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<br />~:, <br />;:~'j <br />~~ <br /> <br />.",;, <br />'::1 <br />;'1 <br /> <br />of the Rio Grande. Irrigation return flows, municipal and industrial <br />wastewaters, and surface runoff also are discharged into the floodways and <br />are not returned to the Rio Grande mainstem. Because of the levees and the <br />floodways, only a narrow portion of the lower Rio Grande valley is included <br />in the assessment study area. <br /> <br />, ~~;! <br />, <br /> <br />" <br />" <br /> <br />Natural water resources conditions vary considerably throughout the <br />Region in accordance with climate, geology and topographic features. A <br />semiarid to arid climate with low humidity and erratic rainfall is charac- <br />teristic of the Region. Average annual precipitation ranges from thirty <br />inches in the high mountains and lower Rio Grande coastal plain to only eight <br />inches in the middle basin area where most of the precipitation occurs during <br />intense thunderstorms. In the upper and middle portions of the Region, run- <br />off is highest during the late spring and summer months when snowmelt and <br />thunderstorm activity are prevalent. In the lower valley, seasonal stream- <br />flow patterns tend to be more erratic as they are influenced not only by <br />upstream conditions, but also by periods of intense rainfall associated <br />with localized tropical storms and hurricanes; streamflows typically have <br />been highest during September and October. <br /> <br />,', <br /> <br />" <br /> <br />Throughout the Region, irrigation diversions and depletions of surface <br />waters greatly influence streamflow conditions. Reservoirs on the Rio Grande <br />mainstem in southern New Mexico (Elephant Butte and Caballo) and in Texas <br />below Big Bend (Amistad and Falcon) provide both flood control and conser- <br />vation storage, and these impoundments play key roles in determining flow <br />levels in the River. <br /> <br />The alluvial valley fill in the upper and middle mainstem reaches of the <br />Rio Grande Basin is the major fresh water aquifer. In the lower valley, the <br />Rio Grande delta is an erratic but locally productive aquifer; however, water <br />quality tends to be poor. With increased competitron for available surface <br />water supplies, ground water development has progressed considerably during <br />the past twenty years. Presently, almost half of the total water usage in <br />the Region comes from ground water sources. <br /> <br />r'. {' r ... ." '" <br />\.;.\)~.... ~ J <br /> <br />4 <br />