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 
Fic. 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). 
Frc. 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 Hydropsycke 
larvae, and a few young stone-fly nymphs. The only forms present were 
those that could be introduced b y terrestrial, egg-laying females. 
