THE WORK OF RUNNING WATER. 



197 



toward the sides of the valley (Fig. 178), but it rarely continues to the 

 limiting bluffs. Since a stream with a well-developed flat frequently 

 shifts its course, old levees and abandoned channels lend variety to the 

 topography of the flood-plain. 



The topographic adjustment of tributaries.^ — The meandering 

 and shifting of a main stream affects its tributaries. If a main stream 

 swings against the bluff through which a tributary enters, the latter 

 brings its channel into topographic adjustment by lowering its end 

 to the level of the main. If now the main stream opposite the tribu- 

 tary swings to the other side of its valley, the tributary must make 



Fig. 187. — A general view of the Mississippi delta. 



its way across the flat with a very low gradient. Not only this, but 

 the flat of the main valley through which the tributary must flow is 

 likely to be aggraded by the main in time of flood. The result is that 

 the tributary stream becomes an aggrading stream at its debouchure, 

 and topographic adjustment is not established until it has filled up 

 the lower end of its valley to some notable extent. The filling of the 

 lower end of the tributary like\Vise affects the lower ends of its lower 

 tributaries. 



Chamberlin. Jour, of Geo!., Vol. X, pp. 747-754. 



