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<br />Mountain Region are not strictly applicable Igiven the high <br />runoff and dissimilar geology of the study w<l.tersheds. The <br />available data and a correlation of a numbe~ of geanorphic <br />factors with channel characteristics using I the so-called <br />"broad level stream channel classificatior1 systan" (see <br />Silvey and Rosgen, 1981) and data provided ~y the san Juan <br />National Forest provided an initial basis, hPwever, for the <br />assUllption that on all watersheds suspended sediment <br />concentrations can exceed 100 I1B/l for short! rerioCls during <br />high flood conditions. At baseflow, it was 9ssumed that the <br />suspended sediment concentrations average 5 ill or less on <br />all watersheds. <br />I <br />The maximum allowable sediment yields for ~ach develo[rl1ent <br />alternative were fixed by assuming that sUlfpended sediment <br />concentrations during the peak flow month co~ld be increased <br />by about 60 mg/l above baseline levels (see Tiilile IV-7). The <br />60 mg/l is based on an analysis of sediment/5ischarge rating <br />curves (regressions of suspended sediment versus discharge) <br />from Frazier Experimental Forest and other: Rocky ~lountain <br />areas. The 60 mg/l is approximately the amoUnt of suspended <br />sediment increase that would occur if chann~l stability was <br />destabilized to the nextl~~r stability cat~gorylstream type <br />(Rosgen, 1981). The maximum allowable s\;lspended sediment <br />concentrations during the low flow months $re estimated to <br />be 10 mg/l on all watersheds. It was assum$1 that increases <br />. of these magnitudes would not seriouslyi reduce channel <br />stability, condition or capacity. I <br /> <br />The total impact from future developnent or( each stream was <br />estimated by: (1) computing flow-induced changes in <br />sediment yield using the assumed rating CUrves for existing <br />conditions, and (2) computing the introduced sediment <br />conponents. In computing the introduced seqiment conponents, <br />it was assumed that the mitigation measures Ifound in Appendix <br />A will be strictly followed. Eased on ca1c4lations using the <br />modified universal Soil Loss Equation (Uf.S. Environmental <br />Protection Agency, 1980) and work by Leaf (11970), a value of <br />0.17 tons per year per acre disturbed was I rea~'onErl to be a <br />liberal estin~te (actual yields will probclbly be less) for <br />the delivered streamside disturbance sediment component, and <br />1.6 tons per year F.er acre was assumed fot road and similar <br />disturbances in close proximity to streams. 1 <br />I <br />Table IV-6 surrmarizes computations ma\:'le in cOIlputing <br />potential on-site and off-site soil loss Ifor the ski area <br />using the modified soil loss equation. I~ was assumed that <br />potential on-site soil loss would be reduC$d at least 90 per <br />cent by rehabilitation inclUding r~egetation and/or <br />mechanical means. I <br />I <br />I <br />I <br />, <br />I <br />I <br /> <br />l74 <br />