72 



Table IV.1.1 shows the magnitude and distribution or 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 Yukon has a 

 drainage area of about 360,000 square miles in Alaska and Canada, 

 about one-third that of the Mississippi, and ranks between the 

 Mississippi and Columbia as one of the three major river systems of 

 the Nation. 



The mouths of these three rivers form estuarine systems unique in 

 the estuarine zone of the United States. The tremendous volumes of 

 water discharged ^ by each of these is the dominating environmental 

 factor where the river enters the sea. 



The Mississippi and Yukon reach the ocean after passing through 

 many hundreds of miles of low-lying, easily erodable land. Immense 

 deltas formed at the mouth of each river as the great volumes of 

 suspended material accumulated in this passage Avere deposited at the 

 place where the river currents were slowed down by the sea (fig. 

 IV.1.12A and fig. IV.1.6). The Columbia collects relatively little sedi- 

 ment in its passage over rocky terrain, and is confined near its mouth 

 to a narrow channel where it has cut its way to the ocean through 

 coastal mountain ranges. The deposited sediments form only an 

 offshore bar which is continually cut away and reestablished by the 

 ocean swells and currents sweeping in over the narrow Continental 

 Shelf (fig. IV.1.12B). 



There are 80 other river basins in the United States having drainage 

 areas of over 1,000 square miles ; these, with the three river systems 

 already mentioned, account for land runoff from 85 percent of the 

 entire land area draining to the estuarine zone. Over half of these are 

 in the Gulf, Alaska, and South Atlantic biophysical regions. There 

 are none in the Caribbean and Pacific islands regions. 



The ratio of drainage basin size to miles of ocean coastline in each 

 region, as shown in table IY.1.1, is an index of the relative importance 

 of upland runoff conditions to the estuarine zone. In the North Atlan- 

 tic biophysical region, for example, runoff comes on the average only 

 from a distance of 30 miles inland. In the South Atlantic region, 

 however, runoff comes from an average distance of 182 miles, thus 

 indicating that large river basins are far more important to the 

 estuarine zone in the South Atlantic region than in the North Atlantic. 



The ratio of runoff to total miles of tidal shoreline is an index of 

 the importance of land runoff in estuarine stratification and water 

 movement patterns. A low ratio means there is little runoff in propor- 

 tion to the size of the estuarine zone, as in the Caribbean region, and 

 water stratification generally is not significant in this region; while 

 high ratios, as in the two Pacific regions, indicate high proportionate 

 land runoff and stratification-dominated estuaries. 



Regional averages like those in table IY.1.1 are important in that 

 they show that there are general unifying criteria through which les- 

 sons learned in one part of the national estuarine system can be applied 

 to other parts of the estuarine zone. 



1 In a Httle over an hour on an average day, the Mississippi discharges into the Gulf of 

 Mexico enough water to supply the domestic water needs of the entire present population 

 of the United States. 



