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1f2 SURFACE WATER SUPPLY Tables 20 through 26 contain data on discharge of the South Platte River, Plum Creek, Bear Creek and Cherry Creek within Water District No.8 from 1940 through 1966. The gauge at South Platte reflects inflow to the district by the South Platte River prior to any diversions by Denver, Aurora, Littleton or Englewood and includes transmountain diversions conveyed by the South Platte. The gauge at Denver measures the outflow from the district after all diversions and includes return flow from irrigation and from the Littleton and Englewood Sewage Treatment Plants, but does not reflect return flo'" from the Denver northside or eastside Sewage Treatment Plants nor does it reflect return flow from the Metropolitan Sewer District Plant which is now treat- ing Denver sewage and sewage from other municipalities, but does not treat Englewood or Littleton sewage. The gauge on Bear Creek reflects the contri- bution of that tributary, to the river except for diversions to the Olson and Bell Ditch which occur between the gauge and the mouth. The gauges at Cherry Creek and Plum Creek reflect within reasonable 1 imits the inflow of those two tributaries to the river. '. Figure 4 is a double mass diagram of the inflow and outflow of Water District No.8 covering the period 1901 through 1966, The ordinate is the cumulative annual discharge at South Platte while the abscissa is the cumulative flow at Denver. Examination of the diagram shows that for the period 1901 through 1966 the average annual flow at South Platte was 260,000 acre-feet and the outflow from the district measured by the gauge at Denver averaged 236,000 acre-feet per year, indicating that Over the long term the South Platte is a losing stream through Water District No.8. The diagram also indicated that the relation between inflow and outflow aver the 66-year period por- trayed has remained relatively constant, there being no period with a really large deviation from the long-time mean flow. Prior to 1913 the flow into the district seemed to increase at a greater rate than did the flow out of the district. From 1915 to 1940 the flow was quite close to the long-time average and the relation between inflow and outflow remained relatively constant. From 1940 through 1949 the flow into the district did not increase at the same rate as the flow out of the district, whereas from 1949 to 1956 just the reverse occurred. The period 1940 through 1949 was probably in- fluenced by the diversion of water through the Moffat Tunnel by Denver, accompanied by a decrease in its use of South Platte River water during the period. From 1950 to 1956 was a period of water shortage and the munici- palities experiencing increased demands were drawing water out of storage, primarily the city of Denver which would have been taking storage water out of Antero, Eleven Hlle Canyon and Cheeseman Reservoirs" This water would have been diverted within the distric( but the return flow would not be reflected In the diagram as it was returned to the stream below the Denver gauge. Since 1956 the relation between inflow and outflow has remained fairly constant and close to the long-time average value. There has not been sufficient time for the effects of importations of Blue River water through the Roberts Tunnel or of the Homestake Project water to have its effects On the data shown In Figure 4, , Transmountain diversIons into the South Platte River basin from the west slope through the Hoffat Tunnel, Berthoud Pass Ditch, Boreas Pass Ditch and the Roberts Tunnel are contained in Tables 27 through 30.