22 Artificial Aquatic Habitats 

 NAVIGATION POOLS 



In some of our larger rivers, locks and dams have been installed to 

 maintain water depths for navigation. Examples of such rivers are the 

 Mississippi, Ohio, and Illinois. Although these relatively shallow impound- 

 ments retain some current, the river rapids (important in the successful 

 spawning of some fishes such as the blue sucker) have been largely 

 eliminated. Navigation locks and dams are under the jurisdiction of the 

 U.S. Corps of Army Engineers, which builds new dams, maintaining the 

 present installations and also a river channel of a specified depth for the 

 movement of towboats and barges. 



Studies of the fishes living in the navigation pools of the upper 

 Mississippi indicate that they support both an extensive sport, and a 

 commercial fishery.^' -^ 



Low dams across a large river will result in the permanent flooding of 

 backwater areas adjacent to the river, except when the level of an up- 

 stream pool may be lowered to furnish water for navigation downstream. 

 When this occurs, backwater lakes may be drained quite rapidly, some- 

 times to the detriment of their fish, particularly in winter when these 

 backwaters are covered by thick ice. 



LATERAL-LEVEE RESERVOIRS 



Low lands in the flood plains of rivers are sometimes protected from the 

 river by levees; these low lands are pumped dry for agricultural uses. 

 However, when these areas are abandoned or reconverted into lakes with 

 the levees still intact, they become lateral-levee reservoirs. These shallow 

 reservoirs are very productive. 



Some of them are supplied with stone or concrete spillways to allow 

 the entrance of water from the adjacent river when it rises above the 

 spillway crest. Then, as the river level recedes, water flows out of the 

 lake until the spillway crest level is again reached. The water in such a 

 lateral-levee reservoir may fluctuate moderately to follow changing levels 

 in the river when the level of the reservoir is below the spillway crest 

 due to slow seepage through the levee. 



These lakes are quite turbid, due primarily to the action of wind.^^ 

 They are productive of hook-and-line fish,'^' -' and at the same time may 

 support a large commercial fishery for such river species as carp, buffalo, 

 freshwater drum, and channel catfish. 



MULTI-PURPOSE RESERVOIRS 



Large impoundments constructed by the federal government in many 

 parts of the United States have been justified on the basis of a combina- 



