545 
Jan. Feb. March | April | May June July 
Channel plankton...) .21 «23 e277, 4.59 6.08 7.22* | 4.23 
Backwater plankton] 1.63 2.06 2.46 11.43 18.91 6.19 | 3.92T 
Raoee tet ce 1to8) rtog | rtog | 1to2.5| 1to3 | 1 to .of |1 to.9|| 
afi Mean 
Aug. Sept. | Oct. Nov. Dec. |annual 
Channel plankton.. 3.88 2.56 1.70 .88 71 271 
Backwater plankton 13.90 12.92 14.78 15.10 | 6.81 9.18 
Rationece ees I 103.6 | 1to5 |1to8.7 | rto17 | 1to9.6 |1to3.4 
*Omitting 1895, 1.42. TOmitting 1895, 2.96. [Omitting 1895, 1 to 4.4. 
|| Omitting 1895, 1 to 1.3. 
months, from November to March, the backwaters have from 
8to 17 times the plankton content of the channel, and in 
the warmer season, from April to October, from 0.8 to 5 times 
as much. Omitting the aberrant record of June, 1895, from 
channel data, it becomes apparent that there is least difference 
between channel and backwaters in April-May and July, pre- 
dominantly the months of greatest run-off of impounded back- 
waters. 
Channel plankton of the [linois River, therefore, has its 
source in a large degree in impounded backwaters, and is main- 
tained to a considerable extent by their run-off. 
INDIGENOUS PLANKTON OF THE CHANNEL. 
It does not seem probable that the channel plankton is 
only the mingled plankton of the tributaries and backwaters. 
Growth and reproduction, modified, however, by the cyclic 
phenomenon, continue in the channel after plankton-laden 
backwaters unite with it. This increase is facilitated by the 
fact that channel waters are generally richer (see Tables 
XXIII.) in nitrogenous matters and the other products of 
decay than the backwaters. During high water, when the cur- 
rent is more rapid, the time for further breeding is considera- 
bly reduced, even to five days, from Utica to the mouth of the 
