OVERFLOW OF RED RIVER OF THE NORTH. 



37 



caused comparatively high stages of the Keel River during the last 

 week in March and the first week in April. This investigation showed 

 for the highest flood of which we have record (that of April, 1897) 

 that just prior to the spring break-up the average depth of snow 

 over the watershed was 26 inches, and that all of this snow melted in 

 approximately 20 days. The data also showed that all of the larger 

 floods that have occurred on the Red River have been due mainly to 

 the melting of snow over the watershed, rather than to precipitation 



DAKOTA 

 Fig. 14. — Storm rainfall on watershed of Red River of the North, June 26-28, 1915. 



at the time the flood occurred, though in some cases precipitation at 

 the time the snow was melting has undoubtedly caused stages some- 

 what higher than otherwise would have occurred. That rains in 

 April prolong the flood period is indicated by the diagrams for 1904 

 and 1916, figure 15. 



Records show that nearly every year more than 25 per cent of the 

 annual run-off occurs in April, although the precipitation for April 

 is seldom excessive and usually is less than 10 per cent of the annual 

 precipitation. This high percentage of run-off during April is due 

 to the melting of the snow stored throughout the winter months and 

 to the more or less impervious condition of the ground at the time 

 of the spring thaw. 



