THE IRRIGATION AGE. 13 



James River Valley, that water will burst forth with immense force. 

 The efficient head may in some cases be nearly 500 feet above the sur- 

 face. 



Because of the unavoidable leakage along its eastern edge there 

 is quite a regular decline of pressure toward the east. This from 

 Highmore to Huron has been found to be about four feet per mile, and 

 a similar grade appears to extend from Sheridan, Wyo., to Marshall, 

 Minn. In some cases it is much steeper and shows also frequent 

 irregularities. This leakage and the opening of wells must gradually 

 exhaust the supply unless it is constantly replenished. 



How is this water supplied? The ready answer is from three 

 sources. First, by the absorption of rain falling directly upon the 

 exposed edge of the Dakota and Jurassic formations. Second, that 

 which seeps into these deformations from streams flowing over such 

 exposures. Third, that which may pass into them from other formations 

 below the surface, particularly from the underlying older formations. 

 If these are equal to the water escaping from wells and springs and 

 are unfailing, the supply of course may continue forever. 



SECTION I Lt,U.TRAT*WG THE ARTESIAN RESOURCES OF T H E NORT H W EST 



IRRIGATION Att 



How, therefore, may we judge of the amount of this supply avail- 

 able for Dakota? Nebraska, Wyoming and Montana, as well as wide 

 areas in the western part of the Dakotas, are too high to obtain water 

 from this source without expensive pumping. Iowa and Minnesota 

 are not to any considerable extent underlaid by these water-bearing 

 formations. Marshall, Minn., obtains water directly from it and pos- 

 sibly some shallow wells in the Red River Valley may do so indirectly, 

 but many of them are known to be supplied from other sources. The 

 Dakota formation doubtless underlies Assiniboia and the Saskatchewan 

 valley, though it may not be so much a water-bearing formation as 

 further south. Moreover, it lies deep there and other supplies of water 

 are readily accessible. We, therefore, feel justified in estimating the 

 adequacy of the supply available in Dakota as follows: 



From geological maps we estimate the exposed surface of the Da- 

 kota and Jurassic formations between the north line of Colorado and 

 Canada to be at least 3,000 square miles. The average rainfall reported 

 in that region is twenty-four inches per annum. Supposing that one- 



