The Regional Aquifer-System Analysis (RASA) Program was started in 1978 following a congressional mandate to develop quantitative appraisals of the major ground-water systems of the United States. The RASA Program represents a systematic effort to study a number of the Nation's most important aquifer systems, which in aggregate underlie much of the country and which represent an important component of the Nation's total water supply. In general, the boundaries of these studies are identified by the hydrologic extent of each system and accordingly transcend the political subdivisions to which investigations have often arbitrarily been limited in the past. The broad objective for each study is to assemble geologic, hydrologic, and geochemical information, to analyze and develop an understanding of the system, and to develop predictive capabilities that will contribute to the effective management of the system. The use of computer simulation is an important element of the RASA studies, both to develop an understanding of the natural, undisturbed hydrologic system and the changes brought about in it by human activities, and to provide a means of predicting the regional effects of future pumping or other stresses.The final interpretive results of the RASA Program are presented in a series of U.S. Geological Survey Professional Papers that describe the geology, hydrology, and geochemistry of each regional aquifer system. Each study within the RASA Program is assigned a single Professional Paper number, and where the volume of interpretive material warrants, separate topical chapters that consider the principal elements of the investigation may be published. The series of RASA interpretive reports begins with Professional Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available..
Volcanic rocks of Quaternary age, chiefly basalt, underlie much of the eastern Snake River Plain in southern Idaho and are a major source of water. Sedimentary rocks are a secondary source but supply most of the ground water in the western part of the plain. The economy of the Snake River Plain is based largely on irrigated agriculture, as attested by the more than 3 million acres of irrigated land. Ground water plays a vital role, both as a source of water to wells and as discharge from springs that sustain flow in the Snake River. Because surface water is totally appropriated in some years, ground water is needed for expanded irrigation development. There is uncertainty, however, about the effects increased withdrawals may have on groundwater levels, spring discharges, and consequently, flow in streams. The U.S. Geological Survey has begun a comprehensive study of the regional groundwater system in the Snake River Plain. The purpose of the study is to refine knowledge of the regional groundwater flow system, determine effects of conjunctive use of ground and surface water, and describe the water quality. Regional groundwater flow models will be used in the study to aid analysis. Hypotheses concerning the groundwater system will be tested, and the system's response to various ground-watermanagement alternatives will be evaluated. This report describes the objectives, approach, and plan of study, and establishes a time frame within which the work will be accomplished.
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