EFFECTS OF DROUGHTS AND FLOODS 



107 



the headwaters at the rate of at least a mile per year, if they were intro- 

 duced into a large stream. This must be a response to both water 

 pressure and current. The small value of such single observations is 

 recognized but they are presented here because the opportunity to secure 

 such data is small. In this river there are also notable relations between 

 especially dry seasons and the distribution of other animals. The 

 season in which the riffles were dry (October 31, 1907) the pools presented 



The Transverse Distribution of Stream Animals 



Fig. 64. — Shows the form of bottom and size of bottom materials in a cross- 

 section of the North Branch of the Chicago River, a-d, natural size (original). 



a, a burrowing May-fly nymph (Hexagenia sp.). 



b, small bivalve (Sphaerium stamineum), two individuals, two views. 



c, viviparous snail (Campeloma integrum) , seen from two sides. 



d, the long river snail, young and full grown (Pleurocera elevatum). 

 Fig. 65. — Cross-section of the stream with reference to a curve. 



an unusual aspect. The standing pools were choked with water-net. 

 The minuter forms, such as protozoa and flatworms, were present in the 

 greatest profusion. Hydra was abundant. All this is in marked con- 

 trast to the conditions which one finds when the stream is running. 



The season following the dry riffles, we found small Hydropsyche 

 larvae, and a few young stone-fly nymphs. The only forms present were 

 those that could be introduced by terrestrial, egg-laying females. 



