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I I I 5.2.2 Soils Fine-textured soils of loam occur in much of the area. These fine- textured soils are potentially erodible. In addition, the permeability of the surface layer is moderately slow to moderate. The permeability of the sub- soils immediately below the surface layer is slow. These low permeabilities restrict infiltration and water movement through the subsoil. I I I 5.2.3 Climate Average annual precipitation amounts to about 14 inches. In general, most of the precipitation occurs between April and September during numerous thunderstorms. Most of the storms are of short duration and moderate inten- sity. However, intense thunderstorm activity has produced floods in the area as indicated by the flood history. I I I 5.2.4 Runoff The quantity and intensity of precipitation determine the amount and discharge rates of runoff. In this area, intense precipitation is infrequent and generally, peak flows and runoff volumes per unit area are considered to be moderate to low. Also, average annual runoff as compared to average annual precipitation is low. I I I 5.2.5 Topography The topography is relatively well dissected with well-developed tributary channels. Upland slopes are usually less than 15 percent and more commonly between one and ten percent. Some of the area could be considered steep. I I 5.2.6 Ground Cover Ground cover is sufficient to slow overland flow and increase infiltra- tion. Cover is defined as anything which reduces the erosive effects of pre- cipitation. On the average, ground cover is estimated to be between 60 to 70 percent and consists largely of short-grass sods and bunch grasses. This amount of cover is adequate to protect the soil and substantially reduce I I I erosion. I I 4l 5.2.7 Land Use Portions of the watershed are grazed and urbanized, but not intensively. 5.2.8 Upland Erosion No signs of surface rilling were seen. 5.2.9 Channel Erosion and Sediment Transport Some gullying is evident along the tributary water courses, as well as occasionaly active headcutting. Sandstone outcroppings in Lang Gulch control channel erosion in localized reaches. The nine factors just described were numerically rated using the PSIAC method. The average annual sediment yield is estimated to be less than 0.2 acre-feet per square mile, which is the lowest sediment yield category. The USGS estimates the upper limit of average annual sediment yield to be about 0.1 acre-feet per square mile for similar areas in Denver (ref. 22). Results of the PSIAC method are close to this estimate. A rough estimate of average annual sediment concentration can be computed based on the estimates of annual sediment and water yields. The assumptions are: (1) ten percent of the 15-inch average annual precipitation runs off as stream flow: (2) annual sediment yield is 0.1 acre-feet per square mile; and (3) an acre-foot of sediment has a porosity of 40 percent. Based on these assumptions, the average annual sediment concentration is computed to be 2080 milligrams/liter. This value is high since the assumed sediment yield of 0.1 acre-feet per square mile is an estimate of the upper limit. Thus, the actual average annual sediment concentration is expected to be less than 2080 mg/l. Measured sediment concentrations in Mail Creek basin during 1980 for low-flow conditions (one to 60 cfs) ranged from 10 to 180 mg/l (ref. 14). 5.3 Stream Bank Stability The stability of stream banks in the study area was thoroughly investi- gated by field and office studies. The banks in the study area are predomi- nantly of a cohesive nature with very few instances of noncohesive bank material. Bank heights ranged from a low of approximately one foot to in excess of 20 feet. Banks were often vertical. Stability in the study area ranged from extremely stable to extremely unstable. Unstable banks, in general, were located on the outside of meander bends. Bank failures were