162 EARTH FEATURES AND THEIR MEANING 



with the bottom or sides of the channel abrade its surface so as 

 ever to deepen and widen the valley. This cutting accomplished 

 by partially suspended debris in rapidly moving currents of water is 

 known as corrasion and the stream is said to be incising its valley. 



As the current is checked upon the lower and flatter grades, 

 some of its load of sediment, and especially the coarser portion, 

 will be deposited and so partially fill in the channel. A nice 

 balance is thus established between degradation and the con- 

 trasted process known as aggradation. The older the river valley 

 the flatter become the grades at any section of its course, and 

 thus the point which separates the lower zone of aggradation 

 from the upper one of degradation moves steadily upstream with 

 the lapse of time. 



The accordance of tributary valleys. It is a consequence of 

 the great sensitiveness of stream corrasion to current velocity 

 that no side stream may enter the trunk valley at a level above 

 that of the main stream the tributary streams enter the trunk 

 stream accordantly. Each has carved its own valley, and any 

 abrupt increase in gradient of the side streams near where they 

 enter the main stream would have increased the local corrasion 

 at an accelerated rate and so have cut down the channel to the 

 level of the trunk stream. 



The grading of the flood plain. All rivers are subject to 

 seasonal variations in the volume of their waters. Where there 

 are wet and dry seasons these differences are greatest, and for a 

 large part of the year the valleys in such regions may be empty 

 of water, and are in fact often utilized for thoroughfares. In the 

 temperate climates of middle latitudes rivers are generally flooded 

 in the spring when the winter snows are melted, though they 

 may dwindle to comparatively small streams during the late 

 summer. In the upper reaches of the river the current velocities 

 are such that the usual river channel may carry all the water of 

 flood time ; but lower down and in the zone of aggradation, where 

 the current has been checked, the level of the water rises in flood 

 above the banks of its usual channel and spreads over the sur- 

 rounding lowlands. As a deposit of sediment is spread upon the 

 surface, the succession of the annual deposits from this source 

 raises the general level as a broad floor described as the flood plain 

 of the river. 



