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N ~ Q N GEOLOGICAL SURVEY RESEARCH 1970 EVALUATION OF A METHOD FOR ESTIMATING SEDIMENT YIELD By LYNN M. SHOWN, Denver, Colo. .1b.'tract.-A method tor estimnting annual ~ediment ;rlE'ld. de\"1'loped by the l'acltlc South1\'pljt Intflr'.\.~l'nC':r COlUlllittf"t'. was testN in 28 smnll wntt'rsh~(]~ rJllgJI1;: in size trom O.O"l to 3i squnre milE'S. TileS€> watersheds tor whIch hydrologic rec- ords e:<:ist are in arid nnd semiarid areos ot western Colorndo. north-<'eDtrnl Xew :\lexico, nod east.(.'entrnl Wyoming, aud repl"E'sent a variety ot geologic. climatic. ecologic. and topo. graphic 'conditions. Estimates ot sediment yit>ld correlnted closely with sediment yields measured in reseryo[rs, but the estimates were usually lower tban the measured yip.lds. The Lnl:'thod was designed to make general sediment c1as~ificntions {or areas greater than 10 square miles. but it also appears to work nearly as welt for \\"ntersheds that are ns small as 0.1 square mile. &!diment-yield records for drainage arens of less than 500 square miles in wildland arens of the Western L nited States are scarce. Jfanagers of land in these areas need estimates of sediment ~'ield to aid in deci. sions which concurrently affect water, soil, timber, forage, and recreation. A sedimentation task force of the Pacific Southwest Inter-Agency Committee (PSI- AC) prepared a guideline method for field e,'aluation of sediment yields, in conjunction with a discussion of factors affecting sediment ~'ields in the Pacific South. ''rest area. The purpose of this investigation was to determine the usefulness of the PSL-\.C method for extrapolating sediment-yield information to arens lacking records. METHOD . The method del'eloped by the PSI.-\.C (1968) consists of rating the watershed on the basis of nine factors shown in table 1. A numerical rating is arrived at by summing the ,'alues nssigned to the factors. A sug- gested range of ,'alues for each factor is included in the table. If a factor would be likely to result in a small sediment yield, a ,'alue near the lower end of the range is assigned. A yalue near the upper end of the range is assigned when the factor would cause the sediment yield to be large. For e:tample, surface geol. T.\BLE I.-Rating range' for the !ocl.orl et'oluated in the -Pacific Southwut Inttr.Agency Committee method for utimating 8tda.. ment yield, u&ing terrain chorocteri8tiu Factor Rot"", =I' Suriace geology.._. 0-10 Soils_..__________ 0-10 Climn.te__________ 0-10 Runoff....._..... 0-10 Topography__.. .._ 0-20 Ground cover_____ -10-10 Land use_........ -10-10 {;pland erosion_... 0-25 Channel erosion 0-25 3lld sediment transport. :\I:un thara.etuistiCS {'Onstder~ Rock In'e. Hardness. , Texture. Aggregation. Shrink.swell. Rockiness. Storm frequency, intensitj.., 3nd durotion. Snow. Freeze-thaw. , V clume per- unit. a.rea. Peak flow per unit area. Steepn~ of upland slopes. Relief. Fan 3nd 600d plain development. Vegetation. Litter. Rocks. Understory development under trees. Percentage cultivated. Grazing intensit,'". Logging. . Roads. Rills and gullies. Landslides. Wind deposits'in channels. Bank and bed erosion. Flow deptlls. Active be::Ldcuts. Channel vegetAtion. Weathering. Fracturing. 5aHoitv. Caliche. Organic matter. ogy would be assigned a value of lOin an area under. lain by soft fissile shale, and a mlue of 1 in an area underlain by se"eral thick basalt flows. Each factor e:t~pt topography is paired with an- other factor that has a similar influence on sediment yield. The pnirings are: surface geology and soils, climate and nmolf, ground co"er and land use, and up- land and channel erosion. Each factor is rated separ- ately, but the one it is paired with is usunlly considered concurrently, and the degree of interdependence of the two is reflected in the similarity or lack of similnrity in their respecti"e ratings. lJ.s. CEOL. St:llVT.Y PROP. PAPEJl ""8. PACES 8.....8:u B245 <1'Yn t.