138 CONSERVATION OF GROUND WATER 



ervoir. Underflow is the only method of movement in certain 

 reaches of some streams, where the surface channel is dry. Be- 

 cause of the low velocity the quantity that moves by underflow 

 in most watercourses is very small in comparison with the sur- 

 face flow and is commonly neglected in statistics concerning 

 outflow from drainasre basins. On the other hand, the storage 

 in the ground-water reservoir of a watercourse may be large 

 and in many places is several times the quantity in the stream 

 even at flood stagres. 



Development of the ground water in a watercourse is not 

 limited by the replenishment from precipitation in a restricted 

 area, as is the case in most ground-water reservoirs close to or 

 beyond the limit of sustained perennial yield. Nor is it limited 

 by the capabilities of an aquifer to transmit water long dis- 

 tances from points of replenishment, because the stream does 

 the long-distance hauling. The basic factor limiting develop- 

 ment at any point is the transmissivity of the rock materials 

 between the stream and the well. Along the Nation's perennial 

 streams are many places where the materials are permeable 

 enough to yield large quantities of water to wells, and where 

 the water withdrawn is replaced by infiltration from the 

 stream. There are also many places where conditions are un- 

 favorable for river infiltration. Assuming that hydrologic con- 

 ditions are favorable for wells dependent upon river infiltra- 

 tion, such developments rely upon the stream as the ultimate 

 source of water. They may be unsuccessful if the water of the 

 stream is unsuitable for use or if it is unavailable for use be- 

 cause of prior downstream rights. 



WHERE THE SURFACE WATER IS USABLE 



Practically every state has some wells which obtain water 

 from its watercourses, and in some states there are large de- 

 velopments for public supply, irrigation, or industry. The 

 most productive wells in the industrialized Northeast are 

 along watercourses, and watercourses are the principal pro- 

 ducers of ground water in New England, upstate New York, 

 Pennsylvania, Ohio, West Virginia, Kentucky, and Indiana. 



