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I I I I I I I I I I I I I I I I I I I Lower Big Dry Creek Hydrologic Study 3.0 OVERVIEW OF WATERSHED CHARACTERISTICS BDCW A has conducted or sponsored multiple studies (Aquatics Associates, 1998, 1999a&b, 2002, and 2005; WWE 1998, 1999, 2000, 2002; Gossenauer and Wachob 2001) of Big Dry Creek over the last nine years to develop a better understanding of the watershed. These reports are the basis of the key watershed characteristic assumptions relevant to this study. Background information describing the watershed location, hydrology, stream characteristics, and extent of current and projected development follow. 3.1 Location The Big Dry Creek basin originates in unincorporated Jefferson County at the mouth of Coal Creek Canyon at an e1,evation of approximately 8,000 feet above sea level. The total drainage area is approximately 110 square miles. The basin drains easterly from the headwaters area across Rocky Flats, where several tributaries including Walnut Creek, Woman Creek, and Upper Big Dry Creek form. Below Standley Lake, Big Dry Creek flows in a northeasterly direction approximately 33 miles to its confluence with the South Platte River near Fort Lupton in Weld County. Interstate 25 roughly divides the watershed between urban and agricultural land uses with agricultural uses dominating east ofl-25. Over the last decade, this "dividing line" has been moving eastward as development occurs along the 1-25 and E-470 corridors. The Big Dry Creek basin is outlined in Figure 1, and Figure 2 shows the boundaries of the municipalities and counties comprising the watershed. 3.2 Hydrology The hydrology of Big Dry Creek is significantly influenced by irrigation releases from Standley Lake, runoff from storm events, discharges from municipal wastewater treatment plants (WWTPs), diversions to irrigation ditches and iITigation return flows. Big Dry Creek has not experienced a "natural'" flow regime in roughly 100 years since Standley Lake was constructed in the early 1900s. In addition, significant population growth in the central portion of the watershed during the latter portion of the twentieth century has resulted in increased base flows from wastewater discharges lmd increased storm runoff from increased impervious area. 971-179.092 June 2005 Page 12 Wright Water Engineers, Inc.