CHAPTER XIII 

 BROOK, CREEK, AND RIVER 



HERE are a number of factors influen- 

 cing the character of the animal and plant 

 life of the stream, though most of them 

 act as indirect determiners affecting the 

 quantity and character of the gases con- 

 tained in the water, its acidity or alka- 

 linity, the temperature or rate of flow 

 of the stream, which are the principal 

 immediate causes of variation in the stream fauna and flora. 



If the sources of the water supply are such that the stream 

 flows intermittently, drying up to form a succession of stagnant 

 pools when the rains cease usually in midsummer but running 

 as a brook or creek during the spring freshets or the fall rains, 

 the life it contains will be quite different from that of the steadily 

 flowing streams. A spring-fed brook, because of its low temper- 

 ature, will support a population quite unlike that of the stream 

 that emanates from pond or swamp. A stream whose waters 

 are distinctly acid from abundant decomposing organic matter 

 or from factory waste will have quite a different fauna and flora 

 from the usual alkaline stream of this area. The population 

 of the Illinois River has suffered very marked change in recent 

 years because of the increase in decomposing organic matter 

 brought to it by the Chicago Drainage Canal. Thorn Creek 

 has largely lost its normal inhabitants on account of factory 

 waste, and a new set of organisms have come in. 



The stream from source to mouth shows a succession of 

 species, those of the small brook giving place to others that appear 

 as the stream widens and deepens. So we may speak of the 

 headwaters or brook society, the midcourse or creek society, 



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