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 debris, 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 debris 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 canons, the absence of debris 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. 



