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<br />. <br /> <br />(:') <br />t.C <br />W <br />~ <br /> <br />. <br /> <br />. <br /> <br />SPECIAL STUDIES (Continued) <br /> <br />(generally above 8,000 feet [2,440 m]) will usually be lower than in- <br />dicated in Table F and Figure 7. Nevertheless, the salinity of the <br />water that ultimately flows from those geohydroloic units into main-stem, <br />streams should, in most cases, be within the range indicated. <br /> <br />For example, runoff in Bitter Creek, which originates on the Green <br />River Formation (in geohydrologic unit 3), is nonsaline in the head- <br />waters area, but is highly saline by the time it discharges into the <br />White River. Part of this increase of salinity is due to concentration <br />by evapotranspiration along the water course; but most is due to the <br />ground water component of the streamflow, which generally is highly <br />saline. This is indicated by chemical analyses of ground water samples <br />collected directly from the G'f3~n River Formation adjacent to Bitter <br />Creek (Price and Miller, 1975). Similarly, the salinity of runoff in <br />Wahweap Creek, which heads on the Kaiparowits Plateau, increases from <br />less than 1,000 mgfl near the headwaters area to more than 2,000 mgfl <br />where the creek drains into Lake Powell. This increase is due chiefly <br />to saline inflow of water that has been in contact with the Tropic Shale <br />and the Straight Cliffs Sandstone of geohydrologic unit 4. <br /> <br />Alluvium (geohydrologic unit 5) in most places along main-stem <br />streams yields nonsaline water,chiefly because it is generally highly <br />permeable and because most highly soluble minerals that it may have <br />contained have been leached. Along many of the intermittent and <br />ephemeral streams that drain geohydrologic units 3 and 4 and parts of <br />geohydrologic unit 2, however, the alluvium (most of which is not shown <br />in Figure 7 contains crusts of salt deposited by evaporating shallow <br />ground water or by receded streamflow. Most of the salt is readily <br />taken into solution by subsequent streamflow and is eventually carried <br />to the Colorado River. <br /> <br />Geologic formations that contribute most significantly to the <br />salinity of the Colorado River are shales, such as the Lewis, Mancos, <br />and Kirtland Shales of Cretaceous age (in geohydrologic unit 4), and <br />those formations made up largely of shale, siltstone, and mudstone, such <br />as the Green River, Uinta, Fort Union, and San Jose Formations of Ter- <br />tiary age (in geohydrologic unit 3). This is especially true in the <br />areas where soils developed on those formations are irrigated, resulting <br />in highly saline return flows to the river system. For example, Spring <br />Canyon Creek, (Price River basin) undergoes a threefold increase in <br />dissolved-solids concentration where it crosses an outcrop of Mancos <br />Shale in an unirrigated area. By contrast, Huntington Creek (San Rafael <br />River basin) and Muddy Creek (Dirty Devil River basin) undergo a <br />fivefold-to-tenfold increase in dissolved-solids concentrations where <br />they cross outcrops of Mancos Shale in irrigated areas (K.M. Waddell, <br />U.S. Geological Survey, written commun., 1977). <br /> <br />112 <br />