miller] running water 79 



In answering this question he will solve a large percentage of 

 the problems of running water. 



Work of Erosion. 



Carrying and using sediment. How does a stream get sedi- 

 ment to carry? How does it carry it? What determines the 

 amount of sediment that it can carry? How does it utilize the 

 sediment in its work of erosion? What affects the rate of 

 erosion? These are typical problems bearing on this phase of 

 the subject for the student to solve. 



The study of a small brook will show (1) the current swing- 

 ing across the stream channel from the outside of one bend to 

 the outside of the next lower bend, (2) that in striking the bank 

 it cuts away small particles, and (3) that small particles are torn 

 loose from the bed of the stream. If there has been a recent 

 rain the muddy water will suggest that wash from the valley 

 sides is an important source of sediment. In arid localities the 

 wind may be as effective as the rain. In determining these facts 

 the student has noticed (1) that small particles are carried in 

 suspension, (2) larger particles of the same material are rolled 

 along the bottom, (3) that some are picked up, carried a short 

 distance and dropped again. A little experimentation shows him 

 that upward currents are produced by irregularities on the 

 stream's bed and that it is these upward currents that enable a 

 stream to pick up and keep up sediment heavier than water ; that 

 if the coarse material is powdered, so that each particle has a 

 large surface in proportion to its weight, the stream can carry 

 it easily. A little further study shows him that the amount 

 which a stream can carry depends upon its velocity, its volume, 

 and the kind of material — coarse vs. fine. It is this material 

 carried by the stream that enables it to erode. It is the tool with 

 which the stream works. Delicate plants may be found growing 

 on the bed of a stream which carries no sediment, yet if the 

 stream acquires a load those plants will be destroyed and erosion 

 of the bed and banks will begin. Combining these facts he con- 

 cludes that the rate of erosion will be affected by (1) the velocity 

 and (2) volume of the stream, (3) the amount and (4) kind of 

 sediment carried, and (5) the character of the material being 

 worn away. 



Forming gullies, ravines, valleys. — How does running water 

 develop a valley? The student has learned already how the 

 run-off from a rain collects in a depression and flow^s off as a 

 streamlet. He will find in the field many examples of the work 



