74 



been cut off from the sea by barrier islands, Mobile Bay illustrates the 

 initial formation of offshore bars, Matagorda Bay shows the full de- 

 velopment of barrier islands, and the marshes around the mouth of 

 the Satilla River represent the ultimate stage in the filling of an 

 estuary. 



The great ice sheet which once covered the estuarine zones of New 

 England, northwest Washington, and southeast Alaska scoured off 

 much of the readily erodable surface material in the coastal water- 

 sheds, thus, natural sedimentation has been a relatively minor factor 

 in modifying estuaries in these areas. Narragansett Bay and Puget 

 Sound, among many others, still maintain the great depths typical 

 of glacially formed embayments. 



Near the edge of the ice sheet, however, where the scoured-off earth 

 and rock carried along under and in the ice finally stopped as the 

 glaciers met the sea and melted, small, shallow bays formed in the 

 glacial debris and subsequently developed offshore sand spits and 

 barrier islands as illustrated by Moriches Bay (fig. IV.1.15) on the 

 south side of Long Island, which is formed of such glacial debris. 



Abundant sediment eroded from the coastal ranges along the Pacific 

 coast of the continental United States has nearly filled several estu- 

 aries, and wide tidal flats are common in the few estuaries along these 

 coasts (fig. IV.1.16). The Columbia, however, collects a proportion- 

 ately less suspended load of sediment as it comes down through the 

 less-erodable volcanic mountains and plateaus of the Pacific 

 Northwest, 



TABLE IV.1.2.- 



-CHARACTERISTICS OF SEDIMENT LOADS ENTERING AND SEDIMENTS RESIDENT IN THE 

 ESTUARINE ZONE 



Biophysical region 



Average annual suspended 

 sediment load 



Tons per 

 square mile 



Tons 



Number of 

 rivers 

 sampled Kinds of sediments in the estuarine zone 



North Atlantic _.. 0) 



Middle Atlantic. _ 220.0 



Chesapeake Bay 130.0 



South Atlantic. 389.0 



Caribbean (i) 



Gulf: 



(1) Excluding 124.0 



Mississippi. 



(2) Mississippi 244.0 



Pacific Southwest: 



Pacific slopes 398.0 



Central Valley 71.4 



Pacific Northwest: 



Pacific slopes 3,610.0 



Columbia 112.0 



Alaska (i) 



Pacific Islands 0) 



(') 



(0 



Glacial debris — Little input from rivers; clay 

 silt in deep areas; sand, gravel around edges. 



Silt, clay in deep areas; fine sand elsewhere. 

 Do. 



Fine sand predominates; organic silt in rivers 

 and swamps. 



Fine sand, except for coral reefs and mangroves. 



(1) Silts and clays with sands abundant around 

 margins only. 

 1 (2) Fine silts and clays, covered by fine sand 

 where delta-making is inactive. 



Fine sand in channels, silts and clays around 

 edges and on tidal flats. 



Do. 



Mixture of gravel, silt, and general glacial debris 

 on southeast, south. Extremely fine "flour" 

 on some parts of south and southwest. 



Sand, coral, slight amounts of silt near rivers. 



■ Not available. 



Reference: The National Estuarine Inventory. 

 Data sources: U.S. Geological Survey. 



