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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 y;!th P'"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 t.he south-facing slopes of Apex Gulch and the streambed It.self. These samples Indicated that t.he slope materials were generally gravelly, silty sands. In t.he rela- t.lvely undisturbed areas, a good grass mat. builds up that absorbs molst.ure and curt.alls erosIon. On t.he north-facing slopes of Apex Gulch exists a large forest.ed area which has a heavy layer of or- ganic mat.t.er and fine silts which have a very high absorption capac 1 t.y. The Important point. to be made Is that wherever the t.opsoil vegeta- tion is disturbed, particularly In areas near the stream and t.he forest areas, great. Increases of r~noff and sedIment loads will oc- cur. Four field t.ests Indicated t.hat. the upper layer has a wide range of infiltration rates. Generally, hOYlever, they have steady rates of about 1.1 inches per hour on t.he open south-facing slopes and 2.7 Inches per hour In the forest covered areas. The general hydrological runoff process Involves Int.ercept.ion of rainfall by groundcover, then Infiltration and travel to the st.,'eam via t.he salls layer above bedrock. This c:"eates a sltuatlol1 whereby Apex Gulch in particular responds slowly t.o raInfall events, creating a steady streamflow down Apex Gulch. The development of Apex and Jackson Gulches should be discouraged becau.e severe Increases in surface and overland flow and st.reamflow will result wIth acceler- at.ed stream degradation. The st.reambed Is generally quite nat.ural and stable with repeated Instances of exposed bedrock between relatively st.eep stream banks cut out. of sedlment.ary mat.erIals. As both .treams pass the toe of the mount.aln slopes, they flat.t.en considerably and are both alt.ered or dlvert.ed by development.s. Sedimentary deposition of large cob- bles and boulders occurs here wit.h decreasing st.ream velocities.