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I I I I I I I I I I I I I I I I I I I Ou2541 APPENDIX C - RIVER WORK AND MAINTENANCE Introduction and Past History Prior to construction of flood control and storage dams on the Colorado River, the lower river from the present site of Hoover Dam to the Gulf was typical of a river carrying a heavy sediment load over an alluvial bed. Before the dams, the river was actively building up the alluvial valleys by repeated inundation when the spring snowmelt flowed from the upper river basin. Each annual flood caused the river to meander across the alluvial valleys, cutting and depositing material in the classical meander patterns. During the ebb of the flood the river typically deposited a remainder of its sediment load on the valley floor. The dams impounded the heavy load of sediment the river historically carried down from the upper basin, and significantly reduced the flood flows which canied most of the sediment through the system. The clear water released from the dams entered the channel practically free of sediment and immediately began acquiring a new sediment load. The dams caused the residual coarse sediment in the river to be redistributed with the result that farther downstream, below each dam, the quantity of sediment was sufficient that the river continued the natural process of meanders and aggradation. Although river maintenance work started near Yuma, Arizona, prior to 1925, Congress did not pass the CRFWLS Act until March 3, 1925. The present authority under which Reclamation operates the Colorado River Front Work and Levee System is the Act of June 28, 1946. This act authorized appropriations for controlling the floods, improving navigation, maintaining the banks of the Colorado River, dredging and straightening the river channel, and conducting studies necessary to fulfill the foregoing objectives. The physical control and training of the lower Colorado River have generally been accomplished by the construction of the system of levees, river realignment, and river control structures. River control structures include bankline reinforcement, riprap, jetties, and training structures. The dredging activities have been used for channel realignment, development of material for levee construction, sediment control, and environmental enhancement. Reclamation's Yuma Area Office (Y AO) is responsible for maintenance of the Colorado River from the SIB to Davis Dam which is approximately 276 river miles. Each year Y AO performs an inspection of the Colorado River and identifies bankline areas, levees, and river structures that require maintenance (Table C-I). For this ongoing maintenance activity on the river and levees, Y AO has a need for up to 60,000 cubic yards of riprap and up to 20,000 cubic yards of gravel per year. Normally Reclamation crews perform the Appendix C - 1