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I I I I I I I I I I I I I I I I I I I 0.0' ....,....')' , " ~ "t 'i Mohave Valley Division The Mohave Valley Division is located between Davis Dam and the Topock Gorge. It is the northernmost of the ten divisions organized under the Colorado River Management Program. It includes the Cities of Laughlin, Nevada, Bullhead City, Arizona, and Needles, California. Hoover Dam significantly reduced the annual floods that purged the lower Colorado River, however, flows were stilllarge enough for scour and developed sediment to remain significant. Subsequent deposition of the sediment in the headwater delta area of Lake Havasu above Parker Dam, created a problem of severe aggradation in the lower Mohave Valley. At Topock, deterioration of the channel induced more deposition, and by 1943, sandbars extended across the entire channel causing water levels upstream to rise and cause serious flooding at Needles. Although emergency protective works were undertaken, channelizing the river was the only permanent solution. Channel stabilization was initiated in 1949 with the dredging of an improved channel between Needles and Topock and the river was diverted into the new channel on June 25, 1951. To prevent the same aggradation process from repeating itself, the Topock Settling Basin was constructed in order to reduce the flow of sediment into Topock Gorge. This work .and associated levee construction eliminated the immediate flood threat to Needles. However, it did not, by itself, provide the river stability between Davis Dam and Topock which was needed to assure that the problem would not recur. Channel dredging, levee construction, and associated bankline stabilization work which reduced the pickup and transport of sediment, were subsequently accomplished upstream from Needles to a point 10 miles below Davis Dam. The continuous dredging in the Topock Settling Basin was suspended in 1982 due to the gradual reduction of the sediment loads being scoured from the river as the bottom material coarsened and the river approached a steady regime level. Reclamation continues to monitor the sediment transport and river conditions. Dredging has continued in the basin since 1982 on an intermittent basis, and there is a likelihood that maintenance dredging may be required within the next ten years. Related work for the improvement of fish and wildlife habitats and recreational features has also been provided. Topock Marsh, which owes its existence to the completion of Parker Dam and the subsequent filling of Lake Havasu in 1938, has been encompassed with a dike to maintain water levels at an elevation of 455 feet above mean sea level (msl). At this level, approximately 4,000 acres of open water are available for fisheries and wildlife management. Inlet and outlet structures were constructed by Reclamation to control water apportioned to the Havasu National Wildlife Refuge. The high flows of 1983 and 1984 flooded parts of the Mohave Valley above Topock Marsh. As a result, a new flood control structure, Topock South Levee, was constructed in order to prevent mainstem floodwater from backing into Topock Marsh. At the same time, revegetation, water control structures, and aquatic habitat development were designed to enhance fish and wildlife conditions. Reclamation also participated in the development of the Needles Marina and Park Moabi near Topock. Popularity of these marinas has increased each year since their construction. Beal Appendix C - 5