452 



and relation to environmental factors shows marked similari- 

 ties to that found in the backwaters. The river is, however, 

 more immediately and directly subject to ihe effect of floods, 

 sewage, and stagnation, and exhibits less uniformity and regu- 

 larity in its planktographs. High water and repeated floods 

 are wont to depress its production, while stable conditions such 

 as prevail more fully in falling levels and low waters often lead 

 to increase in production. The run-off of impounded backwa- 

 ters where plankton breeds, which attends falling levels, and 

 the concentration of sewage in low water likewise conduce to 

 increased production. 



Spoon River. This is always plankton-poor save at lowest 

 levels, when the slackened current renders the stream an im- 

 pounding area. At high levels in spring impounded backwaters 

 of the bottom-lands join it and tend slightly to increase its 

 plankton content. It is immediately and predominantly a dil- 

 uent of channel plankton. 



Qa'iver Lake. This is an area subject to great vicissitudes 

 of production by virtue of the variety of environmental factors 

 operative and their changing efficiency with fluctuations in 

 river levels. Influx of creek and spring water of recent origin 

 and relative dominance of vegetation are increased at low lev- 

 els, and production falls. Backwaters from the channel in 

 floods below 6-8 ft. increase its impounding function, and de- 

 crease current, proportion of tributary water, and relative oc- 

 cupancy by vegetation, and production rises. In floods of higher 

 levels the general current of overflow from submerged lands 

 up-stream courses through the lake, and local factors are great- 

 ly reduced in their effect. As a whole it is the least productive 

 of all the backwaters examined, but like the others has a rich 

 plankton when it is filled with impounded backwaters, espe- 

 cially at the season of the vernal pulse. 



Dogfish Lake. This resembles Quiver Lake in many fea- 

 tures, but is freer from immediate access of tributary water and 

 invasion of channel waters at lower levels. It has accordingly 

 a higher and better sustained level of production. 



