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 



