OVERFLOW OF RED RTVER OF THE NORTH. 39 
OTHER FACTORS AFFECTING RUN-OFF. 
TOPOGRAPHY. 
Some of the main topographic features have been described. The 
general topograph}' of the various watersheds is indicated in figure 
1 ; areas are given in Table 1. 
In the great central portion of the watershed comprising the area 
termed " the valley," the surface is nearly level. The run-off there 
never is rapid. In the highest portions, comprising the areas drain- 
ing to the upper parts of the tributary streams, large areas are flat 
and nearly level while others are somewhat rough. Depressions of 
every size are numerous; some are lakes while others are merely 
marshy, spots and " pot holes." The run-off there is even more slow 
than in the valley unless it is modified by artificial drainage to carry 
water from the flat areas and depressions. Between the two por- 
tions of the watershed just described is located the area just outside 
the most recent shore lines of the ancient glacial lake, Agassiz. Here 
the surface slopes are steeper and comparatively regular and vary 
from a few feet to 25 or even 50 feet per mile. Much of this area is 
well drained naturally. The run-off from it is more rapid than from 
other parts of the watershed, but even here it could hardly be termed 
" quick." Considering the area as a unit, probably on no other 
watershed of equal size within the United States is the effect of the 
topography on run-off more nearly uniform over the whole area. 
SIZE AND SHAPE OF THE WATERSHED AREAS. 
The size of a watershed area is the principal factor to be con- 
sidered in determining the volume of run-off originating on the 
area, for, approximately speaking, volume of run-off varies directly 
with the size of watershed. This relation, however, does not hold as 
between size of watershed and rate of run-off, as the rate of run-off 
per unit of area decreases as the size of watershed increases. This 
is true because the most intense storm precipitation and the most 
rapid melting of snow occur on relatively small areas, and the time 
of occurrence of the maximum rates of run-off from these separate 
small drainage areas is seldom, if ever, such as to concentrate these 
maximum discharges in the main outlet. The relation of size of 
drainage area to rate of run-off per square mile for various areas in 
the Red River watershed is indicated in Table 5. 
The shape of the watershed of the Red River tends to produce 
quick run-off. As the width of the area nearly equals its length, 
the run-off from large portions of the watershed tends to concentrate 
at certain points. The same conditions are found in many subdi- 
visions of the watershed, as for example the area above Grand Forks, 
that above Fargo, and watersheds of tributaries as follows: The 
