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 sucha 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. Jospeh 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 (Ill.), 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, (6) relation to ground water, and (c) slope of stream 
bed play a more important réle than they do in later stages. 
Il. 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 (0) 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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