69 



RtVER FLOW 



The estuarine zone is also shaped through erosion and sediment 

 transport by fresh water making its way to the sea. Along the coast- 

 lines of the Continental Shelf of the United States are streams and 

 rivers carrying water from land runoff to the sea. These waterways 

 range from the Mississippi River down to the tiniest stream trickling 

 across the sands of a beach. 



Figure IV. 1.12 illustrates typical seasonal variation in river flow 

 into the estuarine zones of the United States. Everywhere there is a 

 pronounced annual cycle ; peaking sharply in the sprmg in Alaska and 

 New England, peaking from early summer to early fall along the 

 Atlantic and gulf coasts, and reaching a maximum in late winter along 

 the Pacific coast. 



Annual cycles of river flow depend on the annual variation of tem- 

 perature as well as of precipitation, and the total volumes of water 

 and sediment moved reflect not only the total amount of precipitation, 

 but also the sizes and slopes of drainage basins and the types of soil 

 over which the rivers flow in their fall to the sea. 



All river flows begin as either rain, snow, or ice. While rain moves 

 almost immediately into the hydrologic system as ground water and 

 as surface runoff, snow and ice may remain for several months on the 

 ground until they melt under the warmer temperatures of spring. This 

 sudden influx of several months' precipitation into the hydrologic 

 system frequently results in severe erosion and flooding with heavy 

 transport of sediment into the estuarine zone. 



River basin drainages unaffected by winter f reezeup conditions, such 

 as most of those on the southeast Atlantic and gulf coast, also erode 

 and carry sediment loads, but their effects are distributed more equally 

 around the year. Coasts with low-lying drainage basins tend to have 

 marshes which trap sediments, reducing erosion in coastal areas. 



TABLE IV.1.1— RIVER FLOW IN THE ESTUARINE ZONE OF THE UNITED STATES 



Biophysical region 



Total 



Drainage freshwater 



area runoff 



(square (cubic feet 



miles) per second) 



Drainage Runoff 



area per per mile 



mile of of total 



ocean tidal 



coastline shoreline 



Major river basins (more than 

 1,000 square miles drainage) 



Total gaged 



drainage 



area 



Number (square 



of rivers miles) 



Average 



annual 



runoff 



(cubic 



feel per 



second) 



North Atlantic _. 40,700 72,000 



Middle Atlantic 69,700 106,000 



Chesapeake _- 66,500 79,800 



South Atlantic 149,500 154,000 



Caribbean 10,400 11,500 



Gulf(total) 1,704,000 799,000 



Excluding Mississippi 464,000 249,000 



Southwest Pacific 94,300 83,400 



Northwest Pacific (total) 314,000 368,000 



Excluding Columbia 56,000 133,000 



Alaska (total)____ 700,000 (i) 



Excluding Yukon 340,000 0) 



Pacific islands 6,710 0) 



Total (including Alaska and 



Pacific islands) 3,116,800 .2,000,000 



Total (excluding Alaska and 



Pacific islands) 2,410,100 1,568,700 



Total (excluding Alaska, Pacific 



islands, Mississippi River and 



Columbia River) 912,000 784,000 



30 16 



54 15 



15 



182 16 



7 3 



750 52 



274 19 



79 27 



469 77 



84 28 



47 



22 



6 _- 



124 



264 29 



106 



5 

 6 

 6 

 12 

 

 21 

 20 



16 

 15 

 



18,600 



35,300 



47, 100 



68,600 







1, 394, 000 



249, 000 



49, 000 



275, 000 



38, 000 



345, 000 



86, 000 







30, 900 



51, 400 



55,600 



70, 200 







706, OOO 



156,000 



30, 500 



293, 000 



98, 000 



351, 000 



176, 000 







83 2,232,600 1,588,600 

 67 1,978,600 1,237,600 



65 596,000 492,600 



1 Not available. 



Reference: The National Estuarine Inventory. 

 Data sources: U.S. Geological Survey, U.S. Coast and Geodetic Survey. 



