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Colorado River, with 6.9 X 1010 m3 (56 million acre-ft) within the upper 30 m (100 ft) of saturated thickness. Tremendous demands were projected for Colorado River water by the 1930's for agricultural development and later for residential development. These demands necessitated tremendous con- trol over the Colorado River's water resources. The first attempts at tapping local water resources of the lower Colorado River consisted of levee construction and various struc- tures to direct surface water for irrigation (U.S. Bureau of Reclamation 1948). When Hoover Dam was closed in 1935, the lower Colorado River bed began to degrade below it. Eroded materials were deposited near the entrance of Topock Gorge and formed Topock Marsh. Rising water levels near the towns of Needles and Bullhead City prompted extensive dredging oper- ations and the construction of levees along the river. Parker Dam was constructed In 1938 to entrap sediments and to provide a basin for the intake of water to southern California. Channelization, bank stabilization, and dredging were initiated downstream of Parker Dam south to the lower end of Cibola Na- tional Wildlife Refuge to lower the water table and expedite flows down the river. These operations were mostly completed by the late 1960's. Imperial Reservoir, the third major impoundment developed in the 1930's, was built to provide a set- tling basin and to supply water to the Imperial Valley. Since its formation this shallow reservoir has been dredg- ed, when required, to maintain flows and surface water for recreation. The last diversions of river water occur at Laguna Dam for the lower Gila River Valley and at Morelos Dam for Mexico. Davis Dam, the last of the major dams on the lower Colorado River, was closed in 1954. Lake Mohave partially serves as a storage basin for water eventually destined for Mexico. Davis Dam stopped all sediment movement on the river. Although Hoover Dam al- lowed for the water diversions down- stream, almost all water management activities are below Davis Dam. Thus, the Bureau of Reclamation defines the management boundary between lower and upper reaches of the river at Davis Dam. About 332 km (144 mi) of the lower Colorado River's 444 km (276 ml) below Davis Dam are within levees and have been channelized or dredged. All but 11 km (7 ml) of the leveed banks have been armored with riprap. In 1987, another 11 km (7 ml) was proposed for bank modification in the Yuma Divi- sion. Additional work may soon be delineated for Parker II Division. Extensive river management work was conducted in the Mohave Valley from 1945-1960, Parker Division from 1966- 1968, Palo Verde Division from 1962- 1968, Cibola rechannel ization (straightening the river flow) from 1964-1970, Laguna Division from 1968- 1969, and in the Yuma Division from 1951-1952. Channelized sections of the lower Colorado River average about 150 m (492 ft) in capacity width. Areas not bordered by levees upstream of Imperial Dam and below Cibola Na- tional Wildlife Refuge range from 60 to 500,m (197 to 1,640 ft) wide. Depth capacity also vary with modifi- cations, ranging to about 8 m (26 ft) In the deepest parts of unmodified reaches (such as Topock Gorge), aver- aging 3.5 m (11.5 ft) in channelized segments, and <3.5 m (<11.5 ft) where the river valley is broad (as in the Parker and Limitrophe Divisions). Reservoirs, primarily Havasu and Mohave, can be deep, >18 m (>59 ft), but most places are <8 m (<26 ft) deep. 31