174 CONSERVATION OF GROUND WATER 



long-term climatic trends throughout the world; these trends 

 in the last century would result in decreasing net water sup- 

 plies, both of surface water and ground water. 



Measurements by the U.S. Coast and Geodetic Survey show 

 that there has been a gradual but progressive rise in sea level 

 along the Atlantic, Gulf, and Pacific Coasts of the United 

 States: slight in the first three decades of the century, and at 

 an increased rate since 1930. 16 It is not known to what extent 

 these changes may be due to changes in the relative position 

 of continental blocks and the ocean floors of the earth's crust. 

 However, a decrease in the water held on the continents in 

 streams, lakes, subsurface storage, or ice masses in arctic re- 

 gions would result in a corresponding increase in the water of 

 the oceans. The rise in sea level therefore is not in conflict 

 with other hydrologic evidence that in the past century the 

 trend has been toward decreasing net water supplies on the 

 continents. Indeed, the pronounced rise in ocean level be- 

 ginning about 1930 may reflect the widespread depletion of 

 surface-water and ground-water supplies in the drought of the 

 1930's. It is well known that during the glacial period when 

 vast quantities of ice were stored on the continents, the sea 

 level was several hundred feet lower than it is today. 



The records of water-level fluctuations in wells, shorter by 

 far than the records just cited of other phases of the hydrologic 

 cycle, are not yet long enough to ascertain the effect of these 

 suggested long-term climatic trends upon the storage in 

 ground-water reservoirs. And because the possibility of signifi- 

 cant long-term climatic effects cannot be eliminated, the ef- 

 fects of land-use practices cannot be ascertained satisfactorily 

 from these water-level records, even in many areas where there 

 has been little ground-water development. An example of the 

 difficulty of ascertaining the causes of a long-term progressive 

 decline in water supplies is afforded by Devils Lake, N.D., 

 which has shriveled in area from 140 square miles to less than 

 5 square miles in the past 80 years. 



is Marnier, H. A., Sea Level Changes along the Coasts of the United States 

 in Recent Years, Trans. Am. Geophys. Union, vol. 30, pp. 201-205, 1949. 



