High and Low Water 



87 



This accounts for the continual restocking of a stream 

 whose waters are swifter than the swimming of the 

 animals found in the open channels. In these more or 

 less shoreward places they breed and renew the supply. 

 Except in a stream whose waters run a long course sea- 

 ward, allowing an ample time for breeding, there is 

 little indigenous free-swimming population. 



Fig. 25. Annually inundated bulrush-covered flood-plain at the mouth of Fall 

 Creek, Ithaca, N. Y., in 1908. Clear growth of Scirpus fltiviatilis and a 

 drowned elm tree. The Cornell University Biological Field Station at 

 extreme right. West Hill in the distance. 



High a?id Low Water — Inconstancy is a leading char- 

 acteristic of river environment, and this has its chief 

 cause in the bestowal of the rain. Streams fed mainly 

 by springs, lakes, and reservoirs are relatively constant; 

 but nearly all water courses are subject to overflow; 

 their channels are not large enough to carry flood 

 waters, so these overspread the adjacent bottomlands. 

 Every change of level modifies the environment by 



