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II -3 BASIN DESCRIPTION The followIng basin descriptions are given to relate fIeld Invest!.. gation, various development data and the hydrological sensitivities of the basins. For simplifyIng purposes, any given reach will be identified by the same number as the basin it is ~lDwing through. (I.e., reach 2, 3, 6, 8, 9 and 11 form Lena Gulch). Basin 1. Basin 1 as depicted on Drawing No, 1 is the western-most" mountainous area of Lena Gulch. Apex Gulch and Jackson Gulch join to form Lena Gulch at the bottom of the basin In the vicinity of Heritage Square. The upper 70% of the basin is underlain .lith PI"e- cambrian Gneiss, similar to granite. From the hydrological stand.. point, this forms an impermeable barTler below the surface soil layer. Several soil samples were taken from an area at an eleva.. tlon of approximately 6800 feet on the south-facing slopes of Apex Gulch and the streambed Itself. These samples Indicated that the slope materials were generally gravelly, silty sands. In the rela- tively undisturbed areas, a good grass mat builds up that absorbs moisture and curtails erosIon. On the north-facing slopes of Apex Gulch exists a large forested area which has a heavy layer of or- ganic matter and fine silts which have a very high absorption capac 1 ty. The Important point to be made Is that wherever the topsoil vegeta- tion is disturbed, particularly In areas near the stream and the forest areas, great Increases of r~noff and sedIment loads will oc- cur. Four field tests Indicated that the upper layer has a wide range of infiltration rates. Generally, ho.,ever, they have steady rates of about 1.1 inches per hour on the open south-facing slopes and 2.7 Inches per hour In the forest covered areas. The general hydrological runoff process Involves Interception of rainfall by groundcover, then Infiltration and travel to the stl-eam via the soils layer above bedrock. This c:-eates a situation whereby Apex Gulch in particular responds slowly to raInfall events, creating a steady streamflow down Apex Gulch. The development of Apex and Jackson Gulches should be discouraged because severe Increases in surface and overland flow and streamflow will result wIth acceler- ated stream degradation. The streambed Is generally quite natural and stable with repeated Instances of exposed bedrock between relatively steep stream banks cut out of sedimentary materials. As both streams pass the toe of the mountain slopes, they flatten considerably and Bre both altered or diverted by developments. Sedimentary deposition of large cob- bles and boulders occurs here with decreasing stream velocities.