OVERFLOW OF RED RIVER OF THE NORTH. 



47 



RUN-OFF DUE TO STORM PRECIPITATION. 



Only casual examination of the data is required to make it appar- 

 ent that storm precipitation during summer months often has marked 

 effect on run-off in the Red River watershed. The effect is relatively 

 Greater and is more apparent on small watersheds of tributary 

 streams than on large areas, because the most intense storm precipi- 

 tation occurs on comparatively small areas. In figure 19 the pre- 

 cipitation during four summer storm periods, each on a different 



10 15 20 

 JULY 



20 25 30 5 10 15 20 25 30 5 10 15 20 



JULY AUGUST JUNE JULY 



Fig. 19. — Accumulated storm rainfall and run-off. 



watershed, is indicated in comparison with run-off wh'ich followed 

 the storms. 



On watersheds of 10,000 square miles or less, storm precipitation 

 sometimes causes maximum run-off; but on the larger watersheds, 

 such as that above Grand Forks, spring run-off due to melting snow is 

 greater. Accordingly, figure 19 as well as other data on storm pre- 

 cipitation and summer run-off have been used largely in estimating 

 probable maximum run-off for the smaller watersheds (see Table 5), 



