LEVEL OF GEOUND WATER. 41 1 



streams at the base of the canyons. For instance, in the Grand Canyon of 

 the Colorado many springs issue near the river and along the side canyons. 

 Some of these are large and are loaded with calcium carbonate." 'The rule 

 is here, as elsewhere, that the level of ground water starts at the level 

 of the river and rises toward the divide. But in this arid western country, 

 dissected by deep, steep canyons and broken by immmerable fractures, the 

 level of ground water in some places is as much as 1,000 meters or more 

 below the surface. 



While there is great variation in the depth of the level of ground water, 

 and ground water is found only at a considerable depth in many regions, on 

 the average the depth of ground water in the United States is not great — 

 probably in the neighborhood of 30 to 50 meters. 



FORM OF LETOL OF GROUND WATER. 



From the foregoing it follows that the form and position of the level 

 of ground water are largely dependent upon the topography, upon the 

 character of the openings in the rocks, upon the precipitation, and upon 

 other factors. In general, the more accentuated the topography the greater 

 is the difference between the elevation of the surface and the level of ground 

 water. Where from a lowland a steep ridge rises, the level of ground 

 water also rises, but less rapidly and with considerable deviation between 

 the two. Where from a lowland there is a gentle rise, the level of ground 

 water also rises gently, and it more nearly corresponds with the surface 

 than in the case of an abrupt rise. The position of the level of ground 

 water is largely dependent upon precipitation. In humid regions it is apt 

 to be near the surface ; in arid regions it is farther from the surface. 



The relations of the level of ground water to topography and to surface 

 drainage are illustrated by fig. 5. b Where the openings in the rocks are 

 numerous and large, there are much greater differences between the surface 

 and the v level of ground water than where the openings are few and small. 

 For this there are two reasons: First, the precipitation in a region of 

 large openings more readily makes it way through the openings to the 

 drainage level than in a region where the openings are small, for resistance 

 to movement increases as the openings become small, and therefore on 



"Powell, J. W., Explorations of the Colorado River of the West, 1875, p. 94. 

 b King, F. H., Principles and conditions of the movement of ground water: Nineteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 2, 1899, p. 99. 



