9O Types of Aquatic Environment 



typical of the first class ; the Dismal Swamp of eastern 

 Virginia, of the second; and over the northern lake 

 region of the continent there are innumerable examples 

 of the third. These types are rarely entirely isolated, 

 however, since both marsh and bog tend to be invaded 

 by tree growth at their margins. Such wet lands occupy 

 a superficial area larger by far than that covered by 

 lakes and rivers of every sort. They cover in all 

 probably more than a hundred million acres in the 

 United States; great swamp areas border the Gulf 

 of Mexico, the South Atlantic seaboard, and the lower 

 reaches of the Mississippi, and of its larger tributaries, 

 and partially overspread the lake regions of upper 

 Minnesota, Wisconsin, Michigan and Maine. In the 

 order of the areas of " swamp land" (officially so desig- 

 nated) within their borders the leading states are 

 Florida, Louisiana, Arkansas, Mississippi, Michigan, 

 Minnesota, Wisconsin and Maine. 



Swamps naturally occupy the shoal areas along the 

 shores of lakes and seas. Marine swamps below mean 

 tide occur as shoals covered with pliant eel-grass. 

 Above mean tide they are meadow-like areas located 

 behind protecting barrier reefs, or they are mangrove 

 thickets that fringe the shore line, boldly confronting 

 the waves. With these we are not here concerned. 

 Fresh -water marshes likewise occupy the shoals border- 

 ing the larger lakes, where protected from the waves by 

 the bars that mark the shore line. In smaller lakes, 

 where not stopped by wave action, they slowly invade 

 the shoaler waters, advancing with the filling of the 

 basin, and themselves aiding in the filling process. 



That erosion sometimes gives rise to lakes has 

 already been pointed out; much oftener it produces 

 marshes; for depositions of silt in the low reaches of 

 streams are much more likely to produce shoals than 

 deep water. 



