136 RIVER DEPOSITS 



III. Fluviatile or River Deposits 



In a preceding chapter we learned that the power of a stream of 

 water to transport sediment depends upon its velocity, which, in its 

 turn, is determined by the slope of the ground and the volume of 

 water. Further, we discovered the very surprising fact that the 

 transporting power increases as the sixth power of the velocity 

 (T= V & ). It follows from this that a slight decrease in the swift- 

 ness of a stream will cause it to throw down the greater part of its 

 load of sediment, while a slight increase will cause it to carry off 

 what it had before deposited. Thus, great rivers, like the Missis- 

 sippi, which flow in soft, easily moved deposits, are preeminently 

 whimsical and treacherous. As the volume and velocity of the 

 stream are much subject to change, there will obviously be corre- 

 sponding changes in the scour and deposition at any given point, 

 but there are certain places where deposition is so constant that 

 extensive accumulations may be formed there. As we trace a river 

 downward from its source in a mountain region, we find that in 

 the upper stream, which is a torrent in swiftness, only large stones 

 remain at rest, everything else being swept along. Farther down 

 stream, as the slope of the bed diminishes, the coarse gravel is 

 thrown down, next the coarse sand is deposited, and in the lower 

 reaches of a river, which, like the Mississippi, flows over land that 

 has a very gentle slope, and is raised but little above the sea-level, 

 only the finest silt gathers on the bottom. The exact limits of the 

 different kinds of deposit will vary with the stage of water. 



At points where the velocity of the stream meets a constant 

 check, there will be constant deposition, and thus bars and islands 

 are built up in the channel, which will be permanent unless some 

 change of conditions is brought about. In the gravel and sand 

 banks the material is stratified, or divided more or less distinctly 

 into layers made up of similar fragments, each layer representing 

 uninterrupted deposition. A pause in deposition will produce a 

 division plane, which is, of course, all the more marked if the 

 deposited material be changed after the pause. In the sand bars 

 and gravel spits the up-stream side is a gentle slope, ending abruptly 



