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Gravity ~tain Canars desilting basin is long, .....ide, and deep. It contains diversion .....orks at its do.....nstream end with two sets of gates. An upper set of gates allows water near the sur. face, .....hlch contains little silt and sediment, to f]o..... over the gates into the Gila Gravity Main Canal. A lo.....er set of gates is used to flush the sediment at the bollom 01 the basin, about once a year. Dredaina " " Located In the front of a dredge is a large rotating cutterhead. capable of being lowered into the water to a depth of about 25 feet. The cutterhead's blade rotates and loosens the sedi- mentlying on the bottom of the water. A large pump on the dredge vacuums the loosened materia] from the river boUom There is no room to per- manently deposit the sand and sediment near Imperial Dam, so the sediment that is removed at the desilting basins on both sides of the dam is temporarily deposited into the Califor. nia Sluiceway and is later flushed about two miles downstream 10 the Laguna SettHnlo: BUl'in, one of !.....o settling basins Reclamation operates soulh 01 Davis Dam. The Yuma Projects Office also removes sediment in the Colorado River at the Topock Sellling Basin, Just south of Needles, California, and behind Imperia] Dam. Some areas of the river don't need levees-gorges, mesas, and mountains are high enough to prevent water from overflowing. When sediment arrives at a settling basin, the water slows down, causing the sediment to sel- lie to the bottom .....here floating hydraulic dredges later remove the material. and pumps it through a noat. iog discharge pipeline and dis- poses of It up onto the river's old. dry floodplain along the river. In many cases the dredged materia] is used 10: C> elevate ]o.....-Iying lands near the river that could be flooded if not built up .....lth dredged matcrial, :> improve recreation .....ith sand beaches, and :>- fill other various areas. Each dredge owned by the United States and operated by the Yuma Projects Office is capable o( dredging ap- proximately one million cubic yards per year .....hen .....orking at full capacity. The dredges are usually operated around the clock, three 8-hour shifts a day, for ten days straight; then they are shut down for four days for repairs and preventive main- tenance. Levees A levee system has been con- structed in various locations along the river to help protect communities and agricultural lands. Levees are designed and constructed high enough to contain most floods that might occur. Levees provide sig- nifleant nood prolecllon to land owners located outside the noodway and also permit them to obtain reasonably priced flood insurance. Although dams along the rivcr have control]ed mosl of the an- nual high .....ater flows originat- ing from the upper basin, it is still necessary to maintain a good river channel and levee system !o confine and control those Ilo\\'s. Flash Uoods originating below the dams from thunderslorms must also be regulated. There are some areas of the river that do not need levees- .....here gorges, mesas, moun- tains, and flverbanks are high enough to prevent .....ater from o\.erflo.....ing one or both sides of the river. Armoring levecs with rock nprap helps protect the levees Irom being eroded during high flows. :\lany of the It:vces along the 100H'r Colorado River that were constructed years ago are no..... being armored with nprap.