370 
BULLETIN OF THE BUREAU OF FISHERIES. 
The average annual flow at Keokuk estimated for a stage of 4.9 feet is approxi- 
mately 55,000 c. f. s. The maximum flow at Keokuk is approximately 260,000 
c. f. s. (See statement of D. C. Kingman, Chief of Engineers, U. S. A. Rivers and 
Harbors Committee, Impounding of water above Keokuk Dam, etc., 1917.) The 
average discharge of the river above Lake Keokuk increases from La Crosse, 33,400 
c. f. s., to Keithsburg, 54,000 c. f. s. (see Table 6). During the course of the present 
investigation the discharge probably averaged 40,000 c. f. s. in the upper part of the 
rive'r between Reads Landing and Le Claire, and 55,000 c. f. s. in the lower p&rt 
between Le Claire and Keokuk. 
FLOW OF SEDIMENT. 
The flow of sediment suspended in the water and rolled along the bed of the 
river is of great importance in the Mississippi, as its annual discharge of sediment 
into the Gulf is expressed in the enormous figure of 7,459,267,200 cubic feet of solid 
material. Our knowledge of the matter is based chiefly on many valuable investi- 
gations made by Humphrey and Abbot (1876). Series of observations were made 
also by the Board of Engineers, United States Army, by the Mississippi River Com- 
mission, and by the United States Geological Survey. 
The material carried down a river may be in solution, in suspension, or rolled 
along the bed. In the upper Mississippi the amount of material dissolved in the 
water is greater than that in suspension. The mean of the observations made at 
Minneapolis shows that 200 parts of material per 1,000,000 parts of water are in 
solution and only 7.9 parts per 1,000,000 in suspension (Townsend, 1915). The 
quantity of material suspended in water in the lower Mississippi is considerably 
greater, reaching the ratio of 280 parts per million. The amount of sediment in 
the lower Mississippi depends almost exclusively on the proportion of water from 
the Missouri. In comparison with the Missouri, the upper Mississippi is a clear 
stream and the amount of sediment carried by it is insignificant. 
Observations on the Missouri near St. Charles, Mo., indicate that the so-called 
degree of saturation — that is, the amount of sediment per unit of volume of water — 
depends on the stage of the river. This rule can not be applied to the upper Missis- 
sippi, where there is no such relation between the degree of saturation and the stage. 
An important part in the upper Mississippi is being played by Lake Pepin, in which 
is deposited a considerable part of the sediment brought to the Mississippi from the 
Minnesota River. The amount of sediment in suspension at Winona, below Lake 
Pepin, is about one-third or one-quarter of that at Prescott above the lake. 
The degree of saturation at the same stage may be very different, depending 
on the source of the flood. The maximum degree of saturation in the lower part 
of the upper Mississippi at Hannibal, Mo., is only one-sixth of that in the Missouri at 
St. Charles. The amount of sediment carried in the upper strata is usually less than 
at the depths. This is clearly shown in the following table, the data for which are 
taken from Hooker’s paper (1897). Only the observations at Clayton show an 
amount of sediment at the bottom less than at mid depth. 
