LIMNOLOGICAL, OBSERVATIONS IN THE UPPER MISSISSIPPI. 
369 
Observations of the current were made at every station on Lake Pepin, but 
movement of the water from the river down the lake was observed only at stations 
88 and 89 opposite Wacouta Light, about 2 miles below the mouth of the Mississippi. 
The velocity at these stations ranged from 0.9 f. s., close to the right shore, to 0.34 
f . s., in mid lake. The velocity observed at various other stations on the lake ranged 
from 0.11 to 0.83 f. s., but the directions of the currents fluctuated, depending 
exclusively on the direction of the wind. The maximum velocity, 0.83 f. s., was 
observed on a very windy day. When the water was rough, it was impossible to 
work with the current meter because the vertical movements of the boat revolved 
the wheel of the instrument. The measures of velocity less than 0.2 foot per second 
are probably not exact, because the Price current meter does not work well at low 
velocities. On calm days the drift of Aphanizomenon clumps along the boat often 
could be observed, and the velocity of this movement could even be calculated, but 
the wheel of the current meter was motionless, as the flow of the water was not 
strong enough to turn the cups of the instrument. 
DISCHARGE. 
The quantity of water flowing in a stream is usually expressed in units of dis- 
charge. By the discharge of a stream we mean the quantity of water flowing 
through a given cross section in a unit of time, the most common unit of discharge 
being the so-called second-foot, which is the average number of cubic feet flowing 
in each second of a definite period of time (day, week, month, or year). The dis- 
charge is obtained as the product of two factors— the area of cross section of the 
stream, which depends on the shape and the dimensions of the bed and banks, and 
the mean velocity. As both factors are controlled by the stage, the discharge may 
be considered as a function of the stage. 
The discharge of the Mississippi is exceedingly variable, the minimum varying 
from about 2,000 c. f. s., between Minneapolis and St. Paul, to 20,000 c. f. s., at 
Keokuk. Probably the minimum discharge at Keokuk in winter is even less, 
reaching only 12,000 c. f. s. (See Livers and Harbors Committee, Impounding of 
water above Keokuk Dam, etc., 1917, pp. 78-79.) 
The maximum and minimum discharges in the Wisconsin section of the river, 
according to Martin (1916), expressed in cubic feet per second, are as follows: Near 
Prescott, 134,000 and 3,000; outlet of Lake Pepin, East Winona and La Crosse, 
127,000 and 8,000; Prairie du Chien, 179,000 and 16,000; and Clayton, 179,000 and 
16,000. 
The mean annual discharge is also subject to great fluctuations. Thus, for 
example, the mean annual discharges between Minneapolis and St. Paul for the 
years 1905-1912, according to Meyer (1914), are as follows: 
1905. 
1906. 
1907. 
1908. 
Cubic feet 
per second. 
. 12, 920 
1909. 
. 13, 390 
1910 
. 10, 250 
1911 
. 8, 710 
1912 
Cubic feet 
per second. 
. 6, 965 
. 4, 630 
3, 240 
. 5, 260 
For four months during these eight years the mean monthly flow was below 
2,000 feet per second. 
