THE WORK OF STREAMS 119 



fact that the velocity and volume of rivers fluctuate often from day 

 to day, and the quantity of silt varies with the velocity. Moreover, 

 the material in solution in a cubic foot is greater at low than at high 

 water, since the proportion of spring water is then greater. It is 

 also difficult to measure the quantity of material rolled along the 

 bottom. It is believed, however, that notwithstanding these diffi- 

 culties, the estimate of the rate of denudation of the United States 

 of one foot in about 9000 years is accurate within 20 per cent. 



Deposition 



Causes of Deposition. — Streams bearing a full load will deposit 

 their sediment when their velocities are diminished. (1) A stream 

 flowing from a steep to a gentle gradient will deposit its coarser sedi- 

 ment. (2) When a stream emerging from a straight, narrow channel 

 flows into a wide, winding one, its current is diminished by friction 

 with its bottom and sides, and deposition may take place. (3) When 

 tributary streams with steep gradients flow into slow-moving main 

 streams, they may deposit a part of their load. (4) Since the velocity 

 of a stream increases with its volume, it is evident that, if the volume 

 is diminished in any way, as by seepage or evaporation, its ability 

 to carry sediment will be correspondingly decreased. Consequently, 

 rivers in arid regions are often depositing streams (p. 81), even when 

 they have steep gradients. (5) When slow-moving streams, carrying 

 much fine sediment, meet any obstruction, such as a stranded log or 

 a tree which has fallen from the bank, the slight check to the current 

 produced in this way may cause the formation of a sand bar or 

 island. Many of the islands of the lower Mississippi River began 

 as " snags." (6) When a stream reaches a body of still water, either 

 a large lake or the ocean, all of the sediment soon finds a resting 

 place. The goal of all sediment is the sea, but in its journey ocean- 

 ward it makes many halts, forming the alluvium of the river valley. 



Flood Plains. — Flood plains are formed by graded streams, as 

 a result of both lateral erosion and of deposition during overflow. 

 A broad, flat valley may be formed in this way. Since rivers nor- 

 mally first reach base level where they enter the sea, their flood plains 

 are usually widest there. The lower Mississippi flood plain (which 

 is, perhaps, more correctly described as a delta) is five to eight miles 

 wide and is bounded on the east by clay bluffs iop to 300 feet high, 

 and on the west side, as far as the Red River, by less prominent 



