IV-26 



Regional averages like those in Table IV. 1.1 are important in that 

 they show tnat there are general unifying criteria through which 

 lessons learned in one part of the national estuarine system can be 

 applied to other parts of the estuarine zone. 



SEDIMENTATION 



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

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

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

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

 became zones of mixinn, sediment deposition, and erosion where the 

 rivers and tidal currents met. These erosion and sedimentation 

 processes molded the estuarine zone into its present shape and con- 

 tinue to change it. 



The greatest changes occurred in those reqions where the surface 

 soils and clays on wide, gently sloping coastal plains rapidly 

 eroded from the land and came to rest in the estuarine zone or 

 farther out on tne continental shelf. Least change occurred where 

 coastal plains and continental shelves are narrow or consist mostly 

 of resistant rock. 



flnure 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 lew York to Texas abounds with examples of this 

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



