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J I I I I I I I I I B a I I I I I I D 45 A~ degradation at the mouth of Sand Creek would extend upstream in this major tributary only as far as the first bed~control structure. This is a sheet.pfle wall supporting an inverted syphon on Burlington Ditch across the bed 1300 feet upstream from its confluence with the South Platte River. Sand Creek passes over a spillway on top of the syphon. The drop is approximately 12 feet. Simons, If and Associates reported that the structure was under repair in March 1983. No further increase in degradation is anticipated at the confluence of Cherry Creek and the South Platte Rfver. Thernouth is at the scour hole downstream from the rapids in the South Platte River. In May 1977, Merrick and Company reported that the existing Delgany sanitary sewer main was exposed where it crosses Cherry Creek 14S0 feet upstream from the South Platte River. They proposed a concrete cap for this 1ine and the new sewer main to be built at the same crossing. This cap should be extended downward to prevent any undennin1ng of the pipes. Then, that would be the extent of any degradation extending up Cherry Creek. At its confluence with Bear'Creek, the South Platte River is now aggrading. If1JI1ediately upstream from its mouth, Bear Creek passes over the exposed Denver Fonnation. This rock fonnatlon controls the. riverbed grade upstream from thi s poi nt. The Bea r Creek profl ~ e 15 shown 1 n the flood hazard area delineation report by Gingery AssoClates, Inc., in 1979. 46 VIII. SUMMARY From the studies of the South Platte River from Chatfield Dam to Baseline Road Immediately north of Brighton which have been completed at this time, the preliminary findings are as follows: 1. The study reach is 41.2 miles long with an overall sinuosity of 1.19. The average riverbed slope from Chatfield Dam to the mouth of Cherry Creek is approximately 13 feet per mile. Thereafter, the slope decreased to approximately 9 feet per mile. 2. The rlver has three different fonns which have been deslgnated Rural, Suburban and Urban. I I I I I I I I I I I I I The Rural reaches are immediately downstream from Chatfield Dam and from East One Hundred-Fourth Avenue to Baseline Road. In these, the river is alluvial, sinuous and mobile; bars are prominent, pioneer vegetation exists, there is relatively llttle trash and the works of men are few. The Suburban reaches are from Columbine Valley to West Hampden Avenue and from the Sand Creek confluence to East One Hundred~Fourth Avenue. In these reaches, the river is narrower, less sinuous, and there are fewer gravel and sand bars. Some eroding banks have been stabilized. There are more bridges, weirs, drop structures. intakes, outfalls and buildings. Trash is plentiful. The Urban reach is from West Hampden Avenue to the confluence wlth Sand Creek. This reach has the fonn of a man~made ditch. The channel has been straightened. realigned, narrowed and deepened. Both banks and the bed have been stabilized. Works of men abound and include facilities for people to enjoy the riverine environment. J. The average annual streamflow in the South Platte River at Denver decreased by 25 percent in 1925 and has remained relatively drier ever since. A return to the wetter regime would have severe repercussions in the met ro- politan ",rea. The pattern of monthly streamflow has changed very little. Return flows from irrigation and municipal and industrial use make up some- what for water diverted from the river. Imports of water from the Western Slope negate consumptive use. 4. Flooding In the Metropolitan Area is partially controlled with the Chatfield, Cherry Creek and Bear Creek Dams. Floods are expected to be less frequent but large floods are still possible. 5. The three flood control dams have decreased the supply of sedi- ment to the South Platt!:' R1v!:'r in th~ ~nver Metropolitiln uea by an estimated 80 percent. The result has been iI degradation of the riverbed which is con~ tinuingatthistfme. The rate of degradation has not beendeteminedyet. Degradation has also resulted from the narrowing of the riverbed in the Urban reilch.