CHAPTER VI 



ANIMAL COMMUNITIES OF STREAMS 



I. Introduction 



The conditions in streams from headwaters to mouth have many 

 features in common with lakes, like Lake Michigan. It is therefore 

 appropriate that they follow the discussion of such a lake. The streams 

 belong to two drainage systems — the Mississippi and the Saint Lawrence. 

 All are tributary either to Lake Michigan or to the Illinois River. The 

 principal tributaries of the lake near Chicago are the Chicago River, the 

 Calumet River, Trail Creek, the Galien River, the St. Joseph River, 

 and the Black River. The principal tributaries of the Illinois River, 

 with which we are concerned, are the Fox River, the DesPlaines River, 

 the DuPage River, the Kankakee River, Salt Creek (111.), Hickory Creek. 



The factors of greatest importance in governing the distribution of 

 animals in streams are current and kind of bottom. They influence 

 carbon dioxide, light, oxygen content, vegetation, etc. 



These factors are controlled by age (physiographic), length of stream, 

 and elevation of source above the mouth, all of which are physiographic. 

 The typical stream begins as a gully and works its way into the land 

 (Fig. 68, p. 112). The importance of some of the factors is greater in 

 some stream stages than in others. For example, in the younger stages 

 (a) material eroded, (b) relation to ground water, and (c) slope of stream 

 bed play a more important role than they do in later stages. 



II. Communities of Streams 



I. CLASSIFICATION 



The classification of stream communities is based upon physio- 

 graphic history and physiographic conditions. In the early stages of 

 stream development there are two types to be distinguished: (a) the 

 communities of intermittent streams, and (b) spring-fed streams. As 

 soon as the intermittent stream cuts below the ground-water level, 

 it becomes much like the spring-fed stream. Permanent streams are 

 divided into brooks, swift and moderate, and rivers, sluggish and moder- 

 ate, with communities named accordingly. We undertake a discussion, 

 first, of the history of the communities of streams developing in materials 



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