459 



mingled more or less with the residual and plankton-rich wa- 

 ters of reservoir lakes and sloughs, soon develops an abundant 

 plankton ; or, retained in the channel and mingled with plank- 

 ton-bearing waters from a thousand sources, it develops a phyto- 

 and then zoo-plankton as soon as the requisite time for breeding 

 has elapsed. Illustrations which demonstrate the operation of 

 this hydrographic factor permeate all of the data of the variable 

 environment with which we are dealing. Attention has been 

 called repeatedly in the detailed discussion of the movement in 

 production to specific instances of the barrenness of flood waters 

 of recent origin which invade plankton-rich areas. By dilution 

 and replacement they lower plankton content in channel 

 waters pre-eminently and to a less extent in backwaters, where 

 their diluent action is less pervading and replacement less com- 

 plete. The most perfect illustration of this will be found in the 

 August flood of 1896 (PI. X.). 



Tributary streams such as Spoon River, whose waters are 

 of recent origin, contain but little plankton. Quiver Lake falls 

 in production as spring and creek waters preponderate in its 

 area at low-water stages, while the phenomenally rich waters 

 of Phelps Lake are poverty-stricken only so long as Spoon Riv- 

 er floods maintain a current through the lake. 



The time requisite for the development of the plankton in 

 such waters of recent origin must necessarily vary with the at- 

 tendant circumstances, such as climatic conditions, previous 

 state of the plankton, and the proportions of the mingling wa- 

 ters. The volumetric pulses of plankton are frequently so 

 located as to suggest the agency of floods in determining, to 

 some extent at least, their location and amplitude. The apices 

 of these pulses are found at varying intervals after the pre- 

 ceding depression occasioned by invasion of water of recent 

 origin. Thus the August pulse in 1896 in channel waters makes 

 full recovery in a week from the scouring effect of the flood 

 (PI. X.). In the backwaters this same flood (PI. XXVIL, XXXI., 

 and XXX VII.) was attended by a rising pulse which culmi- 

 nated after an interval of 2 to 3 weeks. In a general way, 10-20 



