OVERFLOW OF RED RIVER OF THE NORTH. 



45 



lent to the percentages shown in the table will occur during any 

 year when annual precipitation is maximum in depth. 



Table 11. — Probable maximum annual run-off. 



Streams. 



Station. 



Drainage 

 area. 



Ratio of 

 annual 

 run-off to 

 annual pre- 

 cipitation. 



Red River 



Do 



Red Lake River 

 Thief River 



Grand Forks, N. Dak.. 



Fargo, N. Dak 



Crookston, Mhxn 



Thief River Falls, Minn 



Sq. mi. 



25, 755 



6,435 



5,320 



1,080 



Per cent. 



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Fig. 17. — Precipitation and run-off. Red River of the North watershed above Grand Forks, 

 N. Dak., for selected months, 1882-1919. 1, Maximum monthly precipitation ; 2, run- 

 off during months of maximum precipitation ; 3, monthly run-off immediately following 

 maximum monthly precipitation ; 4, precipitation during months of maximum run-off ; 

 5, maximum monthly run-off ; 6, monthly precipitation immediately preceding maximum 

 monthly run-off. 



Investigation shows conclusively that run-off can not be determined 

 by considering it merely as a fixed percentage of precipitation. The 

 part of precipitation that supplies demands of evaporation and plant 

 transpiration is much greater and is more nearly uniform in quan- 

 tity from year to year than is the part that goes into run-off. For 

 the watershed above Grand Forks, comparisons of annual run-off and 

 annual precipitation are shown in figure 16. Monthly distribution 



