﻿168 J. LeConte — Elevation of the Sierra Nevada. 



Kiver-beds are most important and accurate indicators of 

 crust-movements. On the margins of continents coast lines are 

 our indicators; in the interior, where this fails us, we use river- 

 beds and find them equally valuable. This fact was first 

 distinctly brought out and its importance shown by Powell* 

 and afterwards fully elaborated by Gilbert in his monograph on 

 " Henry Mountains " and by Dutton in his monograph on the 

 " Grand Canon." It is undoubtedly a most fertile principle 

 and of very wide application in Geology. I give here only a 

 very brief statement of the principle as developed by these 

 geologists in order that its application to different regions and 

 especially to the case in hand may be more readily followed. 



A river seeks ever to find its base-level, i. e., the level at which 

 the tendencies to the opposite processes of erosion and sedimenta- 

 tion balance each other. When this is reached the river neither 

 cuts nor builds up. Suppose then, that a river in its lower 

 course, has reached or nearly reached its base-level. If now 

 the land rises the river immediately begins to cut its bed to 

 lower and lower level until it again reaches its base-level. If 

 on the contrary the land sinks the river will build up by sedi- 

 mentation until it again finds the level of equilibrium. In the 

 Plateau region, for example, we have the singular phenomenon 

 of a river system running far below the general level of the 

 country ; in Holland, on the contrary, we have the converse 

 condition of a river system running above the general level of 

 the country : — in the one case below the feet, in the other above 

 the heads of the people. But both cases come under the same 

 general principle of rivers seeking their base-level. In the one 

 case they seek it by erosion, because the country is rising ; in 

 the other by sedimentation, because the country is sinking. 



But the above statement requires some modification. In a 

 steady crust a stream never completely reaches its base-level in 

 any part because the base-level is constantly changing. The 

 position of the base-level of a stream is determined by the rela- 

 tion of the velocity of the current to the freight of sedimentf 

 and the freight of sediment depends on the slope of the upper 

 tributaries. These upper tributaries are always above their 

 base-levels and therefore always cutting and lowering their 

 slopes. But the lowering of the slopes of the upper parts di- 

 minishes the freight of sediments and therefore lowers the base- 



.* Exploration of the Colorado River, p. 203 and seq. 



|Ina river bearing sediment, the current-energy is consumed partly in carry- 

 ing and partly in eroding. As the quantity of sediment increases, more and more 

 of the current energy is consumed in carrrying and less and less is left over for 

 erosion, until finally all is thus consumed and the river begins to build by deposit. 

 Thus flooded rivers with rapid current will often build up by deposit, because 

 overloaded, and then afterwards, even with much less current will again scour 

 out their channels. 



