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I ] J ] ~ ] B ] ] ] ! ~ g I I I I I I 19 20 Aver~ge Monthly Streamflow (Units are ftJ/s and %ofannual) 0" No. "', J" F,b Mar Ap, "., JUf. M A" Sop Annual 158 150 111 92.2 101 151 343 814 700 441 407 113 308ftJ/s 4.3 4.1 3.0 1.5 1.8 4.1 '.3 22.1 19.0 12.0 11.0 5.8 100.0% The annual streamflow for the period 1927 to 1981 is shown in Figure 5, along with the sequence of deviations from the mean. The mean is 372 cubic feet per second for the 55 years of record. This is equivalent to an annual volume of 270,000 acre-feet. The highest annual streamflow was 1166 cubic feet per second in 1942; the lowest was 104 cubic feet per secondln1954. The characteristics of the annual streamflow at Henderson are very similar to those at Denver. The annual streamflow at Henderson 1s 64 cubic feet per second greater than at Denver. Clear Creek 1s the major con- tr1butorto this water. There are 18 higher than average streamflow years' 35 years were drier than normal years at Henderson. One year was average.' The persistence table is given below. Persistence Table Annual Streamflow at Henderson (Units are % of years) water in the latter years in the months of August through April, and less water in the months of May and June. July remained unchanged. In1900,1915,1921 and1924,theaverageoftheannual streamflows was 704 cubic feet per second. likewise, the same average occurred for the combined years 1959, 1970, 1973 and 1980. In the latter years, the months of May, June and July had more streamflow. The months of August through April had less streamflow. It is concluded that the monthly variation in streamflow pattern at Denver changes as much or more from wet years to dry years than it has changed over time due to development in the South Platte River Basin upstream from Denver. Following year is Wet 0" Wet 17 17 Antecedent year is 0', 17 49 Henderson. The average monthly streamflow in the South Platte River at Henderson for the period 1926 to 1981 is as follows: Average Monthly Streamflow (Units are ft3!s and % of annual) Oct No> "', J," F,b M" Ap' "" J" J,l A" S,p Annual 171 166 149 150 168 118 349 931 '88 S4J 418 134 372ft3!, 3.' 3.' 3.4 3.4 3.5 4.' 7.' 21.1 21.7 12.3 '.5 5.1 100.O~ At Henderson, there is a strong persistence towards dry years fo1lowin9 dry. The others are all equal. Monthly Streamflow Denver. The average monthly streamflow in the South Platte River at Denver for the period 1925 to 1981 is as shown on the following page. May has the hi9hest flow and along with June and July contributes slightly more than half the annual streamflow. The monthly variation is also plotted in Figure 6. In years of high streamflow, every month has more streamflow than the same months in a year with low streamflow. In an effort to determine if there has been a change in the pattern of monthly streamflow due to the pr09ressive utilization of the water in the South Platte River, monthly streamflows for wet and dry periOdS have been compared. At Henderson, June has slightly more streamflow on the average than May. The highest monthly streamflow was in May 1973, for which the average was 5558 cubic feet per second. The lowest monthly streamflow was in 28.6 cubic feet per second in NQvembe~ 1934. Th~ averages for all months are plotted in Figure 6. Floods In 1902, 1903, 1904 and 1906, the average of the annual streamflow was163cubicfeetpersecond,well belownonnal. Likewise, in 1976, 1977, 1978 and 1981, the average was 165 cubic feet per second. There was more General. The principal cause of flooding in the South Platte River at Denver is prolonged rainfall on the high plains and in the foothills, I I I I I I I I I I I I I I I I I I I