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2.3 Watershed Conditions Steep channel slopes creates a potential for Upper Willow Creek to transport large volumes of sediment and debris. Under natural conditions, a bed of cobbles or boulders will develop, and movement of channel materials will only occur during high flow events. Therefore, sediment transport volumes can be highly influenced by the supply of sediment into the channel from the watershed. Natural events such as landslides or beaver dam breaks may lead to localized increases in sediment supply. However, disturbances by man in the watershed can have a much greater impact on sediment supply. Control of these sediment sources may be one way to control excessive sedimentation and improve stream function and health in Willow Creek. 2.3.1 Watershed Physical Characteristics Watershed sediment supply is highly influenced by the physical characteristics of the watershed including watershed slopes, vegetative cover, and geologic stability. Figure 2.3.1 includes a representation of the slope of the Willow Creek watershed using gradations of red. Slopes were quantified on a 30m by 30m grid using USGS DEMs. Much of the Willow Creek watershed is characterized by extremely steep slopes. Amazingly, the average percent slope in the Willow Creek watershed is 45.4 degrees. Unprotected loose sediments or disturbed sediments will easily be mobilized by rain storms or snow melt in many areas of the watershed. Figure 2.3.1 also includes several characterizations of vegetative cover. These maps were derived from vegetative studies performed by the U.S. Forest Service. Maps of the primary cover type, percent forest, and percent barren are included. The GIS coverage also includes the percent cover of the primary cover types as well as individual vegetation species that are present. In addition to the high elevation areas near timberline, a range of barren areas exists from above the flume entrance to above the confluence of East and West Willow Creek. Steep rock talus slopes are predominant in this area, although historic mining disturbance and timber harvesting still influences some bare areas. Riparian vegetation can be seen in upstream reaches of Willow Creek, but is notably absent downstream. Table 2.3.1 summarizes physical characteristics broken down by sections of the watershed. Table 2.3.1. Physical characteristics of Willow Creek watershed areas Area Average Maximum Mean Percent Percent Watershed Area Slope ( acres) Elevation Elevation ( degrees) Forest Barren West Willow Creek 8424 11434 13291 44.8 26.9 37.2 East Willow Creek 13293 11511 13895 48.7 28.2 45.0 Willow Creek above gage 1377 10217 11365 42.2 41.7 32.8 Willow Creek below gage 2375 9202 11014 31.4 9.4 33.3 Total watershed area 25469 11202 13895 45.4 26.8 40.7 2-37