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" " '~ ,"i:"". ,," o4:lo. ..... ~ '-'.:> .. ; . ~. above ancl belo... a Hancos Shale outcrop resulted in a threefold increase in dissolved solicls concentrat;l.ons. . Irrigation return flows .on Mancos Shales in the. HUlltingtonCreek in the San Rafael and' Muddy Creek in' the nit.ty Devil River drainages result in as much as a'tenfold incr"asa in c1issolveclsolids cono<>n1:rations. This. increase is typical of many of the marine shales although generally not as pronounced. Other marine fCitmationswhich conhibu:te signifi- cant amounts of salt are the Wasatch end Green River format:ions and the Uintah- Duchesne formation which 'unclerlies the inigated areas in theUintah and. Big Sandy drainage... ,.-' >> , .. The saline Paradox formation ",'f Pennsylvania Age underlies a large area of western Colorado and eastern Utah, but hasfe...sudace expoeures. The most notable .salt. contribution from this formation is .t'heparlldox Valley on the Dolores' River inwest:ern Colorado.. Natural po:tnt salinity sources like the Glenwood"Dotsero and the Steamboat Springs .are elso the result of geologic conditions. Water moving down"Warcl ..int:oj:hs earth along fracturas and bedding planae increases in temperature ancl . in the ability of the "ater to dissolve.mineralconstitue1't:s. l<hen the saline , 'water's eventually return to ths surface, theiT 'salt content is usually very high. Hagan (1971) reports that the saltclisoharge of major thermal springs ..in theColorsdo Rivllr Basin exoeads SOO,OOO Mgm p"rYEler. Geologie investigations have been made in many parts of tha basin in connection with coal, uranium, oil and gas andotheT minerals. Although the vast majority of these investigations are not hydrologically orientecl, the results can still be useful in the interpretation of data on the'qual;i.ty of' surface and shallow groundwaters. . The geology of the basin is the dominant factor in the chemical quality of the basin's waters. Figure 5 depicts the surface water quelity concentrationsessociat"c1...iththe various ar"as, in the Upper Coloraclo River Basin. Comparing Figures 4 ancl5 "Will inclii::ate the gaolog:ic importance to water quality. WA'l:ER SUPPLY OF TIlE UPPER COLORADO RIVER The largest ancl most prominent constra:!,nts to the water supply in the Upper Basin are the. various treaty ancl eompecS rights. The 1922 Colorado River Compact guarantees a total of 9.25 x 10 ha-m over each eonsecuti;re ten- ~ . 'yeaT periocl to the Lower .Basin for an annuel averageil.f about. 9.25 x 10 ha411. The' 1944 ~exican Water Treaty effectively raisecl this annual average amount to ....1.02. x10 ha-m, assuming that. one-half of.. the "Water promised to Mexico comes ,from the UCRll ellocation (Holburt, 1977). In addition".s'averal other legal, . legislative ancl international obligaUo"s h.ave tiecl the selinity concen.tra- tions and cont~olto.the....ater supply.. Mann et: al.(1974')prodUl:.eclanint!lr~ estinglegal-political history of the Upper, Basin "Which is helpful' in under- standing the davalopment of the Colorado River. .- ,.' .. ~ ";', ...; .:" . -..,',.." Precipitation The majority of the water supply in the Colorado River Basin comes from high mountain sno"Wpaeks. Flow duration tables and curves describing the seaaonal and annual water supply variability of the Upper Colorado River "ncl 16 ..