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Chatfield Dam, which was completed in 1975 has had an effect on South Platte River flows. For 33 years of record prior to 1975 the average annual flow on the South Platte at Littleton was 234 cfs. Similarly, the average daily flow at Littleton from 1975 through 1981 was 206 cfs. The average daily low flow prior to 1975 is 19 cfs and the average annual low flow after 1975 is also 19 cfs. Thus, it appears that the operation of Chatfield Dam as a flood control reservoir has had an effect on average and greater than aver- age flows, but that it has minimal effect on low flows, because water is needed to satisfy downstream water rights and because of imported water from III-5 TABLE I II-6 SOUTH PLATTE RIVER LOW FLOW HYDROLOGY Aver age Monthly Flows in CFS (1972-1981) L itt 1 eton Denver Gage Henderson Gage Month Gage at 19th St. Near Henderson January 51 111 247 February 57 118 272 March 72 153 305 Apr il 192 317 439 May 687 1,021 1,314 June 579 770 1,088 July 400 489 668 August 214 305 460 September 91 141 300 October 62 131 293 November 63 133 254 December 55 118 220 LOW FLOW HYDROLOGY Low flow conditions on the South Platte River are of interest because of recreation, water supply, and water quality concerns. Table 111-6 shows the average monthly flows on the South Platte River based on the 10-years from 1972 through 1981. These records show that the lowest flows occur during the late summer and winter. This same pattern occurred in the dry year, 1978, when the average monthly flow in October was 42 cfs at the Littleton gage. Average monthly flows are shown in Table 111-7 for the dry year, 1978. +"'''' I..................,.. ,...;....... ^+ ......... r,.......................l n~...;........ ..."..... .................." ... ......... ~, ""'"- ......".. II............... ......... ,.......... June 1921 Flood The flood of June 1921 was caused by heavy rains occurring over the entire basin, Although the peak discharge at Denver was a moderate 8,790 cfs, the flood volume was approximately 200,000 acre-feet. Overbank flooding occur- red from Water ton to Brighton with flood widths of from 1/2 to 1 and 1/2- mile in the reach above Denver. TABLE III-7 SOUTH PLATTE RIVER LOW FLOW HYDROLOGY Monthly Average Flow in CFS Dry Year (1978) January 39 78 163 February 44 84 229 March 56 95 168 April 51 102 197 May 82 218 405 June 164 229 539 July 274 294 519 August 146 197 311 September 58 88 215 October 42 67 144 November 60 116 173 December 36 84 221 HISTORICAL FLOODS Flood damages in the study reach have been caused by high peak flows that inundate land adjacent to the river, and by sustained high flows that may not inundate the floodplain, but cause damage by erosion and undercutting of structures. High peak flow floods are most often associated with thunder- storms and sustained rains. Long duration flooding is most often associated with snowmelt. Some significant past floods are described below.