THE BED-ROCK SURFACE AND THE CHANNELS. 345 
The third section on Plate G is introduced for the purpose of illustrating 
similar facts in the important district of Dutch Flat, Gold Run, and You Bet. 
This section, which is six and a half miles in length, is also drawn nearly 
parallel with the crest of the Sierra, and its position may easily be traced on 
the large map illustrating the position of the gravels in that district. The 
peculiarities of each of these regions illustrated in the three sections have 
been set forth with sufficient detail in the preceding pages. 
That the detrital material resulting from the wear of a river upon its rocky 
bed, should occupy lower and lower positions as the work of abrasion goes 
on, seems to be in the ordinary course of events. It is true, however, only 
to a limited extent. As long as the stream has sufficient power to remove 
the detritus which is formed along its course, or brought into it by its tribu- 
taries, so long it must deepen its bed; but when the channel becomes 
covered with abraded material 
gravel, sand, or mud — then, of course, all 
wear of the bottom ceases, and if the velocity of the stream slackens suffi- 
ciently, the height of its bottom will be raised. Thus the Lower Mississippi, 
like many other rivers, accumulates detritus along its course ; and wherever 
it is artificially confined within fixed limits by dikes or levees, this takes 
place so rapidly as to become a practical question of serious importance. 
Thus, some regions are thought to have sunk, because the rivers draining 
them run at higher levels than they formerly did; when in reality it is only 
that their channels have become choked with their own débris, which the 
current has no longer the power to carry away. 
A river remaining permanently of the same size, and not compelled arti- 
ficially to heap up débris along one narrow line, will change its course so as 
to allow its deposits to retain about the same level over the whole width of 
the valley in which it is enclosed. Let the water diminish in quantity, and 
the stream will continually occupy narrower areas, terraces being often 
formed, as they have been along so many streams in New England. 
The peculiarities which reveal themselves on comparison of the Pliocene 
and Recent drainage systems of the Sierra Nevada are, the great depth of 
the present caiions, the absence of débris from the slopes, and their extreme 
narrowness at the bottom. And the essential cause of these differences is, 
beyond question, the comparatively small quantity of water now being 
carried off from the region in question. Auxiliary conditions are, the steep- 
ness of the slope of the range and the peculiar climate prevailing there 
during the latest geological epoch. 
