RIVER EROSION Tat 
water may dissolve with some rapidity, particularly if 
it is flowing over a relatively soluble rock, like a lime- 
stone; but even a granite would be slightly dissolved 
by ordinary river water. 
Even when conditions are favorable, ie action of 
solution is shght when compared with the mechanical 
work of water. This 1s well illustrated in the Niagara 
River where it flows out of Lake Erie. Here it has 
almost no valley, although in a part of its course it 
flows over an easily dissolved limestone; but in the 
same period of time, below the falls, a gorge has been 
carved out by mechanical action. As the river leaves 
Lake Erie, it is clear and free from sediment, and must 
work by solution alone; that it is achieving something 
is shown by the roughened surface of the limestone 
blocks just above the falls; but how slight this action 
is, may be understood from the fact that the deepening 
of the valley has been almost imperceptible. 
An analysis of any river water, will show much 
mineral matter in solution; but it is not to be sup- 
posed that the river has taken all of this from its 
channel. Most of it has come from springs and other 
sources of underground water. Therefore, while not 
coming from the channel proper, it is derived from 
the area which the stream drains, and every bit that is 
thus removed, serves to widen or deepen the river 
valley. That portion which is taken from the channel, 
