OVERFLOW OF EED RIVER OF THE FORTH. 19 



thence out of the watershed. Precipitation is the source of run-off, 

 but many different factors affect the amount and rate of run-off 

 from a given watershed. A portion of the water is absorbed by the 

 soil ; some of this passes through the soil into drainage channels. A 

 part is evaporated from the soil or is transpired by plants and trees 

 or is evaporated from the surface of the soil. A small amount sinks 

 deep into the ground, and the remainder passes over the surface of 

 the ground to a watercourse and through it out of the watershed. 

 The fate and amount of run-off from a watershed are affected 

 principally by the amount and distribution of the precipitation and 

 its form — whether rain or snow. 



After precipitation the principal factors which affect the rate of 

 run-off from a watershed are topography, size and shape of the 

 watershed, prevailing temperature, character of vegetation on the 

 watershed, and artificial drainage. In addition to these and some- 

 what dependent upon them are evaporation, transpiration of plants, 

 absorption by the soil, and storage, all of which tend to regulate the 

 amount of water reaching drainage channels. The effect of these 

 several factors upon the amount and rate of run-off is not constant 

 but is continually changing. It is exceedingly difficult, if not im- 

 possible, to measure the effect of the various factors upon the run-off, 

 and in the absence of measurements from a given watershed the 

 determination of the probable rate of run-off becomes a complex 

 problem. 



The best bases for calculating future run-off are measurements of 

 stream flow extending over a long period of time for the particular 

 watershed under consideration. If such records are available, the 

 amount and rate of run-off can be estimated closely unless the 

 physical features which affect run-off from the watershed are ma- 

 terially changed. If complete data for a long period are not avail- 

 able, then records for a shorter time on the watershed under con- 

 sideration, or on more or less similar watersheds, are most essential. 

 For the watershed of the Red River, and especially for those of its 

 tributaries, the run-off records are meager because settlement and 

 development in the valley are only comparatively recent. 



Daily records of the river stage at Grand Forks, from which the 

 run-off at that point can be computed, are available as far back as 

 1882. From 1882 to 1901 the gauge readings were recorded by the 

 Corps of Engineers, United States Army, and from 1902 to the 

 present time by the United States Geological Survey. Similar 

 records are available for the Red River at Fargo from 1902 to the 

 present time, as are also records for 12 stations on tributary streams 

 for periods of from 1 to 17 years, since 1901. The daily flow of the 

 river at Emerson, Manitoba, just north of the international boundary, 



