73 



SEDIMENTATION 



The general outlines of the estuaries, lagoons, and embayments in 

 the estuarine zone of the United States were formed by erosion from 

 land runoff during the last ice age when sea levels were much lower 

 than they are now. As the sea level rose, the drowned river mouths be- 

 came zones of mixing, sediment deposition, and erosion where the 

 rivers and tidal currents met. These erosion and sedimentation proces- 

 ses molded the estuarine zone into its present shape and contmue to 

 change it. 



The greatest changes occurred in those regions where the surface soils 

 and clay on wide, gently sloping coastal plains rapidly eroded from 

 the land and came to rest in the estuarine zone or farther out on the 

 continental shelf. Least change occurred where coastal plains and con- 

 tinental shelves are narrow or consist mostly of resistant rock. 



Figure IV.1.13 illustrates the evolution of an estuary from a 

 drowned river valley to a coastal marsh. The estuarine zone of the 

 United States from New York to Texas abounds with examples of 

 this evolutionary process (fig. IV.1.14). Delaware Bay has not yet 



Figure IV.1.13 



STAGES IN ESTUARINE SYSTEM MODIFICATION 

 DUE TO SEDIMENTATION 



