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<br />" <br />L") <br />C'ol ' <br />C\! <br />C\I , 600 120 <br />0 <br />r~ <br />-- <br /> 500 100 tii <br /> w <br /> W LL <br /> -' uJ <br /> c.. a: <br /> 0 0 <br /> w <C <br /> c.. LL <br /> LL 400 ,80 0 <br /> 0 (/) <br /> (/) 0 <br /> 0 Z <br /> Z <( <br /> <( (/) <br /> (/) ::l <br /> ~ 300 60 0 <br /> :r: <br /> :r: f- <br /> f- Z <br /> Z <br /> Z ~ <br /> 0 0 <br /> 200 40 -' <br /> i= LL <br /> ::i f- <br /> ::l <br /> ::l 0 <br /> c.. -' <br /> 0 <( <br /> c.. ::l <br /> 100 20 z <br /> z <br /> <( <br /> <br /> <br />o 0 <br />1910 1920 1930 1940 1950 1960 1970 1980 1990 <br />YEAR <br /> <br />FIGURE 2.--Relation between population growth in Las Vegas Valley and <br />surface-water outflow to Las Ve!!as Wash (from Patrick A. Glancy, U.S. <br />Geological Survey, written commun., 1987). Dashed whcre approximate. <br /> <br />The cost of constructing the series of dikes and slurry walls would be considerable; hence, the U.S. <br />Bureau of Reclamation selected the D-14 detention basin (fig. 3) as a test site to determine whether the <br />strategy would be effective in reducing the dissolved-solids input to Las Vegas Wash, The site was chosen <br />because as much as 60 percent of the shallow ground-water flow seeping into the wash was thought to be <br />entering through the Whitney area (0. Art Tuma, U.S. Bureau of Reclamation, oral commun.. 1986). <br />Selection of an area with the greatest quantity of flow would provide an accurate test of the effectiveness <br />of the detention basin as a salt-reduction strategy. <br /> <br />In the fall of 1986, the D-I4 dike was constructed. It is composed of compacted alluvial-fan deposits. <br />The level top of the dike reaches a maximum heighI of about 6 ft above land surface near the southeast <br />corner. The top of the dike is level, so the elevation difference between the top of the dike and land surface <br />becomes less along the western legs as the land surface rises to the west An inlet structure on the north <br />side of the dike (fig. 3) allows surface water (from the Monson Road floodway) and treated sewage effluent <br />to enter the detention basin. The Monson Road floodway is an unlined channel carrying urban runoff and <br />shallow ground-water seepage from the upgradient flood plain of the wash to the study area. At the south <br /> <br />-5- <br />