122 • Wetlands: Their Use and Regulation 



bicides, fertilizers, and animal wastes to down- 

 stream wedand systems. The drainage may change 

 vegetation in adjacent areas; the runoff may cause 

 pollution of adjacent land and open water areas 

 (45). Drainage of wetlands for agricultural uses 

 results in the loss of organic material from the soils 

 due to oxidation. In some parts of the country, this 

 may lead to soil subsidence and increased hazards 

 of fire (9). For example, reclaimed peat-based agri- 

 cultural land in the Sacramento-San Joaquin Valley 

 has subsided through processes of compaction, ox- 

 idation, and wind loss and is now up to 20 ft below 

 sea level (17). 



In some instances, the creation of new habitats 

 has changed the behavior of migrating birds; rice 

 cultivation in southwest Louisiana and eastern 

 Texas has encouraged overwintering of waterfowl 

 that normally overwinter in eastern Louisiana 

 wetlands. Natural filling of drainage ditches may 

 cause an area to revert to a wetland, as occurred 

 on some former agricultural lands in New England 

 (56). 



Forested wetlands are also partially drained to 

 lower the water table and allow harvesting of the 

 forested land. After harvesting, an area may be 

 allowed to regenerate naturally or replanted as a 

 pine or hardwood plantation. Active forest manage- 

 ment can significantly increase the yield of wood 

 from the land but also decrease wildlife diversity 

 within forested plantations, depending on a number 

 of factors. Maki, et al. (31) report that the prac- 

 tice of "high grading," in which only desirable 

 large and shade-intolerant species are harvested, 

 produces extensive stands of shade-tolerant species 

 having less value as habitat. Large-scale drainage 

 and channelization could contribute to decreases 

 in resident invertebrate density and diversity (3). 

 If good management practices are not used, con- 

 structing drainage ditches and channelizing streams 

 in forested wetlands may also increase erosion and 

 sedimentation, which in turn affects wildlife habitat 

 and water quality in adjacent areas (7). In addi- 

 tion, the drainage of wetlands (14) may increase 

 the danger of floods in downstream areas. 



Drainage of wetlands in south Florida has been 

 cited as contributing to flooding, drought, oxida- 

 tion and subsidence of peat, saltwater intrusion, 

 reduction offish and wildlife resources, and water- 



quality problems in Lake Okeechobee — particularly 

 increases in nutrients, suspended solids, and pol- 

 lutants introduced from land uses to which wedands 

 are converted (9). 



Grazing of livestock in wedands has been a com- 

 mon practice because of the relatively rapid and 

 lush growth of some wetland plants, particularly 

 in arid regions. Some wetland vegetation has 

 proved more nutritious for livestock than upland 

 forage (38). Overgrazing leads to trampling and 

 compaction of soft wetland soils and the loss of 

 natural food sources for resident and migratory 

 wildlife. Moderate grazing, on the other hand, can 

 help maintain a wedand by encouraging the growth 

 of annuals and by setting back vegetative succes- 

 sion. 



Other agricultural practices, such as mowing, 

 disking, and burning wetland vegetation to con- 

 trol crop weeds and mosquitoes, are often carried 

 out in the playa basins of the southern Great Plains. 

 The adverse effects of these practices are temporary 

 and, like moderate grazing, can promote the growth 

 of annual wetland vegetation (38). However, such 

 practices conducted late in the growing season may 

 severely curtail winter cover for upland game birds 

 and waterfowl. 



Extensive Flooding 



Permanently inundating wetlands to certain 

 depths will eliminate wetland vegetation. Some- 

 times wedands are flooded to create ponds for grow- 

 ing aquatic organisms, particularly fish and shell- 

 fish. Extensive flooding of wetlands is also 

 associated with agricultural conversions of prairie 

 potholes; development of impoundments for munic- 

 ipal- and agricultural-water supply, hydropower, 

 and flood control in places such as New England, 

 the Lower Mississippi River Valley, the Lower Col- 

 orado River Valley, Nebraska, and Alaska; water- 

 fowl management in South Carolina; for mosquito 

 control in North Carolina; and aquaculture in Lou- 

 isiana. 



Culture ponds for crayfish and shrimp, for in- 

 stance, are prevalent in Louisiana. These ponds are 

 constructed by building dikes to raise water eleva- 

 tions. In addition to its direct effects on the wedand 



