LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
381 
On windy days the stratification of plankton disappears and it becomes dis- 
tributed uniformly. (See Table 29, p. 423. Stations 26, 28, 32, 34.) The strati- 
fication of plankton during calm weather and its uniform distribution from the 
surface to the bottom on windy days show that the wind causes the circulation 
of the whole body of water in the lake. This circulation is possible because there 
is no great difference in the temperature and density of water at the different levels. 
Of the tributaries emptying into Lake Keokuk the most important is Skunk 
River, on the Iowa side of the lake. The mouth of this river is now submerged 
and its water flows slowly. The plankton of the Skunk River is considerably 
Fig. 11.— Distribution of plankton in Lake Keokuk, July, 1921. (Figures on the chart indicate the mean volume of plankton 
per cubic meter of -water. Figures beneath the chart correspond to the serial numbers of stations in the cross sections; upper 
figures refer to the left side stations.) 
richer than that of Lake Keokuk. On July 20 its average volume, measured in two 
different channels of the river, was 17.5 cm. 3 (station 14) and 26.5 cm. 3 (station 13) 
per cubic meter. 
The overflowed islands with submerged vegetation are of particular impor- 
tance for Lake Keokuk. It would be especially interesting to study in a detailed 
manner the microscopical fauna and flora of those parts of the lake where a great 
quantity of old vegetation is now in a state of decomposition. Unfortunately the 
writer was unable to collect the material for quantitative plankton investigation in 
