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<br />Section 2.0
<br />Study Area Description
<br />
<br />channel of Box Elder Creek consists primarily of type A soils, which have high
<br />infiltration characteristics,
<br />
<br />2.1 Topography and Physiography
<br />
<br />2.3 Surface Water Resources
<br />
<br />Box Elder Creek is located east of metropolitan Denver in the rolling plains area adjacent
<br />to Colorado's Front Range, The Box Elder Creek watershed is extremely long and
<br />narrow. The watershed extends approximately 100 miles from its headwaters at the
<br />Monument Divide in El Paso County to the confluence with the South Platte River near
<br />KWler, Colorado, The watershed is generally 3 to 5 miles wide over the entire length of
<br />the creek,
<br />
<br />2.3.1 Stream Characteristics
<br />
<br />Two main north-flowing streams exist in the study area, These are Box Elder Creek and
<br />Coyote Run, a major left bank tributary to Box Elder Creek. In addition, 89 separate
<br />major drainageways flow to these two creeks, All of the creeks in the study area are
<br />ephemeral, flowing intennittently in response to significant rainfall or snowmelt events or
<br />high groundwater inflow,
<br />
<br />The Upper Box Elder Creek study area is shown in Figure 2-1. The study area is located
<br />in the far southeastern portion of the UDFCD boundary. The upstream study limit
<br />coincides with the Arapahoe/Elbert County line and the downstream study limit is located
<br />at the south boundary of the Denver International Airport, approximately 4.5 miles north
<br />of 1-70. The study area is approximately 75 square miles in area, Of this, the Coyote
<br />Run watershed comprises approximately 29 square miles, and the Box Elder Creek
<br />watershed comprises approximately 46 square miles, The study area is under the
<br />jurisdiction of Arapahoe County, Adams County, and the City of Aurora, The area under
<br />Aurora's jurisdiction is located generally in the vicinity of 1-70,
<br />
<br />The mainstem Box Elder Creek channel is wide and shallow, with a sandy bottom and flat
<br />side slopes, Box Elder Creek exhibits a meandering channel pattern over most of its
<br />length in the study area; however, one significant reach of channel has a markedly
<br />straight, braided form, This reach extends from approximately 2 miles upstream of 1-70
<br />to 3 miles downstream,
<br />
<br />The portion of the Box Elder Creek watershed that is upstream of the study area is also
<br />indicated in Figure 2-1. This upstream watershed is located in Elbert and El Paso
<br />Counties and is approximately 127 square miles in area, Box Elder Creek is identified as
<br />Running Creek in this portion of the watershed,
<br />
<br />Local residents have attested that this straight reach has a high capacity to infiltrate base
<br />flows, One resident located 4 miles upstream of 1-70 mentioned that water flowed by his
<br />property for more than 2 months during the summer of 1993, including one runoff event
<br />that produced a flow depth of 2 to 3 feet, but none of the flow reportedly reached 1-70.
<br />The straight, braided form of this reach is most likely the result of aggradation processes
<br />that occur when sediment-laden flows infiltrate in this reach, leaving sediments to
<br />accumulate in fans on the channel bottom,
<br />
<br />The highest point in the watershed on the Monument Divide in El Paso County is
<br />7440 feet above mean sea level (MSL). The lowest point in the study area, at the north
<br />study limit, is at elevation 5390 MSL. Watershed topography shifts from steep, relatively
<br />rugged terrain in the upper portion of the watershed to moderate rolling terrain in the
<br />lower portion of the study area The transitional zone from steep to moderate terrain is
<br />located about 2 to 3 miles south of 1-70, The alluvial floodplain area of Box Elder Creek
<br />is broad and flat in the lower portion of the study area.
<br />
<br />Although infiltration along Box Elder Creek appears to be significant during base flows
<br />and small runoff events, it is not expected to have much of an impact on peak discharges
<br />during infrequent, large flood events. Therefore, stream losses were assumed to be
<br />negligible during the flood events modelled in this study.
<br />
<br />Coyote Run and the smaller tributaries generally exhibit a meandering channel form and
<br />are vegetated with grasses and occasional shrubs and trees, These tributaries generally
<br />have more pronounced side slopes than the Box Elder Creek channel, which tends to be
<br />broad and flat,
<br />
<br />2.2 Soils
<br />
<br />Hydrologic soil types within the watershed study area are shown in Appendix A.
<br />Hydrologic soil types were identified based on soil surveys produced by the U,S. Soil
<br />Conservation Service for Adams (SCS, 1974) and Arapahoe Counties (SCS, 1971),
<br />
<br />Box Elder Creek has an average channel gradient of 0,5 percent through the study area,
<br />dropping 535 feet over a distance of 21 miles, Coyote Run and the smaller tributaries
<br />have generally steeper gradients, ranging as high as 2,9 percent.
<br />
<br />2.3.2 Reservoirs
<br />
<br />Hydrologic soil types B and C are predominant in the watershed, Type B soils have
<br />moderate infiltration rates and type C soils have lower infiltration potential, The main
<br />
<br />There are no reservoirs within the study area, other than a number of small farm ponds,
<br />A detention dam was constructed between 1938 and 1941 near the Town of Elizabeth to
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