330 
tent than the channel, Quiver, Thompson’s, and Phelp’s lakes 
showing respectively 42.14, 51.39, and 76.17 em.’ 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 
which 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 avery large part of the run-off, and predominates over waters 
of tributary streams which contain but little plankton (see 
Oe OLN) 
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 shght. 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 flood waters. The remainder of the decline forms a very 
regular curve in which no break due to flood waters—such, 
for example, as that of August, 1896 (Pl. X.)—ecan be traced. 
This is not due to the presence of an abundant plankton in the 
