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<br />392
<br />
<br />JAMES E. DEACON AND W. L MINCKLEY
<br />
<br />(Gautier, 1935) which certainly are inconsistent with present climatic
<br />regimes, and fossil assemblages of vertebrates along these watercourses at-
<br />test to big river habitats across the presently arid lands (Joleaud, 1935).
<br />Similar situations of ancient, now dry valleys, or remnants of terraces pro-
<br />duced by streams larger than those now present, occur in many other
<br />desert regions (Leopold and Miller, 1954; Wright, 1956; Alimen, 1965).
<br />In some instances, remaining waters of such extinct systems support
<br />remnant fish populations, reconfirming drainage connections long since dis-
<br />rupted and obscured (Pellegrin, 1914, 1931, 1934; Braestrup, 1940;
<br />Miller, 1946a, 1949a; Hubbs and Miller, 1948a).
<br />Man's activities over the past few decades in North America-and far
<br />longer in some other deserts-have accelerated desiccation of streams, es-
<br />pecially larger ones. These have almost all been beheaded by impoundment
<br />and their waters spread over lowlands to irrigate domesticated crops.
<br />At the time of settlement, large rivers such as the Colorado and Gila
<br />of the Sonoran Desert had complex marshes, lakes, and sloughs along their
<br />lower courses, as did their major tributaries (Sykes, 1937; Leopold, 1953;
<br />Miller, 1961 a). Desert creeks also were far more permanent in their lower
<br />. parts. Smaller streams in southern Arizona flowed through broad, marshy
<br />floodplains in multiple channels, or moved by seepage (Hastings, 1959a,b;
<br />Miller, 1961a; Hastings and Turner, 1965; Minckley, 1969a). Dark, or-
<br />ganic soils form parts of the walls of now dry washes. These soils are rem-
<br />nants of marsh, or cienega deposits (Martin and Mehringer, 1965).
<br />Riparian galleries of cottonwood (Populus), and other, broad-leafed, de-
<br />ciduous trees, so conspicuous at present along some desert waterways
<br />(Lowe, 1967), may well have been restricted to places of relatively good
<br />drainage in the recent past. Areas of entrenched channel doubtless were
<br />present (Hastings, 1959a,b), b'Jrdered by mesquite (Prosopis) and forming
<br />bosques on terraces. Under such conditions stream bottoms are typically
<br />of mud and organic debris, except in places of channel degradation or the
<br />presence of stony dikes. Currents are slow, impeded by debris and the
<br />. tortuous nature of channels, and flooding effects are minimized (Mil-
<br />ler, 1961 a; Minckley, 1969a). The present aspect of many streams in'
<br />southwestern United States, with broad, sandy, braided channels, may
<br />therefore be a disclimatic phenomenon.
<br />A trend toward erosion of channels originated in the 1880's with a cycle
<br />of arroyo cutting that may be continuing today (Miller, 1961a; Hastings
<br />and Turner, 1965). Similar erosive events were recorded, at about the
<br />same time, over a large geographic area, and the relative uniformity
<br />"makes it difficult to link settlement. . . with arroyo cutting, the uniform
<br />onset 0 0 . on the other hand, points toward operation of a broad regi"onal
<br />factor like climate" (Hastings and Turner, 1965) 0 However, in southern
<br />
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