CHAPTER IV 



THE WORK OF STREAMS 



It is difficult to over-emphasize the importance of streams, since 

 they carry off the excess of rainfall above evaporation, with the excep- 

 tion of the ground water which enters into chemical composition with 

 rocks or is discharged in underground courses directly to the seas 

 (p. 56). The quantity of water carried in streams is therefore 

 enormous. It has been estimated that the rivers of the world 

 annually discharge 6500 cubic miles of water; a volume which, if 

 spread over Massachusetts, would cover it three quarters of a mile 

 deep. The water of flooded streams is derived largely from rainfall, 

 while the chief source is spring water, when they are low. 



Factors in Stream Erosion 



Material Carried by Streams. — In walking up a small valley 

 one can readily discover the sources of the gravel and sand in the 

 stream bed, and of the mud which renders the water turbid. The 

 small particles which have been broken from the rocks of the banks 

 by the various agents of the weather, and the larger fragments which 

 have been loosened by frost are continually being carried down into 

 the bottom of the valley by gravity (hillside creep, p. 73) and washed 

 down by rains ; deposits of sand and clay through which the valley 

 is cut in places furnish an easy supply during floods ; the solid rock 

 of the valley sides, when undercut by the stream, falls into the water; 

 and some sediment is obtained from the bed over which the stream 

 flows. 



How the Sediment is Moved. — Streams accomplish their work of 

 removing this load of sediment (1) by pushing along the larger of 

 angular rocks, (2) by rolling the rounded and smaller pebbles, and 

 (3) by carrying in suspension the finer sand and clay, as well as such 

 thin, flat particles as mica flakes. This ability of running water to 

 carry fine particles in suspension is due to the fact that the smaller 

 the volume of an object, the larger in proportion is its surface. This 



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