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~'J ::::- ::, ~,..j Gravity :\lain Canal's desilling: basin is long, '....ide, and deep. It .:::ontains di\"ersion works at its downstream end ..dth t...,;o sets of jJates. An upper set or gates allows w.ater near the sur. face. \\.hich contains little slit and seciment. to flow owr the gates into the Gila Gravity :\1ain Canal. A lower set 01 gates is used to flush the sediment at the bottom of the basin. about once a year. Dredging Located In the front 01 a dredge is a large rotating cutterhl"ul. capable of being lowNed Into the water to a depth of about 25 feet. The cutterhead's blade rotales and loosens the sedi- ment lying on the bottom of the waler. A large pump on the dredge vacuums the loosened materiallrom the river bot!om Some areas of the ri{;er don't need leuces-gorges. Tnesas, and mountains are high enough to {Jre{;entlHlter from ocerflowing. There is no room to per- manently deposit the sand and sediment near Imperial Dam. so the sediment that is remowO iLl the desllling b.lSlllS Oll bOlh sides of the dam is leJ~lpor,)rily deposited into the c.tlifor- ni" SluicewilY and is later flushed about two miles downstream to the Laguna Sdlling Bil~in, one ollwo settling basins Recl.1malion operates south of Davis Dam. The Yuma Projects Office also remo'.es sediment in the Colorado Ri....er althe Toro(:k Settling Basin, just south of Needles, California, and behind Imperial D.Jrn. When sediment arrives at a settling basin. the water slows down. causing: the sediment to sd- tle to the botlom where rloatil1!:l hydraullc dredges bIer remove thE' material. and pumps it through a noat. ing dbchaq;::c pipeline and dis- poses of II up onto the river's 010, dry floodplain along the river. In many cases the dredged material is used to: t:> elevate low-lying l.:mds near the river that could be [Iooded if not built up with dredged material. :Jo improve recreation with sand beaches, a.nd :> fill other various areas, Each dredge owned by the Cnited States and operated by the Yuma Projects OHice is capable of dredging ap- proximately one million cubic yards per year when working at lull capacity. The dredges are usually operated around th~ clock, threeo...hour shifts a day for ten days straight: then they are shut down lor four days lor repairs and preventive main- tenance. Levees r\ hove... system h.ts been con- structed in various locations ,]Iong the river to help protect commllnities and ,]gricultural lands. Levees are designed and conslrucled high enough to contain most floods that might occur. Levees provide sig- nHicant nood protection to land owners located outside the nuodway and also permit them to obtain reasonably priced flood insurance. Although dams along the ri\'N have controlled most of the an- nual high water /lows originat- ing from the upper basin, It Is still necessary to maintain a good river channel and levee system to coniine and control those flows, Flash floods originating below the dams from thunderstorms must also be regulated. There are some areas oj the river that do not need levees- where ~orges, mesas, moun- tains, and riverbanks are high enough to prevent waler from overflowing one or both sides of the riyt'r. Armoring levees with rock riprap helps protect the levees from bein~ eroded during high flows. :\Iany of the levees along the lower Colorado River that were constructed yeMs ago are nm\' being armored with riprap_