PROBLEMS FROM LAND OCCUPANCY 207 



may be higher than the stream level, so that ground water is 

 discharged into the stream. 



The effects of river regulation are most marked on those 

 ground-water reservoirs which under natural conditions are 

 recharged chiefly during floods and at other times are insu- 

 lated from the stream by fine sediments in the channel. Obvi- 

 ously if the flood peaks are eliminated and the period of in- 

 sulation extended indefinitely, the replenishment to the 

 ground-water reservoir will be less than under natural condi- 

 tions. The maintenance of "pool" stages for navigation is espe- 

 cially likely to affect recharge of ground-water reservoirs ad- 

 versely, because these pools generally are silt traps, and the 

 stream bed under them may be made relatively impermeable. 



Adverse effects upon ground-water reservoirs should not be 

 considered as arguments against flood control, for the potential 

 damage from floods may far outweigh the value of ground- 

 water reservoirs along the stream. But the ground-water reser- 

 voirs do constitute an important resource, and comprehensive 

 plans for river-basin development should consider the effects 

 upon them of structures designed to regulate the flow of the 

 stream. In places where the only feasible solution to flood dan- 

 gers is a flood-control structure that will reduce the natural 

 recharge of ground water, alternate artificial means of replen- 

 ishment should be sought. 



LEVEES AND FLOOD WALLS 



The intimate relation between surface water and ground 

 water in watercourses has been shown by scientific research 

 at many places where the ground water has been developed 

 (pages 136-160). Efforts to restrict the surface water to certain 

 channels and thus prevent inundation of the adjacent flood 

 plain have also given proof of this intimate relationship. At 

 Lawrenceburg, Ind., a flood wall protects the town from flood- 

 waters of the Ohio River. But the river at flood stage also pro- 

 vides significant natural recharge to the ground-water reservoir 

 under the town. In order to prevent "inundation" from be- 



