414 



also years of higher water, of greater uniformity of environment 

 — because of greater extent of open water, of greater interchange 

 of water in overflow stages, and therefore of greater agreement 

 in the course of plankton production. The similarity in the 

 course of plankton production in different bodies of water is in 

 a large measure a function of the similarity of their environ- 

 ment and the resemblance of their planktons in the matter of 

 constituent organisms. 



STATION F, PHELPS LAKE. 

 (Table IX.; PI, XXI., XL..-XLI1.) 



ENVIRONMENTAL CONDITIONS. 



This body of water lies on the western side of the river about 

 a mile below the city of Havana, in the elevated bottom-lands 

 below the mouth of Spoon River. It trends northeast and 

 southwest for a distance of seven eighths of a mile, has a width 

 of 400-600 feet, and a total area of 50-60 acres. Its bottom lies 

 about 6.5 feet above low-water mark, and the greatest depth re- 

 corded in it at high water at the point of collection was only 

 10 ft. It is but slightly deeper toward the lower end. Its out- 

 let is by a tortuous slough choked with driftwood, which runs 

 for two fifths of a mile in a southerly direction to the river. 

 The elevation of the bottom of this slough at its entrance to 

 the river is 8-9 ft. above low-water mark, so that all run-off 

 from the river drained by this slough ceases when it drains to 

 this level, and it is not reinvaded by floods below this elevation. 

 When the river falls below the level of the outlet and the lake 

 drains as fully as the outlet permits, there still remain about 

 1.5 ft. of water from which no further run-off occurs. The vol- 

 ume is then slowly reduced by evaporation or increased by sum- 

 mer rains. 



The lake is not fed by springs or tributaries of any sort be- 

 yond seepage from the level alluvial bottoms in which it lies, 

 and which nowhere in the vicinity rise more than 10 feet above 

 its bottom and generally very much less than this distance. At 

 river stages of 11 ft. and above, backwater from Spoon River 



