IV-22 



Annual cycles of river flow depend on the annual variation of 

 temperature as well as of precipitation, and the total volumes of 

 water and sediment moved reflect not only the total amount of pre- 

 cipitation, 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 hydro- 

 logic system frequently results in severe erosion and flooding with 

 heavy transport of sediment Into the estuarine zone. 



River basin drainages unaffected by winter freeze-up conditions, such 

 as most of those on the Southeast Atlantic and Gulf coasts, 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 1n 

 coastal areas. 



Table IV. 1. It shows the magnitude and distribution of river flows 

 entering the estuarine zone of the United States. Two river systems, 

 those of the Mississippi and the Columbia, drain 62 percent of the 

 land area of the conterminous United States and account for 50 

 percent of the land runoff passing through the estuarine zone. The 



