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r-", ~ ~ ~- Oescriotlon of Basin Table 1 -Comparison 01 river basin drainage and runoff River basin Area Runoff Runof1 (square miles) (million acre.feet per yearl (inches per year! 244,000 15 'Z 1,234,000 440 6.7 258,000 180 13.1 12.000 14 20_9 COlorado MISSISSippi Columbia Delaware The noV.~ at various pOInts in the Basin for the 19-:1-95 period Jrc gi\'cn in Appendl' A The records of noV. depict wide flucruations from month to month and considerable variations from year to year. RESERVOIR STORAGE Wel and dry cycles have played a significant role In bnnglng about the dc\clopmem of the ColorJdo RIver reservoir comple_, (figure]). HistoriC records show that the annual flow of the river has varied from less than 6 million 10 more than ::!O milhon acre.feet per year. The reservoir system allow~ sufficient storage water 10 mainlaln the llo\vs of the river to meet downslrI:arn needs during dr)' pcnod~. In addition to the major reservoirs. numerous smaller reser-'oirs have been bUIlt on many of the tnbutaries. ~tajor storage Ixgan with lake ~tead m 1935 and concluded with the filling of lake powcll in 1980. The Basin reservotrS now have a combined storage capacity equal to approximately four time~ the total average annual virgin (undepletcd and unregulated) flow of the Colorado River. The now of the San Juan River IS controlled by ~avajo Dam. the Green River by FOnlenelle and Flaming Gorge Dams. and the Gunnison River by Ihe Wayne ~. Aspinall Unit Dams. Glen Canyon Dam is the only major dam on the mainstem of the Colorado Ri\'er above lee FerT)'. but It controls almost all the flow leaving the L'ppa Basm. l:1ke ~lead. fonned by Hoo\'er Dam. supplies most of the storage and regulallon in the Lower Colorado River Basin. Lake ~1ead provides water for irrigation. muniCIpal and industnal uses. power generation. flood control. recreation. and many other beneficial uses_ Lake ~10h::m~. the reservoir fonned by Dans Dam. backs waler upstream at high stages about 67 mIles (Q the tailrace of HoO\'er Powerplant. Storage in Lake ~lohave is used for some reregulalion of releases from Hoover Dam. for meeting treaty reqUIrements with ~texico. and for developing power head for the production of electncal energy at Davis Powerplant. The river flows through a natura! channel for abom 10 miles Ixlow Davis Dam al which point the river enter~ the broad ~tohave Valley 33 miles above the upper end of lake Ha...::L.<'U. 7