40 BULLETIN 1017, U. S. DEPARTMENT OF AGRICULTURE. 
Bois de 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 b} 7 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- 
