208 CONSERVATION OF GROUND WATER 



low, the protection of Lawrenceburg is invested partly in. 162 

 wells. These operate without pumping when sufficient head 

 is developed during floods. The wells, located along the toe of 

 the protection embankment to assure its stability., have served 

 this purpose well during prolonged periods of high water, 

 discharging into a collector main by free flow. However, dur- 

 ing the high water of the spring of 1949, ground water rose 

 high enough to cause two minor sand boils and several areas 

 of seepage at a considerable distance from the embankment. 

 Neutralizing measures were not required at that time. 



Levees and flood walls must of necessity restrict not only the 

 water but the sediment of the stream to certain channels, and 

 the protected flood plain ceases to receive the sediment which 

 under natural conditions would have been dropped upon it. 

 The sediment dropped in the channel necessarily raises the 

 stage of the river for equivalent flows, necessitating higher 

 levees. The inevitable result along many of the larger rivers 

 has been that the stream is now flowing at elevations appreci- 

 ably higher than under natural conditions, and at times ap- 

 preciably higher than the adjacent flood plain. As an evidence 

 of the close relation between the stream and the ground water, 

 the water table has risen rather generally under the areas out- 

 side the levees, and thousands of acres have been waterlogged. 



'OCT 



Navigation Channels 



The dredging of navigation channels has contributed to 

 the deterioration of some ground-water reservoirs along the 

 Atlantic Coast, by facilitating the movement of salt water into 

 them. At Runyon and Cape May, N.J., and at Lewes, Del., 

 pumping from wells has lowered the water table below sea 

 level in shallow aquifers and has thus produced gradients fa- 

 vorable for movement of sea water from the coast. But at each 

 place the aquifers were evidently protected from the sea water 

 by a relatively impermeable clay blanket. Where this clay 

 was removed by dredging for navigation, the ocean water has 

 been able to enter the aquifer, and notable increases in salin- 



