THE WORK OF STREAMS 135 



fastest, other things being equal, when carrying a partial load, 

 so that many blows are delivered, but not so many that all are 

 weak. (What qualities should render rock fragments most 

 efficient tools for stream corrasion? Would tools possessing 

 these qualities long retain them all? Why?) 



Graded streams. When the gradient of a stream is just 

 steep enough to give it the velocity necessary to wash forward 

 the sediment brought to it from the tributary slopes, it is said 

 to be at grade. If it is able to transport more than is delivered, 

 it removes material from its bed until it comes to grade at a 

 lower and gentler slope. If it is unable to transport all that 

 is delivered, part of the load is left as a deposit. By this 

 means the channel is raised and the gradient becomes steeper 

 gradually, until in time the stream grows swift enough to carry 

 away the sediment brought to it. 



Rate of land reduction by stream erosion. Estimates 

 have been made of the rate at which certain river systems are 

 degrading their basins. This may be done as follows : The 

 width, average depth, and mean velocity of the main river 

 at its mouth may be determined at different times by measure- 

 ments, and from these data the average volume of water dis- 

 charged per year may be calculated. The average amount 

 of material contained in a cubic foot of the water, both in solid 

 form and in solution, may also be learned by examination of 

 numerous samples. Knowing the average amount of sedi- 

 ment in each cubic foot of the water, and the average number 

 of cubic feet discharged in a year, the total amount of sediment 

 delivered at the mouth of the river may be computed readily. 

 Finally, the area of the drainage basin being known, one may 

 determine to what uniform depth the sediment removed 

 yearly would cover it. The result indicates the average rate 

 per year at which the drainage basin is being degraded. By 

 this general method it has been estimated, for example, that 

 the Mississippi Basin is being lowered mechanically at the 

 average rate of one foot in about 5000 years, and when the 

 amount removed in solution is considered also, one foot in 3500 



