OVERFLOW OF RED RIVER OF THE NORTH. 41 



tions of the valley in 1904 and 1916 produced higher stages than 

 otherwise would have prevailed in the streams. 



VEGETAL COVER. 



On areas having moderate slopes such as are found in practically 

 all parts of the Red River watershed, vegetation such as small grains, 

 grasses, potatoes, or other plants whose stalks or leaves form a more- 

 or-less intricate mass through, which run-off must pass, tends to 

 decrease run-off. The velocity of flow is retarded by the obstruc- 

 tions thus offered and the time during which evaporation, absorption 

 by the soil, and transpiration of plants may occur is increased. This 

 retarding effect would be greatest from June to September when 

 plant growth is most luxuriant, and would be relatively small during 

 March, April, and part of May, before plants are high enough to 

 obstruct surface flow. 



ARTIFICIAL DRAINAGE. 



The function of artificial drainage is the disposal of excess water. 

 Usually it is essential that such water be removed more quickly than 

 it flows off through natural channels. This fact in itself would 

 seem to imply an increased run-off rate as a result of artificial drain- 

 age. The drainage, by means of open ditches, of areas which have 

 served as natural reservoirs — such as permanent swamps, marshes, 

 ponds, and lakes — removes large quantities of excess water during 

 the initial period of the operation of the drains. After drains have 

 been in operation for a time the amount of discharge is diminished 

 bj whatever water is absorbed by the drained soil and by whatever 

 amount evaporation and transpiration are increased. One effect of 

 tile drainage is similar to that just stated, the run-off being de- 

 creased by whatever amount soil absorption is increased by the action 

 of the tile. However, the influence of tile on run-off will develop 

 gradually and appreciable effects will be long delayed because the 

 installation of such drains will proceed slowly. 



Artificial drainage may increase the rate of run-off by increasing 

 the number of outlets and thereby shortening the time period during 

 which run-off occurs. However, the decrease in run-off due to in- 

 creased absorption by the drained soil may more than compensate 

 for the extra outlets. 



The only important conclusion which has been reached relative to 

 the effect of artificial drainage on run-off is, that when heavy rain- 

 fall occurs or when large quantities of snow melt rapidly, artificial 

 drains may operate to increase the rate of run-off when areas drained 

 are for any reason impervious to water, that is, when the ground is 

 frozen or is saturated. Such increase in rate of run-off will increase 

 flood heights along watercourses of insufficient capacity. Observa- 



