10 



(2) Ocecm Currents. — The major ocean currents passing near or im- 

 pinging on the continent exert strong, if subtle, effects on the estuarine 

 zone through their temperatures, which affect continental land tem- 

 peratures, and through their nutrients, which govern the nature and 

 productivity of offshore and estuarine fisheries. The cold Labrador 

 Current water from Maine to Virginia, warm Gulf Stream water along 

 the South Atlantic and gulf coasts, and the California Current along 

 the Pacific coast all have noticeable effects on coastal land and water 

 (fig.IV.1.1). _ 



(3) Ooastline^ Slope. — The configuration of the coastline itself, 

 even though subject to additional molding by the flow of rivers to the 

 sea, is closely related to the shape and structure of the Continental 

 Shelf. A wide continental shelf is generally associated with lowland 

 next to the coast, while a narrow shelf is associated with mountainous 

 terrain. These associations throughout the estuarine zone of the United 

 States have produced estuarine systems characteristic of particular 

 regions. Glaciation in New England, Washington, and Alaska ; old 

 mountain ranges and a sedimentary coastal plain from New Jersey to 

 Texas; and the young, steep rang:es of the Pacific coast are all con- 

 tinental features having different impacts on the estuarine zone. 



(4) River Floio. — The estuarine zone is also shaped through erosion 

 and sediment transport by fresh water making its way to the sea. All 

 along the coastlines are streams and rivers carrying water from land 

 runoff to the sea. These waterways range from the Mississippi River, 

 which drains 41 percent of the conterminous land mass of the United 

 States, down to tiny trickles across a beach. The volumes of water and 

 sediment moved reflect not onl;)^ the total amount of precipitation and 

 its annual cycle, but also the sizes and slopes of drainage basins ajid 

 the types of soil over which the rivers flow. 



(5) Sedimentation. — The general outlines of many estuaries, la- 

 goons, and embayments in the estuarine zone 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 became zones of mixing, sediment deposition, and erosion 

 where the rivers and tidal currents met. These erosions and sedimenta- 

 tion processes molded the estuarine zone into its present shape and 

 continue to change it. 



(6) Glimate. — Solar energy striking the earth sets up complex cycles 

 of water and energy flow from the oceans to the sky and the land and 

 back a^ain. That part of the energy cycle occurring in the atmosphere 

 gives rise to the various combinations of weather phenomena which 

 make up local climates. Land, sea, and sky are mutually dependent in 

 producing specific climates, and the great ocean currents play their 

 mdirect roles in modifying the climates of the estuarine zone. 



(Y) Tide. — The tide stands alone as a controlling force and symbol 

 of the estuarine environment. The combination of tidal action and 

 river flow gives rise to that unique phenomenon called an "estuarine 

 circulation pattern," which means the fresh water flows in one direc- 

 tion in one layer and the salt water flows in the opposite direction in 

 another layer with various degrees of mixing at the interface between 

 them. This type of circulation pattern is of great importance in some 

 of the estuaries along the Atlantic and Gulf coasts, and to a large ex- 

 tent governs the capacity of such estuaries to rid themselves of waste 

 materials. 



