io6 



ANIMAL COMMUNITIES OF STREAMS 



We have had but little opportunity to study the swift-water forma- 

 tion during floods, though some of the riffles in Butterfield Creek have 

 been studied when the stream was bank full, but no marked changes 

 were noted. It is obvious that the extreme floods which move large 

 stones crush large numbers of swift-water animals. 



b) Droughts. — There was an unusual drought in the autumn of 1908. 

 The data on the distribution of fishes in Glencoe Brook and County 

 Line Creek were collected before this date (Fig. 67, p. 11 1). Table XVI 

 shows the arrangement after the drought. 



TABLE XVI 



Showing the Effect of Drought on Fishes 

 The localities 1, 2, 3, 4 are indicated on the maps of the North-Shore Streams 



County Line Creek was entirely dry except the pool nearest its 

 mouth in September, 1908. This is locality 4 in Fig. 67, p. in. 

 The following spring was one of normal rainfall. The fish proceeded 

 upstream a distance of only three rods. This partially restored the usual 

 arrangement. If this represents the rate, the fish proceed upstream 

 slowly. Glencoe Brook has not recovered its fish. 



As evidence of upstream migration of Mollusca, the following seems 

 to be important. Frequent examination of a section of the North 

 Branch of the Chicago River at Edgebrook, between 1903 and 1907, 

 showed that Pleurocera elevatum and Campeloma occur in this stream. 

 Pleurocera was not found during this period (ending November, 1907) 

 above a certain point. Campeloma was found only sparingly above 

 this point. The spring of 1908 was one of heavy rainfall and the 

 streams were in flood from April to June. On July 6 the snail 

 Pleurocera was found in numbers one-fourth of a mile farther up- 

 stream than formerly. Campeloma had gone nearly as far. The sea- 

 son from November to April was not different from other seasons 

 and there is no reason to assume that the migration began before the 

 spring floods. If this is true the snails could make their way toward 



