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. . " 8 Moving upstre~m, comparison of the Henderson and Denver gages with Julesburg shows much the same pattern, except the severity of the drought years of the 50is was more pronounced at Denver and Henderson than expe- rienced at the Kersey station. Looking at the inflow stations from the tributaries between Denver and Kersey, one sees the probable reason for the sl ight modification of the Kersey flow. The 10-year moving averages over the same time period for Clear Creek, the Big Thompson River, and the Cache la Poudre River at their mouths indicate a gradually increasing trend of flow over this study period. . The pattern of the outflow of St. Vraln Creek does not show this increasing tendency but shows a pattern quite similar to those already discussed for'the main stem stations. The increas- ing flow pattern for the Big Thompson River and the Cache.la Poudre River Is undoubtedly due to additional return flows generated by the transmountain water imported by the Colorado-Big Thompson Project. This new water has apparently not been sufficiently large to, influence the 6utflow of the St. Vrain the same as the other two tributaries. DESCRIPTION OF GRAPHICAL ANALYSIS The analysis of State Line flow at the Nebraska border was analyzed in a graphical manner as shown on the attached ~i~ure, dated July, 1968. The Figure is a double-mass diagram which has been developed to investigate the flow of the river at Julesburg during the past 56 years in an effort to in- terpret the effect of water importations, well pumping and other natural and man-made events on the state I ine flow. The vertical scale of the diagram represents the accumulated annual runoff, in mi 11 ion acre-feet, of the South Platte River at Julesburg. The horizontal scale is also in mill ion acre-feet and represents the accumulated annual runoff of three mOuntain streams measured at their emergence from the mountains and generally before they are affected by diversions. These three streams represented are St. Vrain Creek at Lyons, Boulder Creek at Orodell, and Clear Creek at Golden. A double-mass diagram is a method of identifying alterations of flow patterns for a point of interest, in this case the South Platte at Julesburg, by comparison with the flow pattern of index flow gauges, such as the three streams, which are subject to the same general cl imatic variations as the point of interest but are not significantly affected by man-made events such as importation of west slope water and well pumping. Thus if there were no changes in regime of flow over the years, the points when plotted would form a straight I ine. Conversely, a shift in the slope of the line represents a change in flow pattern which can, in many cases, be related to significant events. Analysis of the double-mass diagram, and the table of average annual flows of the South Platte River at Julesburg for various time periods appear- ing thereon, indicates that: 1. Transmountaln diversions, averaging almost 300,000 acre-feet an- nually since 1958 have not caused a significant increase in the flow at Julesburg over the historic average. Reasons for this Include: " a. Increase in combined surface water and w~ll pumping diversions b. Increased use by municipal itles and industry.