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<br />l;.o.) The computed average unnual s,,(Jiment loads are onl)' estimates <br />C <br />~ of tne mean-annual seCiment 10"0 at these sa.~ing stations under <br />joooo. <br /> <br />pre-oevelo~ment watersned conditions. <br /> <br />r~ost of the s;na II er ..atar Sheds <br /> <br />and all of tne larger ones have Deen mOdified to some extent by <br /> <br />grazing. mining. reservoirs. irrigation. and transmountain diversions. <br /> <br />Each of these activities can alter tne sediment load of a stream. <br /> <br />and ]enerally it is impossible to determine whether the net result <br /> <br />has Deen to increase or decrease the net sediment load. <br /> <br />To the <br /> <br />extent possible. an attempt was made to select the most recent <br /> <br />period of record during ~hicn hydrologic conditions in the Dasin <br /> <br />nave been relatively uniform. <br /> <br />Information collected during the <br /> <br />selected period of record were then useo to compute tne mean-annual <br /> <br />sediment load. <br /> <br />In the San Juan River and tne Green ~iver In ~yoming <br /> <br />study reacnes. the hydrologic conditions of the basin nave oeen <br /> <br />modified consiaeraoly. subsequent to tne selected periOd of record. <br /> <br />Dy tne construction of reservoirs immediately upstream from the <br /> <br />Study reacn. <br /> <br />Nearly all of the seaiment entering these'large <br /> <br />reservoirs will be trapped. <br /> <br />Tnerefore. it was estimated that les, <br /> <br />than 100 tons of sediment per year will oe tranSPQrted past the <br /> <br />San Juan near ArChuleta. and Green River below Fontenelle ?eservoir <br /> <br />gaging stations. <br /> <br />In both of these instances. however. the ~uantity <br /> <br />of sediment trapped by the reservoir is small. Green River 10 <br /> <br />percent and San Juan River 2 percent comparea to the sediment load <br /> <br />transported out of the study reach of the downstream gaging station. <br /> <br />lb <br />