OVERFLOW OF RED RIVER OF THE NORTH. 39 



OTHER FACTORS AFFECTING RUN-OFF. 



TOPOGRAPHY. 



Some of the main topographic features have been described. The 

 general topography 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 



