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I I I I I I :1 I I I I I I I I I I I I B. c. II. DESCRIPTION OF STUDY AREA A. LOCATION First Creek is located north of metropolitan Denver in the plains area adjacent to Colorado's Front Range. The First Creek watershed is long and narrow. The watershed extends approximately 26 miles from its headwaters in Arapahoe County to the confluence with the South Platte River near Commerce City, Colorado. The watershed is generally 2 to 4 miles wide over the entire length of the creek. The Lower First Creek watershed, including Direct Flow Area 0055 (DF A 0055), as wcll as the upper portion of the contributing watershed, is shown in Figure 11.1. The Pena Boulevard crossing defines the upstream limit of the study area. The downstream limit of the study area is the confluence with South Platte River. MAJOR DRAINAGE BASINS First Creek The total First Creek watershed drains an area of 47.2 square miles. Its headwaters are located in Arapahoe County, south of Interstate 70 (1-70) and east of E-470. Runoff from the watershed flows in a northwesterly direction crossing Pena Boulevard just north of56'h Avenue and then crosses through the northeastern portion of the RMA before crossing 96'h Avenue at the upstreanllimit of the study area. First Creek is a right-bank tributary of the South Platte River and out falls into the river at approximately l281h Avenue. The average stream slope upstream ofRMA is about 31 feet per mile, dropping to about 26 feet per mile downstream ofRMA (Reference I). Direct Flow Area (DFA 0055) DF A 0055 is a tributary watershed to lowcr First Creek. DF A 0055 did not originally have a defined channcl, however, several small channels have been constructed as the watershed began to develop. These channels are Arsenal North Drainage, Rolla Tributary, Union Pacific Tributary, Elm Drainage and Birch Run. SOILS Soils information was obtained from the (Soil Conservation Service) SCS report for Adams County and Arapahoe County (Referencc 13 and 14). Generalizations were made on the soil types based on either the predominant local soil type or the greatest hydrologic effect on infiltration rates. Due to this generalization, there are two hydrologic soil types within the First Creek watershed, type "B" and type "C". G. Detailed soils information in the Denver area was not available since the soils are considered imported by the Natural Resources Conservation Service (NRCS), formerly the SCS. For the Denver area, the soils were classitied as "B" soils, based on the predominant adjacent soils (Reference I). A detailed soils map of the First Creek watershed can be found in Appendix C. D. VEGETATION Vegetation in the watershed consists primarily of native grasses typical of high plains areas, with a portion of the prairie grassland being used as pasture. About half of the study area (primarily the south halt) is currently in agricultural use as cropland. Natural riparian vegetation is evident along many of the major streams in the study area and consists primarily of grasses, with isolated areas of shrubs and trecs. In gencral, the natural riparian vegetation is confined to the alluvial channel corridor. In a number of arcas, cropland tillage has encroached on, or totally eliminated, natural riparian vegetation along streams. E. WATERSHED IMPERVIOUSNESS The upper reaches of First Creek are primarily undeveloped, irrigated cropland with wide swales and channels for drainageways. Toward the center of the watershed, First Crcek bisects Green Valley Ranch, which consists of mcdium dcnsity, single-family residences. First Creek then enters RMA with a more incised, low flow channel and widcr flood plain areas (Reference 1). The lower First Creek watershed is the study arca and is located downstream from Pena Boulevard and continues to the South Platte River. The Lower First Crcek watcrshed consists of irrigated farmland with pockets of light industrial and residential properties. F. .FLOOD HISTORY The Fulton Ditch, O'Brian Canal, Burlington Ditch, US.85, 1-76, SH-2, Union Pacific and Burlington Northern Santa Fe railroads, and 96th Avenuc have blocked the flow from First Creek. The crossings are inadequate to pass even the frequent flood events. Flood hazards include general flooding due to a lack of channel capacity, traffic interruption, washout of the Fulton Ditch, O'Brian Canal and Burlington Ditch, and erosion of the channel and bridges. According to information provided in the Adams County Flood Insurance Study (FIS), three major floods have occurrcd on lower First Creek and in nearby watersheds in recent times (Reference 12). WETLANDS An asscssment of the wetlands within the l~wer First Creek watershed was conductcd by ERO Resources Corporation in February 2000 (Reference 23). This assessment found wetlands along the .4.