CHANGES OF LEVEL 107 



tin St. Lawrence is drowned up to Montreal, and the Hudson up 

 to Albany. If the drowned portion of the latter valley were not 

 .so narrow, it would be a bay. Delaware and Chesapeake bays, 

 as well as many smaller ones, both north and south, are likewise 

 tin- drowned ends of river valleys (Figs. 105 and 106). 



Successive rising and sinking. Another peculiarity of valleys 

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

 Fig. 2. The main valleys of this part of the coast were developed 

 \vlu-n the land stood higher than now. Later, the sinking of the 

 coast converted the lower ends of the valleys into bays. The 

 bays were then transformed into lakes or lagoons by deposition at 

 their mouths. Subsequent rise of the land or sinking of the sea al- 

 lowed the drainage from the lakes to cut across the deposits which had 

 converted the bays into lakes. 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 A lesser stream would have been 

 diverted, as many of its tributaries have been. 



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 to 

 base-level. We have seen also that rivers do not always carry their 

 sediment directly to the sea. In many cases it is dropped for a 

 time on land, perhaps to be picked up and carried on again when 

 conditions for its transportation are more favorable. We have now 

 to inquire more particularly into the causes and results of deposition. 



Causes of deposition. When running water drops its load, or 

 any part of it, it is generally because the current has lost velocity. 

 Decrease of gradient is the commonest cause of loss of velocity. 

 The loss may be (i) sudden, as when the water passes from a steep 

 slope to a gentle one, or into a body of standing water; or (2) slow, 

 as in following a valley whose gradient decreases gradually. We 



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



