SANDY BOTTOMED STREAMS IOI 



occasional aquatic insects, and hair-worms (Gordius). In some localities 

 bivalved mollusks (Sphaeridae) and leeches are numerous. 



Under primeval conditions beavers are associated with the pool for- 

 mation. They build dams which contribute to the deepening of the 

 water of the pools. For a good account of their habits see citation 996. 

 An old beaver dam is supposed to have turned the waters of the 

 DesPlaines out of the Chicago River and down the Chicago outlet. 



Characters of the formation: The mores of the pool formation are dis- 

 tinctly those of partially burying the body just beneath the surface of 

 the fine gravel and moving against the current. The few animals that 

 make cases usually use gravel or sand grains. A single caddis-worm 

 makes its case from small sticks such as commonly lodge in eddies. 

 Some of the fishes breeding in these situations cover their eggs (50). 

 Some fishes orient the body and swim upstream as a result of seeing the 

 bottom apparently move forward below as the fish floats down (94). 

 They behave the same if put into a trough with a glass bottom and the 

 trough drawn forward. Some orient also when their bodies rub against 

 the bottom when floating downstream. 



5. THE COMMUNITIES OF SANDY BOTTOMED STREAMS (SHIFTING BOTTOM 



SUB-FORMATIONS) 

 (Stations 22-26; Table XXIV) 



We have studied the upper course of the Black River, the upper 

 course of the Calumet River, and the Deep River, and two or three 

 tributaries of Lake Michigan near South Haven. The kind of material 

 eroded is of the greatest importance in determining the mores present in 

 a stream. The streams of the eastern part of our area are in till which 

 is sandy and their bottoms are sandy. This material is always slipping 

 and moving downstream. There are few large stones. The bottom is 

 not suitable for animals. The swift-water animals are almost entirely 

 absent. The forms present are those which belong to moderately swift 

 water. 



Composition and subdivisions. Such streams are poorly populated. 

 Their mores resemble those of the formations of the pools of streams 

 eroding coarse material, but the shifting is so much more general and the 

 species found so different, that it has been thought wise to separate the 

 two. In the Michigan streams there are in summer a few scattered 

 plants, which support a considerable number of insects; some of the 

 brook beetles (Parnidae) are found attached to them. The logs and 

 roots that happen to be in the water are important; they are the only 



