362 
BULLETIN OF THE BUREAU OF FISHERIES. 
yet formed the delta that now separates the body of the lake from the river the 
conditions in Lake Pepin were similar to the present conditions in Lake Keokuk. 
As a result of the physiographical relations the river exerts more influence on Lake 
Keokuk than on Lake Pepin, the latter being more definitely separated from the 
river. This is of great importance to all organic life of both lakes. 
STAGES. 
The stages of The river are subject to considerable fluctuation. Usually the 
river is at its lowest in the warmest season (July to August) and in winter (December 
to January), when it is covered with ice; the highest stages occur in spring and fall. 
The heaviest rainfall over the upper Mississippi Basin occurs in May and June, and 
the highest stages often coincide with these months, but the time and duration of 
high water are subject to much variation. The fluctuations of the stages of the 
upper Mississippi during 10 years (1911-1920) are shown in Table 4, where the 
highest and the lowest gauge readings and the dates on which they occurred are 
given for 8 points from St. Paul, Minn., to Keokuk, Iowa. (The data are taken 
from “Stages of the Mississippi River,” Mississippi River Commission, 1911-1920.) 
The greatest difference between the lowest and highest stage is about 17 feet 
(Prairie du Chien, 1920). 
The stages of the river during the present investigation are given in Table 5, 
which contains the daily data for 14 points from St. Paul, Minn., to Warsaw, 111. 
(5 miles below Keokuk), obtained from the United States Weather Bureau. The 
only considerable rise of water during the three-month period of investigation 
occurred in the latter part of September (beginning the 16th) in the lower part of 
the river below Le Claire. 
The stages at Lake Keokuk are subject to daily fluctuations, depending on 
the operation of the dam. Sudden rises and falls, ranging from 12 to 18 inches, 
brought many complaints from the local population against the Mississippi River 
Power Co., and the question was even investigated by a special committee of Con- 
gress (see Rivers and Harbors Committee, Impounding of water above Keokuk 
Dam, hearings on the subject of House Resolution 468, 1917). From a biological 
point of view the daily fluctuations of the water level in the lake are of importance 
because every fall and subsequent rise of water causes a decrease in plankton. 
The water running into the lake is considerably poorer in plankton than that in 
the lake, and therefore every sudden rise diminishes the quantity of plankton in 
the lake. During September 16 to 26, when the river was rising, the plankton of 
Lake Keokuk disappeared almost entirely. 
