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1 I I I I I I I I I I I I I I HYDROLOGY II-30 EROSION AND SEDIMENT A TION The erosion and sedimentation process in the South Platte River upstream of Union Avenue is affected by natural phenomena and the works of man. With the closure of Chatfield Dam and Reservoir in 1972, flow in the river at Union Avenue is practicallY controlled by releases through the dam. The erosion process involves both the detachment of particles and the movement of the eroded particles from the site. The variables involved in the process include the type and size of partiCles, and the size, shape, roughness, and slope of the conveyance surface channel. Just as a channel of a certain size, shape. roughness, and slope has a predictable flow capacity, this same channel also has the capability to convey a certain amount of sediment. If the channel is carrying less than its sediment capacity, it will attempt to pick it up from the bed and banks of the channel. If the channel is carrying more than its sediment capacity, it will deposit the excess. Since the 1972 closure of Chatfield Reservoir, the upstream sediment yield being carried by the South Platte River has been trapped behind the dam. Downstream of the dam, the rlver is starved for sediment. Union Avenue is about seven miles below the dam. The 4800 feet of channel immediately downstream of the dam to highway C-470 was built in conjunction with Chatfield Dam and is a stable 100-foot wide channel, essentially free of erosion or deposition. The reach from C-470 to Columbine Valley is about three miles in length. It has a slope of about 11 feet per mile for a total drop of 33 .feet. This reach is rural in nature. The definition of rural is that the river meanders and is sinuous with one high bank and one low bank; it is shallow with a gravel and sand bed with bars.