132 PHYSICAL GEOLOGY 



islands or being built into flood plains, from which it is re- 

 moved later, to be carried another stage on its journey to the 

 sea. Just as in the atmosphere (p. 87), material sinks more 

 slowly in proportion as the surface it exposes to the friction 

 of the water is great in relation to its weight. Sediment of a 

 given sort settles faster in salt water than in fresh water. 



The amount of the load. The amount of the load which 

 any given stream is moving depends upon (1) its velocity, 

 (2) its volume, and (3) the amount and nature of the ma- 

 terial to which it has access. Obviously, the swifter and 

 larger a stream, the more and larger the material it is capable 

 of moving. But many swift streams carry little material, 

 because little loose material is available, or because it is in 

 pieces too large to be moved. The Mississippi River carries 

 on the average over 1,000,000 tons of sediment per day into 

 the Gulf of Mexico. It has been calculated that it dis- 

 charges sediment sufficient to fill the basin of Lake Superior, 

 the largest lake in the world, with an area of 32,000 square 

 miles and an average depth of 550 feet, in about 66,000 

 years. It has been estimated, also, that the work performed 

 each year by the Missouri River in transporting sediment 

 is equivalent to 275,000,000,000 mile tons, or tons carried 

 one mile. The railroads of the United States carried 

 236,600,000,000 mile tons in 1907. 



Questions 



1. Why are many mountain streams clear? 



2. Why in many streams are narrows in the channel floored with 

 coarse material, while broad parts of the channel are lined with fine 

 sediment ? 



3. Do two streams of the same velocity and volume necessarily 

 carry the same amount of sediment ? Reasons ? 



4. Can a given stream carry a greater weight of coarse or of fine 

 material ? Why ? 



5. (1) Just why do streams carry more sediment after heavy 

 rains ? (2) Would the effect of a given rain in northern United States 

 be likely to be the same in January as in July ? (3) Make a general 

 statement concerning erosion, in keeping with the answer to (2). 



