2023-10-31_REVISION - M1973021 (2)

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Table 1-1. Major Drainageway Characteristics Summary Major Drainageway Drainage Area (square miles) Strain Gulch 0.5 Turkey Creek 51 Bear Creek 164 Strain Gulch is the only major drainageway directly adjacent to the Site. Where Strain Gulch meets the upstream extent of the property boundary, it has a drainage area of approximately 330 acres, which is routed through a 72-inch culvert beneath the north quarry and processing area. This culvert discharges into the existing stream channel at the eastern property boundary and north of the entrance haul road. Downstream of the Site, in the Town of Morrison, Strain Gulch flows into Bear Creek. There are no known issues or improvements needed for this drainageway. Turkey Creek, a tributary to Bear Creek, begins in the mountains in the southern portion of the Bear Creek watershed and has a drainage area of 51 square miles. The basin elevations range from 5,566 feet at Bear Creek Lake to 10,700 feet at Black Mountain. Turkey Creek flows east along US-285, south of the Site, and into Bear Creek Lake from the south. Bear Creek, which begins in the mountains southwest of Denver, is a tributary of the South Platte River. The total drainage area is 261 square miles, of which 164 square miles are upstream of Morrison. The basin includes parts of Jefferson, Clear Creek, and Park Counties, and ranges in elevation from 5,780 feet at Morrison to 14,264 feet at Mt. Evans. Bear Creek flows into Bear Creek Lake just east(downstream) of the Dakota Hogback geologic formation at Morrison. This facility is a major flood control reservoir constructed and operated by the U.S. Army Corps of Engineers (UDFCD 2009). None of the major drainageways have known FHAD study areas or Outfall System Planning studies that will affect the proposed mine plan for the Site. Both Bear and Turkey Creeks have floodplains delineated as shown in the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) (see Appendix A) but that are well outside of the Site boundary. No known channel improvements are expected because minimal impacts are anticipated for the major drainageways. There are no known irrigation facilities that will be influenced by Site drainage patterns. 1.4 Sub-Drainage Basins There are nine sub-basins on Site as shown on the existing Drainage Area Maps (see Appendix B). The subbasins either report to a permitted outfall or are off-site basins included to compare with proposed conditions. Table 1-2 shows a summary of the sub-basins characteristics. 1670 Broadway,Suite 3400,Denver,CO 80202-4824 hdrinc.com (303)764-1520