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<br />w <br />...... <br />...... <br />l-' <br /> <br />~ <br /> <br />I <br />I <br />~ <br /> <br />Executive SUmmary <br /> <br />Large areas of the Upper Colorado River Basin are underlain by <br />saline geol9gic formations which contribute to the dissolved load or <br />salinity of the Colorado River. The marine Mancos Shale is the most <br />extensive saline formation outcropping in the Upper Basin and it is a <br />major diffuse source of salinity. The soluble mineral content (SMC) of <br />Mancos Shale can be as high as 20% but averages about 6% for areas of <br />shale without interbedded sandstones. The major soluble mineral is <br />gypsum. <br />Determining the SMC of weathered shale or alluvium samples may <br />require modification of standard procedures because of the slow <br />dissolution rate of gypsum. A method is suggested whereby the <br />electrical conductance (EC) of ll99 sediment-water mixtures is measured <br />after the sample has been agitated a suitable amount of time. Slow <br />dissolution of suspended sediment in ephemeral streamflow may cause the <br />salinity of the flow to be low which may not reflect the potential for <br />further dissolution of particles carried as suspended sediment. <br />The Mancos Shale is composed of three landform typesl badlands, <br />pediments, and alluvial valley floors. Pediments and pediment remnants <br />are relatively stable landforms. Channel incision of alluvial valley <br />floors can produce large quanti ties of sediment, however, salt <br />contribution will be lower because alluvium has been leached and it has <br />a lower SMC than in-situ Mancos Shale. Badland erosion produces large <br />amounts of sediment and, therefore, large amounts of salt either in <br />solution or in undissolved particles of suspended sediment. Sediment <br />and salt yield from badlands can be highly variable due to sediment <br />storage and flushing effects, but overall erosion is rapid and runoff <br />salinity is high. <br />Experimental erosion studies on Mancos hillslopes reveal a positive <br />relationship between salt production and sediment yield as well as a <br />positive relationship between slope angle and sediment and salt <br />production. Long-term studies of Mancos Shale watersheds show that <br />seasonal changes in the soil surface have a significant effect on <br />infiltration capacity, runoff, and sediment yield. Disturbance of the <br />fragile surface in the spring by animals or off-road vehicles will cause <br />a significant increase in sediment and, therefore, salt yield. <br /> <br />ix <br /> <br />;10 <br />,J'l <br /> <br />'-I' <br /> <br />:1 <br />,~ <br />;,~ <br />.~ <br />~ <br /> <br />g <br />'~ <br />,:; <br /> <br />, <br />'l <br /> <br />"~ <br /> <br /> <br />,t <br /> <br />,; <br />, .~ <br />,~ <br /> <br /> <br />".," --~ ~ <br />