372 BULLETIN' OF THE BUREAU OF FISHERIES. 
Table 12 . — Transparency of Mississippi River and various tributaries during rise of water, September, 1921. 
Locality. 
Date. 
Trans- 
parency. 
Locality. 
Date. 
Trans- 
parency. 
Stations: 
Cm. 
Tributaries: 
Cm. 
Reads Landing 
Sept. 10 
79 
Beef Slough 
Sept. 10 
27 
Winona 
Sept. 11 
50 
Black River 
Sept. 12 
43 
La Crosse 
...do 
46 
La Crosse River 
...do 
20 
Near Lansing 
Sept. 13 
28 
Root River 
...do 
10 
Prairie du Cliicn 
Sept. 14 
17 
Wisconsin River 
Sept. 14 
29 
Near Cassvfile 
Sept. 15 
23 
Turkey River 
...do 
2 
Near Bellevue 
Sept. 16 
12 
Rock River. 
Sept. 18 
8 
Near Clinton 
Sept. 17 
12 
Iowa River 
Sept. 20 
Rock Island Rapids 
...do 
8 
Des Moines River 
Sept. 23 
5 
Near Davenport 
Sept- 18 
6 
New Boston 
Sept- 20 
12 
Burlington 
...do 
10 
Alexandria 
Sept. 23 
9 
Fig. 10. — Transparency of water in the Mississippi River and Lake Keokuk, between New Boston, 111., and Alexandria, 
Mo. (The figures on the lines represent the transparency in centimeters; the figures under the lines represent the 
day of observation.) 
The transparency of the water may depend on the quantity of detritus or silt 
in suspension, as well as on the quantity of plankton. It has often been observed 
that the fluctuations in the transparency of water in lakes have closely followed 
fluctuations in the quantity of plankton (Apstein 1896, Le Roux 1899, Galtsoff 
1913-14). In the Mississippi River the transparency of water depends principally 
on the amount of sediment in suspension. When the river is dammed and its flow 
becomes slower, a part of the sediment is deposited and the water becomes clearer, 
provided that the decrease in the amount of sediment is not compensated by the 
increase of plankton. 
