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<br /> <br />5 art 2?jver <br /> <br />lJ' <br />J(Y <br /> <br /> <br />The upper Sa[t 2?jver is a boisterous stream tumbling out ofthe <br /> <br />White Mountains of eastern Arizona. west towards its confluence with the Gila River near <br />Phoenix. The first 125 miles of the Salt. unimpeded by dams. are good examples of an <br />unregulated river. The Salt River Canyon, is a major east-west trending gash in central <br />Arizona. Downstream the river's character changes dramatically. Beginning at Roosevelt <br />Lake reservoir, the Salt becomes a "working" river. A series of four reservoirs that supply <br />water for irrigation. industrial, and municipal use in Phoenix marks the end of the free. <br />flowing river. <br />A natural river has an annual cycle of floods and low-flow periods. depending on <br />climate and season. The upper Salt River basin receives moisture from several types of <br />storms. The greatest amounts of moisture are delivered to the basin by fall and winter <br />storms that roll in from the Pacific Ocean. this moisture is stored as snow pack in the high <br />country. If warm rain falls on previously fallen snow. the Salt River can suddenly snap from <br />trickle to torrent. On December 17, 1978 a severe winter storm dumped lOin. of rain on <br />parts of the Salt River drainage (Aldridge and Hales. 1984). Before the storm. the Salt River <br />was flowing at 3,460 cubic feet per second (ft3/s). twenty-two hours later the river surged <br />to 95,000 ft3/s. Prehistoric floods have reached 160,000 ft]/s (Partridge and Baker. 1987). <br />yet flow of the Salt is sometimes less than 100 ft3/s in early summer. This river may be an <br />extreme example of variability in discharge, but many western rivers probably rose and fell <br />just as quickly before they were dammed. <br /> <br />'*' <br /> <br />Big floods. little floods. Wet years. dry years. So what? Geoscientists must put all these <br />numbers into perspective when they try to understand how a river works. They study <br />basin and river geometry in order to understand how floods sculpt the river's natural <br />channel. Hydrologists try to analyze historical patterns of flow in order to anticipate floods <br />that can be expected in the future. Biologists study aquatic organisms and riparian plants in <br />relation to these patterns of flow. <br /> <br />11 I <br />