted from diversions, impoundments at their headwaters, pumping of underground aquifers, or perhaps a natural climatic trend toward drought. Many such habitats support fishes in their permanent headwaters, and sometimes downstream where water moving through the coarse-grained sands and gravels of the stream bed is forced to the surface in canyons or elsewhere where the channel is crossed by stony dikes. In the recent past; when the lower-elevation grassland or desert streams were more permanent, they often flowed through broad, marshy floodplains in multiple channels (Hastings, 1959; Hastings & Turner, 1966), and were repeatedly dammed by beaver (Emory, 1857; Pattie, 1905; Lockwood, 1929). Dark, organic sediments form parts of the walls of many present-day "washes" in the American Southwest, remnants of marsh or "ci~enega" deposits (Martin & Mehringer, 1965, and references cited). Great galleries of cottonwood trees, so conspicuous at present along many waterways (Fig. 4), were most likely absent or localized in places of relatively good drainage, or were perhaps suppressed by beaver cutting and/or lack of great success in reproduction. Areas of entrenched channel doubtless were present, bordered by mesquite "bosques" on terraces. Under such conditions, much of the stream bottoms must have been of mud and debris, currents would have been slow, impeded by debris, beaver dams, and the tortuous ! nature of channels, and flooding effects would have been minimal (Fig. 5). The present aspect of many desert and grassland streams therefore, with a tendency for broad channels bordered by cut banks (Figs. 4, 6), orig- inated in the later 1880s with a cycle of arroyo cutting. Similar erosional activity was recorded in or around this period over a large area, and +~ the uniformity "points toward operation of a broad regional factor like ., climate (Hastings & Turner, 1966)." However, in southern Arizona at Fk :~._ _..~a .~~,,,n~~.-~......a,n~.a ~. ~.._ x . '" ~".'~ ~~. _ ~-~ - x ~. ~~ __ ~_~ ~ K, - -~ ~~ ~ ~: figure 6. Santa Cruz River and associated washes, Pinal County, Arizona, down-stream from Tucson; snow-dry stream bed obviously greatly incised and eroded.