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~ ] ~ I 2 I I Ii n u ~ ~ ~ 3 9 ~ I I 1 35 these islands no longer shifted because of the fixed position of their vegetated banks. Through channel abandonment and fsland attachment to the floodplain, the South Platte River became a slightly sinuous sfng'e~thalweg channel. In 1977, theJrean bankfull width of the South Platte River between Greeley and Fort Morgan was only 360 feet. The bankfull depth had not changed, rema~ning approximately 5 feet. Sinuosity increased to 1.25. the riverbed gradlent decreased slightly to approxi~ately 6.5 feet per mile. and thesed1mentproperties ". . . do not appear to have changed sign1fi. cantly during the past 150 years" (Nadler and Schumn, 1981, p. 101). C.ur- rently there are more cottonwoods, w111o~ and other shrubs and t~e vegeta- tion is thicker. . Developments tn Metropolitan Denver Up to 1953, developments in the Sout~ Platte River Basin upstream from Metropolitan Denver were similar to those in the Greeley to Fort Morgan reach. It is presumed that the river response in the rural parts of the Denver Metropolitan area was the same as downstream from Greeley. In 1953, closure was made of the Cherry Creek dam decreasing the supply of sediment to the South Platte at their confluence by approximately 40 percent. With the c10sures of the Chatfield Dam in 1975 and Bear Creek Dam in 1979. the sediment supply is estimated to have decreased byapproxi- mately 95 percent in the Urban reach and approximately 80 percent down- stream from the Clear Creek confluence. Developments since 1895 have changed the streamflow at Denver but there appears to be more c~ange from wet to dry years than from developments. T~at is, prior to 1925, average streamflow at Denver was approximately 23 percent greater t~an after 1925. This change to a drier regime in the recent record is unexplained at t~is time. A return to more streamflow would result in increased stresses to t~e river regime in the Denver area. Conversely, the National 'Research Council's carbon dioxide assess- ment corrrnittee (see Kerr, 1983) has reported that a 2 degree C warming of the atmosphere combined with a 10 percent decrease in precipitation would decrease runoff into Western rivers between 40 and 75 percent. "T~e impact would be especially severe in the Missouri, Rio Grande. Upper Colorado and "Lower Colorado regions. ..." There is a 50 percent possibility that by some time between 2050 and 2100. the present-day carbon dioxide concentra- tion will have doubled. The increase will be due mainly to the burning of oil, gas and coal and to a lesser extent by the destruction of tropical forests or other biomass sources. The result WQuld be a wanning of the sur- face of the globe. most likely 1.5 to 3.0 degrees C. I'ost floods in Metropolitan Denver are nO'tlfT"()derated by the three flood control reservoirs. Tile frequency of flooding is reduced. Under the current operations plan, the peak flow in the South Platte River between Chatfield and Henderson is held to 5000 cubic feet per second until the 36 I I I I I I I I I I I I I I I I I I I flood storage capacity is depleted. The morphology of the South Platte River has been changed due to developments along its banks. At bankfull stage in downtown Denver, the river is half as wide and twice as deep as it is in the rural area down- stream. Moreover, the river has been straightened, with check structures on 1ts bed and leveed banks in the bu11t-up areas. In the suburban and rural areas, mining for gravel is affecting the river regime. Future developments in the South Platte River in Metropolitan Denver are for the most part under the control of man. Annual streamflow is not expected to change greatly in the 1rrrnedfate future. Increased consumptive use in the basin will be made up by importations of water from other catchmen~s. The monthly distribution of streamflow may change slightly due to changlng water use patterns. Over a long period the mean annual flood peak is expected to be the same or smaller than before the dams were con- structed; however, the duration of some of the rainfall generated floods is expected to increase due to the routing effects of the reservoirs. Currently, the Corps of Engineers are designing a realigned and channelized section of the South Platte through Littleton to accommodate larger flows without damage. The reach is between the south side of Columbine Valley and West Oxford Avenue. With the completion of this project, the South Platte will be channelized from Columbine Valley to COTTl'1le~ce C~ty. leaving only short reaches upstream and downstream in which the rlver 1S not controlled. It is these latter reaches which are of prime interest here. Responses Cllanges in river stability, alignment, form, width and bed profile due to past developments are not yet completed. These as well as the changes due to new developments are addressed below. In general, the main factors influencing the form of the South Platte River in Metropolitan Denver are the great reduction in sediment load beginning 1n 1953 and the mining of gravel along parts of the river. Other factors are the reduction in mean annual flood peak, the narrowing of the river channel by developers and the reduction of overbank flooding due to levees or the raising of the floodplain (backfilling). In the unmined rural sections of the study reach, the response of the river to developments in the basin prior to 1975 is presumed to be the same as ilTlllE.'diately downstream from Greeley. Here, the reduction in flood peaks and the increase in base flow combined with some wash load of silt and clay resulted in the change from a wide, low, braided stream to the present-day meandering single-thalweg channel. Where the river is completely free to change, the anticipated long- term responses to the reduction in sediment load and flood peak would be as follows: