541 
impounding action of the backwaters of the Illinois River as 
compared with that of other streams. It thus.seems probable 
that its plankton exhibits to an unusual degree the effect of 
such impounded backwaters upon the potamoplankton as a 
type. 
The amount of water impounded, as compared with that in 
the channel, can be estimated roughly from data given by 
Cooley (’97). The area of the bottom-lands is 704.3 square miles, 
of which 76.6 are water and 60.6 marsh. The area of the river 
itself presumably included in the above area, from the mouth 
to Utica, is 32.4 square miles, having an average width of 755 
feet and a length of 227 miles in the limits given. The aver- 
age width of the bottom-lands is 3.1 miles exclusive of the 
river. At low-water stages the marshes have little water in 
them, and the average depths of the bayous and lakes are slight 
—probably less than one half that of the river, which at this 
stage has, according to the averages of Cooley’s estimates, an 
-average depth of 7.5 feet. From these data it follows that 
the total volume of impounded waters at low-water stages is 
somewhat less than that in the river channel itself, and much 
of this is cut off from the river at that stage. Rises in the 
level of the river up to ten feet above low water—which is 
about the average bank height—increase the impounded vol- 
ume. In lower levels this increase is slight, but it becomes rela- 
tively greater as the level rises. When a height of ten feet is 
reached, connection has been established with the greater part 
of the permanent waters and marshes of the bottom-land, and 
the impounded volume must be at least double or triple that of 
the channel. At this level, overflow begins, though the higher 
bottoms are not covered until a depth of sixteen feet is reached. 
At a stage of overflow each additional foot of rise in the im- 
pounded area is equivalent approximately to the volume in the 
channel. Thus, at a maximum flood of eighteen feet there is 
about ten or eleven times as much water in the impounding 
area as is found in the channel. These estimates accord in a 
general way with the observed facts of current and run-off. A 
