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) .i . . Long-range plans for water development in the Upper Basin must, of course, be consistent ,dth river flows at Lee Ferry. Gages at Lee Ferry have been oper::.teLl continuously since 1922. Flowtl as far back as 1896 have been carefully estimated, however, by using records at three principal upstream stations and with Checks against do,!Ils~renm stations. Over the 59-year period, 18<;6-1954" inclusive, actual or histor::.c flows at Lee Ferry have averaged about 13.8 million acre-feet annually. Present upstream use depletes the annual Lee Ferry flow slightly more then ~JO mil- lion acre-feet, but gradually increasing depletions over the 59-year period have averaged only about 106 million acre-feet. Had there been no upstream use of weter, this 1.6 million acre-feet would have been added to the actucl flow of 13.8 million acre-feet at Lee Ferry, making an average virgin flow of 15.4 million acre-feet annually. Close scrutiny of the records over t,;,is 59-year span will show that for certain periods of 10, 20, end even 30 years the average vir6in flow was less than 15 million acre:-feet. Thi~ illustrates that only through efficient long-time holdover reservoir operations can the Upper Basin use the woter alloceted to it, fulfill its obligation to the Lo,~r Basin, and share in meeting any requi~ed deliveries to Mexico. The approximate reservoir capacity required for ultimate river regulation in the Upper Ba~in was detennined ;'y engineerinc analyses. A simplified illustration will demonstrate the method. From 1931 to 1940, inclusive, the driest de calle of record, ~~e virgin flow at Lee Ferry would have totaled only 118.3 million acre-feet. Had the Upper Basin been deplet- ing the stream by its full allotment of 7.5 million acre-feet annually, the residual flow at Lee Ferry for the 10-year period would have been only 43.3 million acre-feet. This would indicate a demand for 31.7 million acre-feet of Upper Basin holdover storage during that period in order to meet compact requirements exclusive of any required deliveries to Mexico. Actually, how- ever, Upper Basin use could not have depleted the flow by 7.5 million acre~ feet in eaCh year of that extreme dry period. Upper Basin diversions, prin- cipally high on tributaries of the river, "ould have been below nonnal in these dry years. For instance, in the driest year (1934), the virgin flou at Lee FerF~ without any Upper Basin use of water would have been only 5.6 million acre-feet. If this water had all been consumed in the Upper Basin, it would have supplied only three-fourths of the Upper Basin allotment with no allo~ance for the Louer Basin. Irrigation projects are planned to oper- ate with shortages of ,~ter supply during such critical periods. It was logically assumed that Upper ;lasin depletions during tho critically dry decade would have been roughly 10 percent less than the compact allowance. Under this condition the residual flow. at Lee Ferry during the 1931-1940 period would have totaled about 52 million acre-feet and the demand for active storage releases I-lould have approximated 23 million acre-feet. Since the water supply studies sho,! tiwt the reservoirs could have been full at tile beginning of the dry period, the ultimate project plan ~s described in the Bureau's 1950 report is based on a tot~l of about 23 million acre-feet of storage capacity for river regulation excluding an indeterminable amount for use in connection ,dth the Mexican Treaty. Streamflow records for the last few years chou that the regulatory capacity of project reservoirs ,lOuld \'1\.~ 4