18 CONSERVATION OF GROUND WATER 



water enters a stream only if it falls directly in the channel or 

 if it cannot get into the ground by infiltration or if it is dis- 

 charged into the stream from ground-water reservoirs. Even 

 after water has reached the stream, it may be lost by evapo- 

 transpiration or disappear by seepage into underlying ground- 

 water reservoirs. 



The great differences in ground-water resources and in 

 stream-flow characteristics in various parts of the country are 

 traced not only to differences in rates of rainfall and other 

 climatic factors but to differences in the materials in and be- 

 low the soil zone, through which the water may pass. In some 

 places the soil is like a blanket over the earth, absorbing the 

 rainfall even of intense storms until it can hold no more, so 

 that some starts moving downward into underlying rock mate- 

 rials; in other places bare rock or other impermeable material 

 or frozen or compacted ground cannot absorb the water even 

 of moderate storms or of gradual snow melting, and the sur- 

 plus may cause a stream to flood. 



The underlying rock materials may be very permeable and 

 form part of a ground-water reservoir capable of transmitting 

 large quantities of water for considerable distances, finally 

 discharging the water at a fairly constant rate into streams. 

 In other places, downward percolation may be stopped within 

 a few feet or even a few inches of the surface by an impermeable 

 layer, and the water collected above that layer may quickly 

 reappear in streams only a short distance away, perhaps soon 

 enough to contribute to floods in those streams. 



Inventory of Our Water Resources 



Systematic inventories are maintained of the reserves of 

 most of the nation's minerals of economic value, such as gold, 

 petroleum, coal, uranium, iron. We know from our continu- 

 ing discoveries of new oil fields or mineral deposits that this 

 inventory is by no means complete, and we hope for posterity's 

 sake that the reserves still unknown may be large. Neverthe- 

 less for each known ore body there are established methods 



