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. PART 11. DESCRIPTION OF BASIN ..... ()1 o o A. Climate Ext~emes of temperature In the Colorado RI....er BaSin range from -50. F to 130. F. The northern portion of the baSin IS charactenzed by short, warm summers and long, cold Winters, and lIlany mountain areas are blanketed by deep snow all WInter. Much of the Intermountain area consists of high baSins or valleys With cold winters and hot dry summers. The southern desert portion of the baSin has long, hot SUlmIlers, prac- tically continuous sunshine, and almost complete absence of freeZing temperatures. Rainfall a....erages 2.5 Inches In the southern end of the baSin ....hlle total preCipitatIon In the mountains reaches 40 to 60 Inches annually. B. Geography . The Colorado RIver begins where peaks nse more In the north....est portIon of Colorado's Rocky MountaIn mtles north....est of Denver. It meanders southwest for the Upper BaSIn to Lee Ferry (FIgure II-I). than 14,000 feet Nat lanai Park, 70 640 mIles thr~ugh The Green RIver, the Colorado RIver's major trIbutary, rIses In western Wyoming and dIscharges Into the rIver In southeastern Utah--730 river mIles south of Its origin and 220 mIles above Lee Ferry. The Green River draIns 70 percent more area than the Colorado River above theIr junctIon but produces only about three-fourths as much water. The GunnIson and the San Juan are the other princIpal trIbutarIes of the Colorado RIver In the Upper BaSin. The Dam, the Gunnison the only It WIll flows of the San Juan RIver are now controlled by the Navajo Green River by Fontenelle and Flaming Gorge Dams, and the River by the Io'ayne N. Asplnal ( Unlt Dams. Glen Canyon Dam IS major dam on the malnstem of the Colorado above Lee Ferry, but permit control of almost all flows leaVIng the Upper BaSIn. The natural drainage area of the Colorado RIver below Lee Ferry and above Imperul Dam 15 about 75,100 square mIles. ThIS section of the river 15 no.... largely controlled by a series of storage and dIverSion dams startIng with Hoover Dam and ending at Imperial Dam. Lake Mead, formed by Hoover Dam, regulat Ion In the Lower Colorado Rt.....er MoI (munl':lpa1 and Industrial) uses, recreatIon. and other benefiCIal uses. pro.....ldes most of BaSIn, prOViding power generat lon, the storage and for lrrlgat lOll, flood control, . Lake Mohave, the reserVOIr stages about 67 mdes upstream Storage In Lake Mohave IS used formed by DavIS Dam, backs water at hIgh to the tallra.:e of Hoover Powt'!rplant. for sam.. rer~gulatlon of rt::leases from 5