THE WORK OF RUNNING WATER. 171 



Lawrence River 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 hkewise the drowned ends of 

 river valleys (see figures. Chapter VI). If all parts of a drainage basin 

 sank equally, the velocities of the streams above the Hmit of drowning 

 would not be changed, for the gradients would remain the same as 

 before. The fact that a river's channel is below sea-level is not to be 

 taken as proof that the valley is drowned. Thus the bottom of the 

 channel of the Mississippi is as much as 100 feet below the level of 

 the Gulf, some 20 miles above New Orleans.* 



Differential movement. Warping. — Where a land surface on which 

 a river system is estabHshed suffers warping, some parts going up 

 and others down, the opposite movements being either absolute or 

 relative, various phenomena would result. This may be illustrated 

 by the accompanying diagrams (Figs. 161 and 162), where the profiles 

 of the streams are represented as warped from the positions represented 

 by the dotted lines, to the positions shown by the full lines. The 

 velocity will be accelerated below the points of differential elevation 

 (between a and 6, Fig. 161, and between a and h, and c and d, Fig. 162), 

 but checked above (above a, and between h and c, Fig. 162). Above an 

 elevation which notably checks its flow, a stream is ponded. If the 

 ponding is sHght, a marsh may develop above the obstruction; if 

 more considerable, a lake is formed. Lakes of this class are likely to 

 be short-lived, since the ponded waters are Hkely to soon overflow 

 and lower their outlet so as to drain the lake. The elevation which 

 ponds the stream may be great enough and rapid enough so that the 

 resulting lake finds an outlet by some course other than that originally 

 followed by the stream. Where a stream holds its course across an 

 uplift athwart its valley, either with or without ponding, it becomes 

 an antecedent stream (see p. 169), since it has a course assumed be- 

 fore the latest deformation of the crust and in apparent disregard 

 of present surface configuration. Thus the Columbia River holds 



of the United States, pp. 169-202; Hayes, the Southern Appalachians, op. cit., pp. 

 305-336; Hayes and Campbell, The Geomorphology of the Southern Appalachians, 

 Nat'l Geog. Mag., Vol. VI, pp. 63-126, and Hayes, Physiography of the Chattanooga 

 District, 19th Ann. Rep. U. S. Geol. Surv., Pt. II, pp. 1-58. 



1 This is the case at Davis and Lone Star. Capt Howell, Miss. Riv. Commission. 



