LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
373 
Such an increase of transparency of water can easily be observed in Lake 
Keokuk, where the water near the dam is more than twice as transparent as at 
Bur ling ton at the head of the lake. This gradual increase of transparency from 
33 cm. in the upper part of the lake to 84 cm. near the dam is graphically represented 
by Figure 10. Attention should be called to the fact that such relation exists only 
when there is no considerable change in the stage of the river. At the end of Sep- 
tember, when the river was rising, the water near Burlington was as muddy as that 
near the dam (fig. 10, lower line) . 
In Lake Pepin fluctuations of transparency of water are not so evident as in 
Lake Keokuk. The water of the river above the lake is nearly as transparent as 
in the lake, although the most transparent water was found in the lower part of the 
lake. In the upper part of Lake Pepin the transparency of water varied from 46 
to 61 cm. (Opposite Wacouta and Point No Point.) In the middle part of the lake 
opposite Point au Sable and Lake City and in the lower part of the lake the trans- 
parency was 76 to 87 cm. The maximum transparency, 102 cm., was found in the 
outlet of the lake opposite Reads Landing. The transparency of water in the 
northern shallow part of the lake, near Bay City, was 28 to 46 cm. and even less, 
19 cm., opposite the mouth of Isabel River. The water of St. Croix Lake was con- 
siderably clearer, its transparency on September 2 being 150 cm. 
TEMPERATURE OF WATER. 
The water of the Mississippi River in July and August was exceedingly warm. 
The highest temperature noted in the main channel, 33.3° C. (91.9° F.), was observed 
on July 13, at 5 p. m. Probably the temperature in bayous and sloughs was even 
higher, because other observations show that the water just below the dikes and in 
sloughs with a slow current was usually 1 to 2 degrees higher than in the main chan- 
nel. The highest temperature observed in Lake Keokuk was 31.1° C. (station 22) ; 
at most of the lake stations the temperature at the surface varied from 27 to 29° C.; 
at the end of September it was 20° C. In Lake Pepin the temperature of the upper 
stratum of water during the period from August 15 to September 10 varied from 22.1 
to 28.9° C. The highest temperature observed was in the shallow places in the 
northern part of the lake; in the middle part of the lake on calm days the temper- 
ature of the upper stratum sometimes reached 27.8° C. (station 76). The surface 
temperature in Lake St. Croix on September 2 was 23.3° C. There was no great 
difference between the temperatures of tributaries and that of the main stream. 
With regard to the vertical distribution of temperature both Lake Keokuk and 
Lake Pepin belong to the type of lake that is characterized by the absence of a 
thermocline. The uniform distribution of temperature at different depths facili- 
tates the vertical circulation of water during a warm season, and therefore has 
great influence on the distribution of plankton. The maximum difference between 
surface and bottom temperature in Lake Keokuk at the time of the investigation 
was only 3.5° C. (Table 13), and on windy days the distribution of temperature 
became more uniform. The greatest difference between the surface and bottom 
temperatures observed in Lake Pepin was 6.5° C. (Table i3, station 76), but the 
difference between the temperature at the 1.5 m. stratum and at the bottom was 
only 0.9°. The greatest difference usually occurred at the afternoon observations. 
