40 BULLETIN 1017, U. S. DEPARTMENT OF AGRICULTURE. 



Bois cle Sioux above Wahpeton, the Buffalo River, the Goose River, 

 the Red Lake River above Crookston, Two Rivers, and the Minne- 

 sota Wild Rice River above Twin Valley. On certain other tribu- 

 tary streams where the ratio of width to length is small, the 

 shape tends to prevent a concentration of water in the lower reaches 

 since the water from lower part runs off before that from the 

 upper portion reaches the main stream. On such streams the period 

 of storm run-off is prolonged, but the rate of discharge is not so high 

 as from equal areas where breadth and length are approximately 

 the same. The Sheyenne River is the largest tributary having a 

 long, narrow watershed. The Otter Tail, Sand Hill, and Pembina 

 watersheds are also of this type. 



TEMPERATURE. 



The relation between prevailing temperature and precipitation 

 and run-off was studied in some detail. As has been noted previously 

 a large amount of precipitation accumulates on the ground in the 

 form of snow. In the spring, owing to the rise in temperature which 

 may sometimes be accompanied by rain, this accumulation is sud- 

 denly carried into the tributaries thus causing maximum floods. 

 The relation which temperature bears to the magnitude of the spring 

 flood depends on, (1) the rate and the amount of the temperature 

 changes; (2) whether the temperature rise occurs simultaneously 

 over the whole or large portions of the valley, or progresses from the 

 upper to the lower portion of the watershed. Since the Red River 

 flows north, if snow covers large portions of the watershed a rise in 

 temperature that moves gradually northward will tend to cause a 

 greater flood than will a simultaneous temperature rise in all parts 

 of the area. Temperature is an important factor as affecting evapora- 

 tion, and evaporation limits run-off quantities very materially. 



In the spring of 1897 when occurred the greatest flood for which 

 reliable records are available, there was a snow covering on March 

 15 of approximately 26 inches. The temperature rose slowly between 

 March 14 and March 25 ; after March 25 the temperature rose rapidly 

 and by April 5 the snow had disappeared. On April 10 the maxi- 

 mum flood stage occurred at Grand Forks. There was very little 

 rainfall in the last 10 days in March, approximately one-half inch 

 in the first five days in April, and the precipitation for April was 

 only one-half the normal. High temperature and the warm rains 

 melted the deep snow covering and caused the surface water to move 

 so quickly into the streams that the amount which percolated into the 

 soil probably was comparatively small and opportunity for loss by 

 evaporation was limited. In a similar manner rapid rise in tem- 

 perature occurring simultaneously over central and northern por- 



