OUR WATER SUPPLY— ME INZER 303 



friction of the rock interstices. Underground reservoirs of some sort 

 are almost universally present and are chiefly responsible for the 

 sustained flow of streams. The ground-water run-off carried by the 

 streams is relatively constant as compared with the very erratic un- 

 controlled direct run-off, but it is nevertheless sensitive to various 

 weather conditions and is generally greatly reduced by severe drought. 

 Streams differ greatly in the quantity and fluctuation of their ground- 

 water run-off according to the geology and other natural conditions 

 of the drainage basins. A subject that has received little investigation 

 but is of much scientific and practical interest is the relation of geology 

 to stream flow. 



The underground reservoirs function lil^e artificial reservoirs with 

 controlled outlets only when they are tapped by wells that extend 

 considerably below the water table. Shallow wells that merely skim 

 off ground water from the top of the zone of saturation are likely to 

 fail when the water table is lowered by drought, but the wells that 

 extend deeper into the water-bearing formations and have access to 

 their great stores of water are not appreciably affected by drought. 

 Reports of failure of such wells are commonly due to mechanical 

 defects in the wells or pumps, or to attempts made in times of drought 

 to increase the rate of pumping beyond the normal capacities of the 

 wells. By drawing water from wells in proper amounts the storage 

 facilities of the underground reservoirs are utilized and ground-water 

 recharge is increased. 



Yield of the artesian reservoirs. — A problem of great practical sig- 

 nificance relates to the perennial yield of the underground reservoirs. 

 To what extent is the water that is annually being drawn from pumped 

 or flowing wells derived from annual recharge and to what extent is it 

 taken out of storage, with the prospect of ultimate serious depletion? 

 From which of the water-bearing formations can additional perennial 

 water supplies be developed and where can these developments be 

 made? These questions are more intricate for the artesian forma- 

 tions, which are under confining covers, than for the water-bearing 

 formations that have water-table conditions and hence have their 

 wells in or near their intake areas. They are also more intricate for 

 the extensive artesian sands and sandstones, which transmit their 

 water through small intergranular interstices and exhibit considerable 

 volume elasticity, than for the artesian limestones and lava rocks, 

 which have much larger water conduits and are more rigid. 



Among the large artesian sandstones of the United States are the 

 Cambrian sandstones of the interior, the St. Peter sandstone, the 

 Dakota sandstone, and the series of thick sands or sandstones of the 

 Atlantic and Gulf Coastal Plain. In these sandstones centuries may 

 be required for water to percolate from the intake areas to the localities 

 of the wells. The total quantities of water that they hold in storage 



