OVERFLOW OF RED RIVER OF THE NORTH. 
37 
caused comparatively high stages of the Red 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 2G 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 
CANADA 
SOUTH 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. 
