413 
Thompson’s Lake and presumably other bottom-land wa- 
ters of similar character, by virtue of their impounding func- 
tion, are reservoirs in which flood waters are stored for a great- 
er or less time, permitting the development in general at all 
seasons of the year of a plankton exceeding in volume from 
1+-fold to 18- fold that coincidentally developed in channel 
waters of the adjacent river. The run-off from this and like 
areas elsewhere thus serves to enrich and maintain the river 
plankton proper. The slightly developed tlood-plain of the [li- 
nois and the consequent considerable area of sach bottom-land 
waters—which equalize the floods, prolong the run-off, and favor 
the production of an abundant plankton in the impounded areas 
—become, accordingly, factors of great importance in causing 
the richness, abundance, variety, and long continuance of the 
unusual plankton production of the Illinois River. 
The similarity in the course of plankton production in 
Thompson’s Lake aud elsewhere in our field of operations is 
shown in the following tabular summary, which gives the num- 
ber of instances of agreement and disagreement in the direc- 
tion of the changes in production in the four localities. 
SIMILARITY IN DIRECTION OF CHANGE IN PRODUCTION IN THOMPSON'S LAKE 
AND AT OTHER STATIONS. 
Illinois River|Quiver Lake|Dogfish Lake] Flag Lake | Total 
er Sd lode] Sa lade! Sz lasz) Sz lea) 2 |ad2 
tx = a oy = FR -u mn ee 
ae Amel Se Abe] fe APe] Se Reel Me ARs 
ASOS ectel eseieie ele 9 4 7 5 9 3 2 I 27 13 
W8OO! 5.6.0 cases 18 8 12 13 I 38 16 9 64 
TSO Pfiverstceseterctsiese 14 4 10 3 5 I 5 2 34 15 
LSGO ieee eee 21 4 22 Ste || Incase Neico] |FeaaOs| Laooce 43 7 
TOG isseys/crerere erase 3 4 3 Ae Wares lla ansreve alls areletesc| ears eve 6 8 
STA) spaces boee OS eed: 54 33 32 12 23 12 | 174 81 
This gives a grand total of 174 instances of similar direc- 
tion of change in production out of a possible 255, or 68 per cent., 
for the 5 years included. It is noticeable that the years differ 
considerably in the degree of agreement detected, the latter 
years of fuller records exhibiting fullest agreement. They are 
