86 



estuarine reg^ions of dissimilar Environmental characteristics (fig. 

 IV.1.19). 



CHARACTERISTICS OF THE BIOPHYSICAL REGIOTSTS 



North Atlantic estuarine region. — ^Canadian border to Cape Cod. 



Cool, fertile waters with a large tidal range strike a steep, indented 

 coast with deep water close inshore, but protected from the fnll force 

 of the ocean waves by a wide continental shelf. Moderate precipitation 

 with heavy snowfall leads to heavy spring river runoff which domi- 

 nates local circulation. Natural erosion and sedimentation are not 

 severe problems, and the evolution of drowned river valley estuaries 

 is in an early stage in this region. 



Middle Atlantic estuarine region. — Cape Cod to Cape Hatteras, 

 exclusive of Chesapeake Bay. 



A wide, gently sloping continental shelf with a smooth shoreline is 

 cut by the entrances of several major river systems carrying moderate 

 amounts of sediments. The same cool, 'fertile waters as in the North 

 Atlantic estuarine region wash this coastline but with a smaller tidal 

 range. The evolution of drowned river valleys into coastal marshes 

 is in a secondary stage in the larger estuarine systems, Avith sand spits 

 and barrier islands forming. 



OhesapeaTce Bay estuarine region. — All of the Chesapeake Bay sys- 

 tem from Cape Charles and Cape Henry inland. 



Isolation from direct oceanic effects in much of the greatly branched 

 system, the many subsystems with major river flows, and the reduced 

 concentration of the ocean salt throughout the bay and its tributaries 

 make this a unique estuarine system. This is a drowned river valley 

 with numerous similar tributary systems in various stages of evolution. 



South Atlantic estuarine region. — Cape Hatteras to Fort Lauder- 

 dale, Fla. (about 26° north latitude) . 



The generally wide continental shelf is brushed by the warm waters 

 of the well-defined Gulf Stream. The low-lying coastal plain termi- 

 nates in barrier islands and marshes in which large amounts of sedi- 

 ments are being continually deposited by moderate sized rivers fed 

 by heavy summer rainfall. Many of the drowned river valley estuaries 

 have evolved all the way to coastal marshes. Tidal ranges are small to 

 moderate, depending on local conditions. 



Caribhean estuarine region. — Fort. Lauderdale to Cape Romano (the 

 Florida peninsula south of 26° north latitude) , plus Puerto Eico and 

 the Virgin Islands. 



High temperatures, heavy rainfall, and warm ocean currents along 

 practically nonexistent continental shelves result in tropical estuarine 

 environments throughout this region. Coral reefs and mangrove 

 swamps are the typical coastal features of south Florida, while the 

 islands are mountainous and are fringed with coral reefs and beaches. 

 Tidal ranges are small. 



Gulf estuarine region. — Cape Romano to the Mexican border. 



A wide continental shelf extends all the way around this large em- 

 bayment, in which warm tropical waters are moved gently by weak 

 currents and small tidal ranges. Heavy rainfall over most of the area 

 brings sediments from the broad coastal plain to be deposited in the 

 estuarine zone. Most of the drowned river valleys have evolved to a 

 point intermediate between those of the Middle and South Atlantic 

 regions — ^barrier islands are extensive and have large shallow bays 

 behind them. 



