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<br />. <br /> <br />. <br /> <br />. <br /> <br />o <br /><0 <br />I\) <br />~ <br /> <br />PART X. SPECIAL STUDIES <br /> <br />A. Geologic Factors that Affect Salinity <br />in the Upper Colorado River Basin <br /> <br />In 1977 the U.S. Geological Survey compiled a report for the Bureau <br />of Land Management that describes the effects of geology in the <br />Colorado, Utah, and Wyoming parts of the Upper Colorado River Basin on <br />salinitr2tn the river system (U.S. Bureau of Land Management, 1978, p. <br />15-23). A summary of that report, revised to include the Arizona and <br />New Mexico segments of the Upper Basin, is given below. <br /> <br />1. Stratigraphy <br /> <br />The source of nearly all the dissolved ions (salinity) in water <br />that enters the Colorado River is the mineral assemblage of the rocks <br />that underlie its drainage basin. Mineral constituents are taken into <br />solution by both overland runoff and ground water runoff (the ground <br />water component of streamflow). The slower moving ground water, having <br />longer contact with the rocks before discharging to streams, dissolves <br />larger amounts of mineral constituents than does overland runoff. The <br />ground water, therefore, contributes significantly to natural salinity <br />of the streamflow. <br /> <br />Principal properties of rocks that affect the natural salinity of <br />water that flows over or through them include their mineral composition, <br />texture, and permeability. These properties are related to the origin, <br />age, and degree of deformation and induration of the rocks. <br /> <br />Some sedimentary rocks of marine and lacustrine origin commonly <br />contain widespread accumulations of such highly soluble minerals as <br />gypsum and halite; whereas most igneous, metamorphic, and sedimentary <br />rocks of terrestrial origin are composed largely of less soluble <br />minerals such as quartz and various silicates. Consequently, runoff <br />from certain rocks of marine and lacustrine origin (especially the <br />shale, mudstone, and marlstone strata of Mesozoic and Cenozoic age) <br />generally has a higher salinity than does runoff from most igneous and <br />metamorphic rocks or sandstone strata of terrestrial origin. <br /> <br />Fine-textured rocks afford more surface contact to water that flows <br />over or through them than do coarse-textured ones. Runoff from fine- <br />textured rocks (such as shale or siltstone) therefore has more oppor- <br />tunity to dissolve mineral constituents, and such runoff generally has a <br />higher salinity than does runoff from coarse-textured rocks (such as <br />conglomerate or coarse-grained sandstone). <br /> <br />110 <br />