VIII RIVERS AND LAKES 331 
to prevent rivers from following their natural 
law of oscillation. But rivers are very true to 
their own laws, and a change at any part is 
continued both upwards and downwards, so 
that a new oscillation in any place cuts its 
way through the whole plain of the river both 
above and below. 
The curves of the Mississippi are, for in- 
stance, for a considerable part of its course 
so regular that they are said. to have been 
used by the Indians as a measure of dis- 
tance. : 
If the country is flat a river gradually 
raises the level on each side, the water which 
overflows during floods being retarded by 
reeds, bushes, trees, and a thousand other 
obstacles, gradually deposits the solid matter 
which it contains, and thus raising the sur- 
face, becomes at length suspended, as it were, 
above the general level. When this elevation 
has reached a certain point, the river during 
some flood bursts its banks, and deserting its 
old bed takes a new course along the lowest 
accessible level. This then it gradually fills 
up, and so on; coming back from time to 
