106 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. 111). 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 
(Fig. 67, p. 111). P=before drought. *P=after drought. 
Name ea ee eornion Scientific Name I 2 3 4 
Glencoe; Broola(no: fish) neninsanorincsse ns =e ee |) Os lai heal eeeiele§ eee 
County Line Creek 
Homedidaceya. see sie Semotilus atromaculatus...| P Pe ii} Bd =e 
Black-nosed dace...... Rihinichihys Gironasuse ee |-- 42 |e le le 
Common sucker........| Catostomus commersonii ..|......|....-.|....>- AP 
County Line Creek was entirely dry except the pool nearest its 
mouth in September, 1908. This is locality 4 in Fig. 67, p. 111. 
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 
