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INDIANA UNIVERSITY STUDIES 



work before it; that is, there is still a great amount of upland. 

 The streams in this case are small, usually straight, swift, heavily 

 loaded with sediment, and characterized by falls and rapids. A 

 stream with these characteristics is termed a young stream. He may 

 see stages in which the work is half completed; that is, in maturity, 

 in which the plain is so cut up that it is all ridges and valleys. 

 He may see stages in which the work is almost finished; that is, 

 in old age, in which the valleys are wide; the streams have many 

 meanders; a few monadnocks rise above the general relief; and the 

 stream is sluggish, and is building up its lower course instead of 

 lowering it. These, in a few words, are the charactertistics of 

 streams in the three stages of youth, maturity and old age. 



If the general leveling of the land is the function of a stream, 

 then we must next see how and in what manner it does this. As 

 the rain falls it beats on the ground and gathers particles of soil; 

 then, uniting into small rivulets, flows away in response to the 

 force of gravity. These little rills, turbid with sediment held in 

 suspension, unite into brooks, and these in turn combine to form 

 larger streams, which are also turbid. The particles held in sus- 

 pension have a tendency to fall to the bottom, but are kept up 

 by the various upward currents that are to be found in flowing 

 water, due to the unevenness of the bed of the streams, or to rocks 

 or other debris on the bottom. The sediment may rest on the bot- 

 tom for a time, but it will be gathered up and carried on down stream 

 and will finally arrive at its resting place in the ocean. 



Not only is the sediment gathered up by the little rills, but 

 the main stream is constantly widening and often deepening its 

 channel. This process also furnishes another source for the der- 

 ivation of sediment. For instance, the Mississippi River car- 

 ries into the Gulf more sediment than the tributaries bring into 

 the main stream. (Dole and Stabler, 'Water Supply Paper, 234.') 



The ability of a stream to carry sediment depends upon the 

 velocity, the volume, the nature of the material to be carried, and 

 the presence of upward and cross currents. Any one who has 

 observed a stream knows that the velocity is not continuously 

 the same, and that the velocity is less at the sides than at the middle, 

 and less on the bottom than on the surface. The thread of swiftest 

 flow is ordinarily in the center of the stream and about one-third 

 of the distance from the surface to the bottom. (I. C. Russell, 

 'Rivers of North America.') The bottom of the current is held 

 back by the friction on the bed, and the surface by the friction of 

 the air. If the stream is heavily loaded, the highest per cent of 



