IV-24 



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 throuqh 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 tMs passage were deposited at the 

 place where the river currents were slowed down by the sea (Fioure 

 IV. 1,124 and Fiaure IV.l.fi). The Columbia collects relatively 

 little sediment in its passaoe over rocky terrain, and is confined 

 near its nouth to a narrow channel where it has cut its way to the 

 ocean throunh coastal mountain ranqes. The deposited sediments 

 form only an offshore bar which is continually cut away and re- 

 established by the ocean swells and currents sweeping in over the 

 narrow continental shelf (Fiqure IV. U2b). 



*In a little over an hour on an averaqe day, the Mississippi 

 discharges Into the Gulf of Mexico enouqh water to supply the 

 domestic water needs of the entire present population of the 

 United States. 



