142 BULLETIN OF THE BUREAU OF FISHERIES 



Station No. 10 is on the Cannon River, a tributary that enters the Mississippi 

 about 3 miles above Red Wing. The sampling station is some distance above the 

 mouth of the river. The water at the station is clear, shallow, and flows very rapidly. 



Station No. 11 is on the Mississippi River at the lower end of Lake Pepin, a 

 short distance above the mouth of the Chippewa River. By the time the water 

 reaches station No. 11 it has passed through Lake Pepin and has lost much of the 

 solid material held in suspension. That Lake Pepin acts essentially as a settling 

 basin is shown by the bottom deposits at the head of the lake and by the greater 

 transparency of the water when it leaves the lake than when it enters it just below 

 Red Wing. 



Station No. 12 is on the Chippewa River where the latter enters the Mississippi 

 at the lower end of Lake Pepin. (Lake Pepin is the result of the delta formed by 

 the Chippewa across the bed of the Mississippi.) The discharge of the Chippewa 

 is given in Table 1. 



Station No. 13 is on the Zumbro River. This station was not visited during 

 the biological survey. 



Station No. 14 is on the Mississippi River just above Winona, about 110 miles 

 below St. Paul. The station here was chosen above the city to avoid the effect of 

 local pollution. The Mississippi River receives the discharge of the Zumbro River 

 above this station. The hydrometric data for the Zumbro are given in Table 2. 



HYDROMETRIC DATA 



The hydrometric data shown in Table 2 were prepared from the gauging station 

 records furnished by Mr. Soule, of the United States Geological Survey. The 

 gauging stations on all the tributaries are at some distance above the mouth of 

 the rivers; therefore, the data, as given in the station records, do not show the 

 actual discharge at the mouth of the tributary. The figures in Table 2, however, 

 do give the approximate mean discharge at the mouth of each tributary. Values 

 are shown for each month of the period October, 1925, to October, 1926. The 

 writer obtained these values by multiplying the mean, as given in the station records, 

 by the ratio of the total drainage area of the tributary to the drainage area above 

 the gauging station. 



The hydrometric data, where they extend over the entire year, bring out the 

 important fact that on each river there are two periods during the year when the 

 discharge reaches a minimum. The first minimum occurs either in January or 

 February and the second comes in July or August. With one exception — the 

 Minnesota River — the winter minimum for the last year was lower than the summer 

 minimum. From a biological standpoint these low-water stages may become very 

 significant. 



It is possible that the large amount of water during the high-water stages may 

 so dilute the sewage and other wastes dumped into a river that their deleterious 

 effect is reduced to a point where no harm results to fish and other aquatic life, 

 but that during the low-water stages this pollution becomes so concentrated that 

 all life is destroyed in the contaminated areas of the river. The periods of minimum 

 discharge, then, may be limiting factors that determine whether fish or other aquatic 

 organisms can survive in the polluted river. 



