350 



The immediate effect of the access of the tributary waters 

 of the stream is as a whole diluent upon the plankton content 

 of the Illinois. A mixture of equal volumes of each would re- 

 sult in a reduction in the Illinois to 1.33 cm.^ per m.^ from 2.19 

 — a falling off of 39 per cent., or even of 49 per cent, if we omit 

 the low-water period of 1897. If we consider the areas of the 

 drainage basins as an index of the relative volumes of water 

 carried by the two streams, and determine the effect of Spoon 

 River contributions, we find the net result, based on the aver- 

 ages of our collections, to be a decline in plankton content in 

 the Illinois from 2.19 cm.^ per m.'' to 2.00 — a decline of 9 per 

 cent. If the low-water period of 1897 is omitted, the decline 

 is even greater, namely, from 2.19 to 1.96 — a fall of 11 percent, 

 in the plankton content as an immediate result of the contri- 

 butions of this tributary. Spoon River thus exerts in the econ- 

 omy of the Illinois an immediate diluent function upon its pro- 

 duction, which, qualitatively, is approximately 10 per cent. 



STATION C, QUIVER LAKE. 



(Tables V., XII. ; Pi. II., IV.. XV.-XVII., XXV.-XXIX., XLVIII., XLIX.) 



ENVIRONMENTAL CONDITIONS. 



This lake lies on the right bank of the Illinois (PI. II.), ex- 

 tending parallel to the river for a distance of three miles, in- 

 cluding Quiver Chute. This is the lower end of the lake, which 

 is separated from the Illinois River only by a low mud bank 

 submerged at levels of 4 ft. and crossed just below our plankton 

 station in the main stream by two "cut-offs" which bring a va- 

 rying volume of river water into the chute, the amount de- 

 pending upon the relative levels on the two sides of the spit. 

 The close connection of the lake and river makes the former 

 responsive to all changes in level in the latter at all stages of 

 water. This lower spit extends northward as a low bank 5 to 

 40 rods in width and generally less than 6 to 8 feet above low- 

 water mark, lying between the lake and river. This is covered 

 with low willows and, to the northward, with heavy forest (PI. 



