75 



The southern part of the Florida peninsula is far from the sources 

 of coastal plain sediment which has filled estuaries immediately to 

 the north. Locally derived sediments, combined with the results of 

 plant and animal activity, are the great estuarine modifiers in this 

 region. Mangrove swamps on the southwest coast and coral reefs on 

 the southeast (fig. IV.1.17) are typical coastal formations. 



Table IV.1.2 gives estimated total quantities of suspended sedi- 

 ments entering the estuarine zone and shows the kinds of sediments 

 typical of each region. The data leading to this table include the effects 

 of human activity as well as natural sedimentation. The most signifi- 

 cant thing about this table is the paucity of data leading to these esti- 

 mates. The sediments carried by only 26 of the rivers entering the 

 estuarine zone have been measured sufficiently well to permit even 

 these estimates (IV-1-2). 



The great volume of sediments carried by the Mississippi, as con- 

 trasted to the quantity carried by the Columbia, illustrates one of the 

 major differences between a river forming a delta and one not forming 

 a delta. The contrast between the sediment loads being carried by the 

 rivers of the middle Atlantic and Chesapeake regions and those of the 

 south Atlantic and gulf also illustrate why the evolution of drowned 

 river valleys has progressed farther in the latter regions. 



The two Pacific coast regions are striking in that rivers with drain- 

 age only from the coastal mountain ranges carry much greater sedi-* 

 ment loads than those which drain the interior ranges. 



CLIMATE 



Solar energy striking the earth sets up complex cycles of water arid 

 energy flow from the oceans to the sky and the land and back again. 

 That part of the energy cycle occurring in the atmosphere gives rise 

 to the various combinations of weather phenomena which make up lo- 

 cal climates. Land, sea, and sky are mutually dependent in produc- 

 ing specific climates, and the great ocean currents play their indirect 

 roles in modifying the climates of the estuarine zone in addition to 

 their direct effects discussed earlier. 



The annual distributions of temperature, precipitation, sunlight, 

 and prevailing winds as well as the total amounts of each are of the 

 greatest significance. Table IV.1.3 and figure IV.1.18 summarize the 

 major climate characteristics in the estuarine zone of the United 

 States. 



Precipitation may fall as rain, snow, or other forms of ice, depend- 

 ing on temperature ; the form of i^recipitation has not only local im- 

 pact, but also affects annual patterns of river flow in rivers draining 

 to the coast. There is a tendency for precipitation along the northern 

 Atlantic coast to be heaviest during the cooler months and for much 

 of it to fall as snow ; the Pacific coast, except for Alaska, has a similar 

 precipitation pattern with much less snowfall. The southern Atlantic, 

 Gulf, and Alaskan coasts receive their heaviest precipitation in the 

 summer and fall, as do Puerto Rico and the Virgin Islands. 



