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<br />.
<br />
<br />. .
<br />
<br />through the Grand Canyon before impoundment averaged about 374td-', and
<br />much higher rates occurred in floods. Spring runoff in 1927 generated a peak
<br />flow of 3538 m3 s -I, carrying 27 164 t d -I through the canyon (Dolan et at.
<br />1974). Sediment deposition on the delta (Plate 1) varied from a few tonnes dailv
<br />to more than 1000 tonnes each second during floods (Sykes 1937). The virgi~
<br />Colorado was the greatest conveyor of sediments of any river in the world (Dill
<br />1944),
<br />Present flows are unlike those of the virgin river. Dams and diversions haYt:
<br />virtually de-watered the lower Gila River and many of the channels on the delta
<br />(Figs I & 3), where flows now occur only in response to infrequent, localist:d
<br />rainstorms. The only significant free-flowing tributaries in the entire basin nOlI
<br />are the Little Colorado and White rivers. In the Upper Basin more than 117
<br />reservoirs of capacity greater than 1.0 km3 have been built or are under con.
<br />struction, and there are proposals for others on the Yampa, White and
<br />Gunnison rivers. The total usable storage in the Upper Basin is more than
<br />36 km3 (Bishop & Porcella 1980), or twice the annual virgin flow at Lee Ferry
<br />In addition, nearly I km3 of water annually is exported from the Upper Basin
<br />by more than 40 trans-basin diversions (e,g. Fig. I). Irons et al. (1965) reportt:d
<br />that 29% of the water reaching Lake Powell was from upstream storagt:
<br />reservoirs, but that percentage now undoubtedly is much higher. Elsewhere,
<br />flows are almost totally controlled by reservoirs. The one reach retaining a near
<br />pre-regulation pattern is that above Lake Powell, including the lower Green and
<br />mainstem Colorado rivers to near the confluence with the Gunnison River
<br />(Fig, 1),
<br />Regulation has had profound effects on flow and sedimentation between
<br />Lakes Powell and Mead. Flows from Lake Powell are 130-764m3s-1 year.
<br />round, thus eliminating the annual freshet (Fig. 5). Sediments are retained in
<br />Lake Powell, except those discharged by the Little Colorado River and othel
<br />sideflows, and the median suspended sediment concentration in the Grand
<br />Canyon has been reduced by a factor of 3.5 (Dolan et ai, 1974; Fig. 6),
<br />Fluvial processes likewise have been affected. Before regulation, alluvial
<br />rubble from side-flows formed long, sandy bottom pools separated by short.
<br />steep rapids. Dolan et af. (1978) showed that side-flows enter along faults (II
<br />geologically unstable points, where there is often major downcutting by tht:
<br />main river, creating the rapids. Debris-laden floods from side-flows also buil:
<br />sand terraces near the high-water mark, and formed extensive channels and
<br />backwaters wherever the floodplain permitted, Post-regulation flows hay c
<br />stabilised the river bottom (Simons 1979; Stanford & Ward 1984; Plate I), and
<br />lack force to move much of the side-flow debris; hence the rapids are growin~
<br />in length and are more turbulent. Sand bars and terraces are no longer reworhd
<br />by flood waters, nor are many, of the flood channels re-watered.
<br />Thus, the regulated channels of the Colorado system are stabilised by Cfln.
<br />stant (once variable), clearwater (once silt-laden) flows (Dolan et al. 1974, 1977,
<br />
<br />160
<br />
<br />.~,
<br />
<br />~""~,
<br />
<br /> 100
<br /> 50
<br />.. 0
<br />~ 150
<br />.2
<br />c
<br />0
<br />j;
<br /> 100
<br />
<br />
<br />I April-June ,
<br />
<br />50
<br />
<br />
<br />I Annual ~
<br />
<br />o ,
<br />1945
<br />
<br />.
<br />1950
<br />
<br />.
<br />1955
<br />
<br />,
<br />1960
<br />
<br />.
<br />1965
<br />
<br />.
<br />1970
<br />
<br />.
<br />1975
<br />
<br />Year
<br />
<br />Figure 6, Mean suspended solids loads annually and during spring runoff (April-June) in the
<br />Colorado River at Lee Ferry, Lake Powell was impounded in 1963 (after Paulson & Baker 1981),
<br />
<br />Carothers & Minckley 1981; Stanford & Ward 1983, 1984). The delta receives
<br />essentially no flow from the mainstream and Gila River, due to diversions.
<br />Today the delta. once a maze of channels (Plate 1), resembles the surrounding
<br />desert.
<br />I
<br />
<br />The Salton Sea
<br />
<br />The Colorado Delta is formed as a "T", with arms extending 320 km south from
<br />near the Salton Sea to the Gulf of California, and base near the Gila River
<br />confluence (Fig. 1). During the Pleistocene, a late Tertiary cryptodepression, the
<br />Salton Sink, filled with water to form Lake LeConte (Fig. 4). Subsequent
<br />deposition may have accelerated subsidence of the delta, and the river period-
<br />ically may have flowed into the sink (Sykes 1937; Hubbs & Miller 1948).
<br />Explorers in 1853 found the Salton Sink (Imperial Valley) dry, with the valley
<br />floor 83 m below sea level. Between 1900-04 the valley was connected to the
<br />Colorado River via irrigation canals. In January 1905 flood waters from the
<br />Colorado broke through the canal gates, and by late summer virtually the entire
<br />river was flowing into the "Salton Sea". The inflow created a lake of 1060 km2
<br />area and 26 m depth (Sykes 1937), and the river was not returned to the gulf
<br />until dike construction in February 1907. Since then diversions have been
<br />controlled, but the irrigated lands have expanded to more than 2000 km2.
<br />Imperial Valley now is the largest expanse of irrigated agriculture in the Western
<br />Hemisphere (Pillsbury 1981), and uses most of the remaining river flow via
<br />diversions at Imperial Dam (Fig. I),
<br />
<br />361
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