CHAPTER XIII. APPLICATION TO NATURAL STREAMS. 



INTRODUCTION. 



The flow of a river is a complex phenomenon. 

 The transportation of debris by it involves 

 intricate reactions. The quantity of debris 

 transported depends on a variety of conditions, 

 and these conditions interact one on another. 

 Direct observation of what takes place at the 

 base of the current is so difficult that the body 

 of information thus obtained is small. In the 

 work of the Berkeley laboratory the attempt 

 was made to study the influence of each con- 

 dition separately, and to that end all the con- 

 ditions were subjected to control. This in- 

 volved the substitution of the artificial for the 

 natural; and while the principles discovered are 

 such as must enter into the work of natural 

 streams, their combinations there are different 

 from the combinations of the laboratory. It is 

 the province of the present chapter to consider 

 the differences between the laboratory streams 

 and natural streams, and in view of those 

 differences the applicability of the laboratory 

 results to problems connected with natural 

 streams. 



FEATURES DISTINGUISHING NATURAL 

 STREAMS. 



KINDS OF STREAMS. 



Classification necessarily involves a purpose, 

 or point of view, and there are in general as 

 many scientific, or natural, or otherwise com- 

 mendable classifications as there are functions 

 to be subserved. The classification of streams 

 here given has no other purpose than to afford 

 a terminology convenient to the subject of 

 debris transportation. 



When the debris supplied to a stream is less 

 than its capacity the stream erodes its bed, and 

 if the condition is other than temporary the 

 current reaches bedrock. The dragging of the 

 load over the rock wears, or abrades, or cor- 

 rades it. When the supply of debris equals or 

 exceeds the capacity of the stream bedrock is 

 not reached by the current, but the stream bed 

 is constituted wholly of debris. Some streams 

 with beds of debris have channel walls of rock, 



which rigidly limit their width and otherwise 

 restrain their development. Most streams with 

 beds of d6bris ha,~ve one or both banks of pre- 

 viously deposited debris or alluvium, and these 

 streams are able to shift their courses by 

 eroding their banks. The several conditions 

 thus outlined will be indicated by speaking of 

 streams as corroding, or rock-walled, or alluvial. 

 In strictness, these terms apply to local phases 

 of stream habit rather than to entire streams. 

 Most rivers and many creeks are corrading 

 streams in parts of their courses and alluvial 

 in other parts. 



Whenever and wherever a stream's capacity 

 is overtaxed by the supply of debris brought 

 from points above a deposit is made, building 

 up the bed. If the supply is less than the 

 capacity, and if the bed is of debris, erosion 

 results. Through these processes streams 

 adjust their profiles to their supplies of d6bris. 

 The process of adjustment 'is called gradation; 

 a. stream which builds up its bed is said to 

 aggrade and one which reduces it is said to 

 degrade. 



An alluvial stream is usually an aggrading 

 stream also ; and when that is the case it is bor- 

 dered by an alluvial plain, called a flood plain, 

 over which the water spreads in time of flood. 



If the general slope descended by an alluvial 

 stream is relatively steep, its course is relatively 

 direct and the bends to right and left are of 

 small angular amount. If the general slope is 

 relatively gentle, the stream winds in an intri- 

 cate manner; part of its course may be in direc- 

 tions opposite to the general course, and some 

 of its curves may swing through 180 or more. 

 This distinction is embodied in the terms direct 

 alluvial stream and meandering stream. The 

 particular magnitude of general slope by which 

 the two classes are separated is greater for small 

 streams than for large. Because fineness is one 

 of the conditions determining the general slope 

 of an alluvial plain, and because the gentler 

 slopes go with the finer alluvium, it is true in 

 the main that meandering streams are associ- 

 ated with fine alluvium. 



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