178 PHYSICAL GEOLOGY 



flood waters. Artificial embankments have been built upon 

 the natural levees of many rivers, in order to reclaim their 

 bottom lands. As deposition continues along the bed of the 

 river, such embankments must be built higher from time to 

 time in order to confine the stream. 



The low margins of many wide flood plains are marshy. 

 In such marshes, the dead leaves, twigs, and branches of 

 the swamp vegetation gather in the shallow water, along 



FIG. 186. Meanders of the upper Green River, Wyoming. (Baker.) 



with minor quantities of silt. This vegetal matter, preserved 

 by the water from complete decay, may be transformed 

 slowly into peat. Some coal beds are thought to represent 

 similar marshes which existed ages ago (pp. 378, 380). 



Many tributary streams on entering an aggraded valley 

 are prevented by the natural levees from uniting with the 

 main river at once, and flow greater or lesser distances down 

 valley before joining it at some point where it swings to their 

 side of the flood plain (Plate IX). 



Braided rivers. In some cases the waters of rapidly 

 depositing rivers flow in numerous channels which meet and 

 divide repeatedly. Deposition along the floor of a given 

 channel reduces its capacity. When the channel is presently 

 unable to hold all of its water, a part breaks over the side 

 and follows a new line. The new channel, becoming choked 

 like the old one, gives off branches which in turn divide. 

 The overflowing waters follow the lowest accessible lines of 



