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II II 11 II II II II II II II II II II II II II II II II Lower Big Dry Creek Hydrologic Study The hydrologic regime of Big Dry Creek is complex because of the significant inflows and diversions from the cret::k (WWE 2002). 3.3 Stream Characteristics The main stem of Big Dry Creek below Standley Lake dam to the confluence with the South Platte River can generally be described as a well defined, meandering channel. Loess-type soil deposits are mapped along the majority of the creek's tributary area. These deposits are wind- deposited, unconsolidated materials that are typically erodible (WWE 2002). The upper eight miles of the channel 'between Standley Lake and the Broomfield WWTP can be generally characterized as a tran5:itiona1 foothills-plains stream type, with the lower portion of the stream characterized as a plains stream type (Aquatics Associates 1999b). Transitional foothills-plains stream characteristics include a diminishing gradient, with rubble and gravel settling out and sand becoming a key constituent of stream substrate. These streams are usually restricted to a single channel and shaded by riparian vegetation. Pools, riffles and runs are present in these streams (Propst 1982). In the transitional foothills-plains segment of Big Dry Creek between Standley Lake and the Broomfield WWTP, the stream banks are well vegetated with grasses, willows, cottonwood and Russian olive trees. The streambed consists of small cobble interspersed with gravel and sand in riffle areas, and sand and silt in pool and run areas. In addition, large cobble, small boulders and large woody debris are also present at a few locations in the stream, but are not widespread. The stream meanders through open space and residential areas (Aquatics Associates 1999b). In this segment, the stream channel varies from being shallow to being deeply incised with steep cut banks as high as 25 feet in the Westminster Open Space and below the WWTP discharges. Some portions of the bed and banks are expenencmg erOSIOn. Plains stream characteristics include shifting channels, eroding banks and bar migration. Point bars create temporary, quiet pools in their lee. In this type of transient system, there is little structural stability (Propst 1982). Below the Broomfield WWTP in the plains portion of Big Dry Creek, the stream bank is covered with overhanging grasses and the streambed is primarily sand and silt. East of 1-25" similar conditions exist with some reaches of the channel being more 971-179.092 June 2005 Wright Water Engineers, Inc. Page 15