529 



pronounced during the spring and early summer at the time 

 of the plankton maximum. Save in 1897 the contrast between 

 the two streams is also well marked during the autumn maxi- 

 mum. The phenomenon of these maxima is thus a function of 

 the main stream rather than of the tributary. 



The great irregularities in the Spoon River plankton as 

 contrasted with the main stream are due to the more profound 

 effect of floods upon its life. In the main stream, floods less 

 frequently and less thoroughly replace its waters owing to its 

 greater volume and, especially, to its reserve of densely popu- 

 lated impounded backwaters. The small tributary, on the other 

 hand, is scoured very completely by every considerable rain, 

 and has no reserve waters to maintain its equilibrium. The 

 oscillations in the quantity of life in its waters are thus more 

 frequent and more extreme than in the main river. 



The quantitative effect of the plankton of a tributary upon 

 that of the main stream may be expressed in the following 

 simple formula : 



DP+dp 

 JJ + d ? 



in which D and d represent the discharge from the catchment- 

 basins of the main stream and tributary, P and p, the plankton 

 per cubic meter of their respective waters, and P' that of the 

 main stream below the junction with the tributary. In the 

 case of Illinois and Spoon rivers the average discharge is ap- 

 proximately in the ratio of the areas of the catchment-basins. 

 Both lie in the same storm belt, and the slightly increased rain- 

 fall in Spoon River basin, which is the more southerly, is more 

 than counterbalanced by the accession of water from the Illi- 

 nois and Michigan Canal due to the pumpage from Chicago 

 River at Bridgeport. Accepting these areas as a basis for the 

 ratio of discharge, we find that the average amount of plank- 

 ton in the Illinois is reduced from 2.19 cubic centimeters per 

 cubic meter to 2.00 if the low-water period of 1897 is retained — 

 a decline of 9 per cent. If the low-water period is rejected 

 from both averages the amount falls to 1.96 cubic centimeters — 



