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<br />. . <br /> <br />....,.. ....,'. ..-:....~: , <br /> <br />, . <br />. <br /> <br />........ <br /> <br />~ <br /> <br />, <br /> <br />'" <br /> <br />~ <br /> <br />... <br /> <br />. <br /> <br />. <br />, <br />,: .. <br />, ;". <br /> <br />.' <br /> <br />,. <br /> <br />.. <br /> <br />, <br /> <br />f ':~,"-A.' <br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br />1 <br />i <br />1 <br /> <br />J <br />.,j <br />'. .1 <br />I <br />j <br /> <br />f' <br /> <br />j <br />i <br />I <br />~ <br /> <br />:... <br /> <br />, <br />i <br />,~ <br />1 <br />j <br /> <br />. "; <br />-, <br /> <br />.~ <br />'~1 <br />. ":. .'. ':-1 <br /> <br />.. <br /> <br />I <br />.. <br /> <br />1 <br />i <br />i <br />, , <br /> <br />j <br />.! <br /> <br />r <br />~ <br />'j <br />. · 1 <br />r <br />~ <br />i <br /> <br />~ <br /> <br />'I <br /> <br />! <br /> <br />~ <br /> <br />-;1 <br />, <br /> <br />-. <br /> <br />'~" <br /> <br />, <br />" <br /> <br />. <br /> <br />. <br /> <br />. .. <br /> <br />I' <br /> <br />'0 <br /> <br />lo <br />, <br /> <br />" <br /> <br />%1 <br /> <br />. '. <br /> <br />~ ~->, ~~...!b.-C.' .J....... :~.-~ i'~ <br /> <br />. <br />'j:, ...a....~ <br /> <br />~ 5/. .~.~ ;;'. <br /> <br />...~i~~.~:.._~.__~__ <-, <br /> <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 /> <br />" <br /> <br />. <br /> <br />~ <br /> <br />. . <br />