Figure 7.4 



Average Annual Precipitation 



of our total reservoir storage capacity, or more than 

 the Mississippi River has discharged into the 

 Gulf of Mexico over the last 200 years. About half 

 of the country is underlain by rock material that 

 could yield at least 50 gallons per minute from 

 wells. 



Ground water also provides the base flow of 

 streams; in some regions, ground water flows pro- 

 vide streams with a continuity of flow that they 

 would not otherwise possess. The water supply infor- 

 mation presented in the section on surface water 

 includes considerable water that enters from ground 

 water aquifers. Part of this ground water resource 

 does not get into the surface water supply naturally, 

 and can be developed only by drilling. 



The Atlantic and Gulf Coastal Plains contain the 



largest reserve of ground water in the nation (fig. 

 7.6). Present pumpage is but a small fraction of the 

 supplies that could be developed. Even so, saltwater 

 encroachment along the Gulf and Atlantic coasts 

 is a limiting factor in ground water development. 



Another significant area for ground water potential 

 is the series of alluvial basins in the West. These 

 are alluvium-filled valleys that receive runoff recharge 

 from surrounding mountains. The surface is very dry, 

 but the alluvial deposits are usually very thick and 

 they now store the equivalent of centuries of charging. 

 In this area, conjunctive development of streamflow 

 and ground water in storage is becoming a necessity 

 because of heavy water use for irrigation and domestic 

 needs in large cities such as Los Angeles, Phoenix, 

 and Albuquerque. 



288 



