330 



tent than the channel, Quiver, Thompson's, and Phelp's lakes 

 showing respectively 42.14, 51.39, and 76.17 cm.^ per m.^ of 

 plankton. The first two collections were taken on the same 

 day as that in the channel; the last, two weeks later. On May 

 3 the river was 11.1 ft. above low water, and thus above bank 

 height in almost all localities. Flood water had invaded the 

 bottom-lands in the middle of February, two and a half months 

 before, and the invasion had continued for a month and a half 

 before the run-off of the impounded flood commenced. Time 

 for breeding a plankton had thus elapsed, and hundreds of 

 pools, ponds, lakes, and swamps which had dried up in the 

 drouth of the previous fall afforded a proline seed-bed of rest- 

 ing stages of plankton organisms to populate the impounded 

 and submerging waters. The slight current, shoal and warm 

 waters, and abundance of organic debris in these impounding 

 regions contribute also to the great development of plankton 

 w^iich the rapid decline in levels draws off into the channel to 

 mingle with and enrich the plankton content of its waters. 

 The proportion of impounded water entering the channel at 

 times of such rapid decline from the high levels of April, 1898, 

 is a very large part of the run-off, and predominates over waters 

 of tributary streams which contain but little plankton (see 

 PI. XXIV.). 



The conjunction of the vernal rise in temperature and 

 the run-off of the impounded flood from reservoir backwaters 

 in which the plankton has had time to breed are thus dominant 

 factors in determining the amplitude of this unusual vernal 

 pulse of plankton in channel waters. 



The effect of the flood of the last part of May upon plank- 

 ton production is apparently slight. The decline from the 

 maximum of 35.68 cm.^ on the 3d was well under way, reaching 

 10.31 on the 10th, and 5.22 on the 17th with the first entrance 

 of fiood waters. The remainder of the decline forms a very 

 regular curve in which no break due to fiood waters — such, 

 for example, as that of August, 1896 (PI. X.)— can be traced. 

 This is not due to the presence of an abundant plankton in the 



