Ch. 5— Wetland Trends • 89 



tions were the greatest factors associated with wet- 

 land alteration in Massachusetts between 1951 and 

 1977; however, development activities were respon- 

 sible for far more actual losses of wetlands. 



Also, changes in sea level, sedimentation, ero- 

 sion, subsidence, and overgrazing by birds or mam- 

 mals all have played a role in the loss of wetlands 

 in coastal Louisiana (2). Because of the many fac- 

 tors involved, it is difficult to determine the signif- 

 icance of losses from natural processes relative to 

 those from man's activities. However, there is evi- 

 dence that until artificial hydrologic changes were 

 made, such as containment of the Mississippi River 

 and canal dredging, there was a slow, long-term 

 net gain of land (including wetlands) in the region 

 (2). The dramatic reverse of these gains implies that 

 much of the loss is man-induced, resulting from a 

 combination of sediment starvation; canal construc- 



tion; saltwater intrusion from navigation channels; 

 and freshwater pumping for rice irrigation, marsh 

 impoundment, and cattle grazing (2). Losses re- 

 ported by NWTS are discussed in more detail be- 

 low, followed by a discussion of wetland trends 

 reported in regional case studies. 



The average annual net-loss rate for the Nation's 

 vegetated wetlands in the lower 48 States during 

 the 20-year period of NWTS was about 550,000 

 acres/yr, or about 0.5 percent of the Nation's wet- 

 lands each year. It must be recognized, however, 

 that the rate of loss is not uniform throughout the 

 country. For example, the Lower Mississippi Al- 

 luvicd Plain lost nearly 190,000 acres/yr, or about 

 1 .6 percent of the region's wetlands each year. The 

 Pacific mountains lost 19,000 acres/yr, but this also 

 represented about 1.6 percent of the region's wet- 

 lands lost each year. These two regions had loss 



Photo credit: OTA Staff. Joan Ham 



A combination of levee and canal construction, saltwater Intrusion from navigation channels, freshwater pumping for 

 rice irrigation, marsh impoundments, and cattle grazing have led to major wetland losses in coastal Louisiana 



