392 



but in general throughout most of the seasonal changes. Its 

 run-off therefore serves generally to enrich the channel waters. 



The greater production is due to the decay of the abun- 

 dant vegetation which the lake contains, to the absence of trib- 

 utary water of recent origin, to the relative freedom from the 

 general current of overflow which largely takes the line of less 

 resistance through Thompson's Lake (PL II.), and, consequent- 

 ly, to the greater time afforded for breeding an abundant plank- 

 ton in this impounding area. 



The dominance of the abundant vegetation is inimical to 

 large plankton production. Other things being equal, plankton 

 production is greater when the relative occupancy of the water 

 by vegetation is decreased. 



The movement in plankton production in this area is in 

 the main similar to that in the river and in Quiver and Dog- 

 fish lakes. Pulses of production tend to coincide, though their 

 amplitude may differ widely in the several localities. This sim- 

 ilarity is least when local environmental factors such as vege- 

 tation, stagnation, or local exhaustion of the food supply are 

 most potent. It is greatest when these are least, that is, during 

 high water. 



station g, Thompson's lake. 



(Table VIIL, PI. II., XX., XXXV.— XXXIX., L.) 



ENVIRONMENTAL CONDITIONS. 



This body of water lies in the bottom-lands on the right 

 bank of the Illinois, above Spoon River, midway between the 

 bluff and the main stream. It trends in a northerly and then 

 a northeasterly direction, following somewhat the curve of 

 the Illinois. It is about five miles in length at low water, with 

 a width in three fourths of the distance of about two thirds of 

 a mile, while the northern end is less than one third of a mile 

 in width. At this stage it contains about 1,400 acres. As levels 

 rise, its margins spread rapidly — owing to the slight gradient 

 of the shores— northward to Grass and Slim Lakes, westward, 

 through Mud Lake, towards the bluff, to the south, towards 



