186 GEOLOGY. 



Flood-plains, chiefly the result of planation, but partly of aggra- 

 dation, are a normal feature of river valleys, after a certain stage of 

 development has been reached. This stage is that at which down- 

 ward erosion becomes shght in comparison with lateral erosion. It 

 follows that an alluvial plain normally begins its development where 

 the valley is first brought to grade, that is, in its lower course. As the 

 development of the valley goes on, the head of the flood-plain advances 

 up-stream, and at the same time its older parts become wider. 



Flood-plains due to alluviation only. — Exceptionally, an alluvial 

 plain is developed by deposition only. Thus if a stream becomes 

 overloaded while its valley is still narrow, as sometimes happens, 

 deposition follows, and, as aggradation proceeds, the narrow valley ac- 

 quires a progressively wider bottom 

 (Fig. 175). Wide valley plains are 

 sometimes developed in this way. 

 Flood plains developed wholly by 

 alluviation are sometimes formed 

 under conditions which are inde- 

 pendent of the stage of a valley's ^^«- 175. -Diagrammatic representation 

 , , i rrn . r- . 1 of a flood plain developed by alluvia- 



development. ihus it a stream sud- ^^^^ ^ i 



denly acquires an exceptional supply 



of detritus in its upper course, the development of an alluvial plain be- 

 gins immediately below the point of overloading. 



The overload might be acquired in various ways. (1) If a stream 

 taps another (piracy) which carries a large quantity of sediment, carry- 

 ing off both water and sediment to a channel with a lower gradient, 

 deposition may take place where, imder the earHer conditions, there 

 was none. (2) Again, when a stream cuts through a barrier near its 

 head waters, its velocity, and, therefore, its eroding power, may be 

 so increased in its upper course that sediment enough is acquired to 

 occasion deposition below, where none took place before. (3) In work- 

 ing back through formations of varying degrees of resistance, a stream's 

 head may presently reach a formation or a region which yields abundant 

 sediment, even though there was no especial barrier below. (4) If an 

 advancing glacier should reach the head waters of a stream, its dis- 

 charge to the stream would greatly increase the load of the latter, and, 

 although its volume would be augmented at the same time, deposition 

 might result. As a matter of fact, streams carrying glacial drainage 



