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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Grande Coastal Basin, and most of the associated return flows are discharged <br />into drains and noodways which flow to the Gulf through Laguna Madre [17]. <br /> <br />GROUND WATER <br /> <br />Ground water occurs in much of the Rio Grande Region in confined and <br />unconfined aquifers composed of sand, gravel, silt, clay, sandstone, carbon- <br />ates, evaporites, and volcanic rocks. Figure 6 depicts the general distribu- <br />tion of geologic units in the Region. For the most part, valley fill <br />comprising unconsolidated medium to coarse grained stream alluvium, very <br />coarse grained alluvial fan deposits, and relatively fine grained lacustrine <br />sediments is the dominant type of aquifer in the upper half of the Region <br />(i.e. in Colorado, New Mexico, and part of Texas above Big Bend). These <br />deposits are accumulated along the mainstem and major tributary valleys and <br />in closed or partially closed basins called bolsons. Volcanic lavas, ash <br />flows, and breccias commonly are associated with the valley fill and serve as <br />aquitards; locally, however, the volcanic rock along with sandstone, <br />limestone, and gypsum also are aquifers. Generally, the alluvial and valley <br />fill aquifers are hydraulically connected to surface streams. <br /> <br />Consolidated sandstone, limestone, dolomite, and gypsum are the dominant <br />aquifers in the middle portion of the Region where alluvial aquifers are <br />either absent or of limited extent. Fracturing, faulting, and, particularly, <br />solution in the carbonate and evaporite aquifers have enhanced the normally <br />limited development of porosity and permeability. Some of these aquifers are <br />hydraulically connected to surface streams; most are at least partially <br />confined. <br /> <br />In the lower part of the Region, where the River crosses the Coastal <br />Plain and discharges into the Gulf of Mexico, alluvial and deltaic sand, <br />silt and clay deposits constitute the principal aquifer. This aquifer is <br />hydraulically connected to the Rio Grande. <br /> <br />Estimates of ground water occurrence in terms of available supplies <br />within each of the designated WAU's are listed in Table 4. These figures <br /> <br />r~ <br />>000 <br /> <br />23 <br />