88 PHYSICAL GEOLOGY 



When a stream has developed a slope which gives it just sufficient 

 velocity to carry its load, leaving no energy for deepening its bed, 

 it is said to be graded. If a stream has less sediment than it can 

 carry, it will remove material from its bed. If it is unable to trans- 

 port all the sediment brought to it, part of this will be left as a deposit, 

 the channel will be raised, and the gradient will be increased until the 

 stream becomes swift enough to carry away its load. When a stream 

 is at a temporary base level (p. 86) above a fall or rapid, there are 

 often smooth reaches where the stream is at grade. If the land 

 through which a river flows has not been elevated or depressed for a 

 long period of time, few falls will exist, and it will be at grade for long 

 stretches. If, however, a long-continued uplift or several uplifts 

 have occurred, even large rivers may be unable to erode their beds 

 to grade. Even when the last uplift was so remote that the large 

 rivers have been able to develop well-graded courses, the tributaries 

 may, and usually do, have a steep slope. 



Factors Affecting the Rate of Erosion. — The rate of erosion of a 

 stream depends upon a large number of factors, (i) Loosely com- 

 pacted rocks, or rocks with a soluble cement, are easily eroded. If, 

 for example, the grains of a sandstone are held together with lime, 

 the solution of the cement will cause the grains to fall apart and thus 

 render the work of the stream easier. (2) Rapid erosion is further 

 favored if the rock has numerous joints and is thin-bedded (p. 24). 

 Usually sedimentary rocks are more readily eroded than massive, 

 crystalline rocks (p. 330) such as granite. (3) The greater the velocity 

 of a stream, the greater, other conditions remaining the same, will be 

 the erosion. Since the velocity of a stream depends upon the volume 

 of water as well as upon the slope of its bed, the cutting power will 

 be greater during floods (p. 82). (4) Under any of the above con- 

 ditions erosion will be favored if the stream has sufficient sand and 

 gravel with which to cut its bed but not so much that a large part 

 of its energy is expended in carrying it. When the amount of sedi- 

 ment is increased without an increase in the volume of water (p. 86), 

 or when the quantity of sediment remains constant but the volume 

 of water decreases, erosion may cease and deposition take place. 

 Rapid erosion by abrasion requires some sediment, but not too much, 

 a steep slope, and a considerable volume of water. 



Scour and Fill. — A stream at flood may be deepening (degrading) 

 its channel where its velocity is great, at the same time that it is build- 

 ing up (aggrading) its flood plain where the velocity is slight. After 



