LAND WATERS STREAMS 177 



Successive rising and sinking. Another peculiarity of valleys 

 and streams resulting from changes of level is illustrated by PI. XII. 

 The main valleys of this part of the coast were developed when the 

 land stood higher than now. Later, the subsidence of the coast 

 converted the lower ends of the valleys into bays or fiords. The 

 bays were then transformed into lakes by deposition at their 

 mouths. Subsequent rise of the land or depression of the sea 

 allowed the drainage from the old lagoons to cut across the deposits 

 which had converted the bays into lagoons. The result is an 

 older, wider valley above, succeeded by a younger one near the 

 debouchure. 



Differential movement. Warping. A land surface on which 

 a river system is established may suffer warping, some parts going 

 up and others down. Above an upwarp which notably checks its 

 flow, a stream is ponded. If a stream holds its course across a 

 notable uplift athwart its valley, it becomes an antecedent stream. 

 The Columbia River has been thought to hold its antecedent 

 course across areas which have been uplifted (differentially) hun- 

 dreds and even thousands of feet 1 . Some of the striking scenic 

 features of this noble valley are the result of these changes in the 

 country through which it flows. A lesser stream would have been 

 diverted, as many of its tributaries have been. Even its course 

 across the Cascade Range has been interpreted as antecedent. 



THE AGGRADATIONAL WORK OF RUNNING WATER 



We have seen that rivers carry mud, sand, gravel, etc., from 

 land to sea, and that their goal is the degradation of the land nearly 

 to the level of the sea. We have also seen that rivers do not always 

 carry the sediment derived from the land directly to the sea. It 

 is often dropped for a time on the land, perhaps to be picked up 

 and carried on again when the conditions for its transportation 

 are more favorable. We have now to inquire more particularly 

 into the causes and especially the results of deposition. 



Causes of Deposition 



When running water drops its load, or any part of it, it is gen- 

 erally because the current has lost something of its velocity. We 

 1 Russell. Rivers of North America, p. 279. 



