4 BULLETIN 180^ U. S. DEPARTMENT OF AGEICULTUKE. 



the stream bed. Where the velocity is greatest, generally midway 

 between the crest and the foot of the hill, the erosion is greatest. 



The modes of transportation as determined by experiment^ show 

 that some of the particles carried by a stream slide along, some roll, 

 and many make short leaps, the process being called "saltation." 

 Saltation itseK grades into suspension. In saltation the particles 

 are just on the verge of suspension; a slight difference in one of the 

 factors may cause the particles to remain in suspension. 



At the foot of the hill, where the slope becomes less steep, there 

 is a deposition of the coarser material. The gravel and sand are 

 deposited first, the silt next, and the clay particles last. The quan- 

 tity of coarse material carried by a stream is greatest in times of 

 floods, while during periods of normal flow the silt and clay particles 

 greatly predominate. The smaller particles are carried in suspension 

 until the plains are reached, or are transported to the sea. 



SOURCE AND QUANTITY OF MATERIAL. 



The source of the material transported by water is in the hills. 

 The streams may deposit it at one place, later to take it up and move 

 it to another; but this is only a part of the process of bringing the 

 soil from the uplands and depositing it in the lowlands. A detailed 

 discussion of the factors affecting sedimentary formations is given 

 by Mather. 2 



The quantity of soil material moved during one year by the streams 

 of the United States is very large. The great depths to which some 

 of our rivers have cut represent the ultimate effect of the removal 

 of soil material by the action of water. The Columbia River and 

 the Colorado River have cut gorges to depths of 2,000 and 5,000 

 feet, respectively. Under more favorable conditions, where the 

 soil is loose or iacoherent, the action may be even more marked. 

 The quantity of material carried in suspension to the sea by the 

 Mississippi River, which drains over one-third of the area of the 

 United States, has been variously estimated at 370,000,000 tons ^ to 

 680,000,000 tons * per annum. The total amount of soil material 

 carried to the sea annually by the rivers of the United States is esti- 

 mated by Dole and Stabler^ as 783,000,000 tons. Estimates of the 

 United States Geological Survey place the quantity of material 

 carried in suspension annually by the Hudson River at 240,000 tons; 

 by the Susquehanna, 240,000 tons; by the Missouri above Ruegg, 



1 C. K. Gilbert and E. C. Murphy, U. S. Geol. Survey, Prof. Paper 86 (1914); Jour. Wash. Acad. Sei., 4, 

 154 (1914). 



2 W. W. Mather, Physical Geography of the United States East of the Rocky Mountains; Am. Jovir. 

 Sci., 49, 14284 (1845). 



3 Humphrey and Abbott, Surveys of the Mississippi. . 



4 Dole and Stabler, U. S. G. S., Water Supply Paper 234, 84 (1909). 

 6 Bui. Geol. Soc. Am., 2, 130 (1894). 



