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Walnut Creek The Rocky Flats Plant Core Area is located over the original upland and steep gully portion of the basin. Development has changed the original land form to a generally flat area sloping gently to the east. The area is heavily developed, with high imperviousness. Walnut Creek is the most developed of the three major drainages because the plant site Core Area lies primarily in this watershed. It slopes eastward at approximately 2.6 percent. Many detention and retention basins exist in the Walnut Creek basin. Walnut Creek aggregate basin characteristics are listed in Table III-2. The Walnut Creek drainage basin (approximately 3110 acres) extends eastward from the base of the foothills near the mouth of Coal Creek Canyon, through the Rocky Flats Plant site, to Great Western Reservoir. The portion of the basin within the study area (headwaters to Indiana Street) contains approximately 2376 acres. The basin includes much of the northern portion of the plant site, including the majority of the Core Area. Woman Creek The Woman Creek drainage basin extends eastward from the base of the foothills near the mouth of Coal Creek Canyon to Standley Lake. The portion of the basin that lies within the study area (headwaters to Indiana Street) contains approximately 2884 acres. The basin includes the southern portion of the plant site. Walnut Creek has several small tributaries whose defined stream channels begin at approximately the center of the Rocky Flats Plant site. These include North Walnut Creek and South Walnut Creek which head in the Core Area. They combine to form Walnut Creek approximately 2500 feet west of Indiana Street. The basin is similar to the Walnut Creek basin with relatively flat uplands, followed by steep gullies draining to Woman Creek, followed by broader valleys adjacent to Woman Creek that drain to Standley Lake. The topographic and hydrologic characteristics of the basin vary considerably. The western portion of the basin from the mouth of Coal Creek Canyon to approximately the center of the plant is relatively flat, sloping approximately 2 percent toward the east. There is no defined stream channel in this part of the basin. Most of the soils in the western portion of the plant have a high infiltration rate (6.0 inches per hour initial infiltration rate). It is presently undeveloped with the exception of the highly developed Core Area that lies at the astern edge of this flat terrain. The north central part of the Woman Creek basin directly south of the Core Area has been removed from its historic pattern of drainage to Woman Creek by the construction of the South Interceptor Ditch. This ditch routes runoff from the extreme southern part of the Core Area and the Buffer Zone between the South Interceptor Ditch and the Core Area to Pond C-2. The main channel of Woman Creek is routed to the north around Pond C-2. The center portion of the basin consists of gullies with up to 20 percent side slopes and 4 percent channel slopes leading into the Walnut Creek tributaries. It is presently mostly undeveloped with the exception of the highly developed Core Area that lies at the western edge of this terrain. Most of the soils in this portion of the plant site have a low infiltration rate (1.0 inches per hour initial infiltration rate). The soil infiltration characteristics vary according to the topography. The soils of the uplands area have high infiltration. The soils of the steep gully area have low infiltration. The soils of the broader valleys have low to medium infiltration. The basin generally is undeveloped except for the extreme southern portion of the Core Area that is tributary to the South Interceptor Ditch. The aggregate basin characteristics are listed in Table III-3. UDDer Bi2 Dry Creek The eastern portion of the basin, including the north-eastern portion of the Buffer Zone and the area east of the plant boundary to Great Western Reservoir, consists of broader valleys with about 5 percent side slopes and 2 percent channel slope contributing to Walnut Creek. The soil has low to medium infiltration characteristics (1.0 - 3.0 inches per hour initial infiltration rate). The eastern portion of the basin is presently undeveloped. The Upper Big Dry Creek drainage basin (shown on Figure 111-2) extends eastward from the base of the foothills near the mouth of Coal Creek Canyon to Standley Lake. It also includes an area lying south of Coal Creek tributary to Spring Creek (basin SL 6) that has been included in previous analyses by UDFCD. Site surveys of basin SL 6 indicate that the