188 K. Bryan — Rock Tanks and Char cos. 



Akt. XI V. — Origin of Rock Tanks and Char cos; by Kikk 



Bryan. 1 



Table of Contents. 

 Introduction. 

 Summary. 

 Rock Tanks. 



Definition. 



Rock tanks away from stream channels. 



Rock tanks in stream channels. 



Physiographic relations of rock tanks. 



Falls due to differing resistance of rocks to erosion. 



Palls due to changes in stream grade. 



Falls due to renewed uplift. 

 Sand Tanks. 

 Charcos. 



Introduction. 



In arid regions small water supplies are very impor- 

 tant. That a few gallons of water can be obtained at a 

 certain place for a period as short as two or three days 

 may make possible a journey which otherwise could not 

 be undertaken. In the Papago Country, a great area 

 in southwestern Arizona lying along the Mexican border 

 between Nozales and Yuma, west of Santa Cruz River 

 and south of Gila, such small water supplies are, in 

 many localities, the only dependence of travelers. In a 

 reconnaissance made in 1917 for the purpose of pre- 

 paring a guide to desert watering places 2 considerable 

 attention was given to rock tanks or "tinajas," and to 

 charcos, or "mud holes." Both types of watering places 

 are pools of water which remain in small depressions or 

 basins after rains. The origin of these basins presents 

 interesting problems in minor geology. The distribution, 

 also, of both charcos and rock tanks is controlled by the 

 physiography of the region. For these reasons it 

 seems advisable to give an account of the origin of these 

 watering places in advance of the publication of the 

 final report. 3 



Summary. 



Rock tanks are located both in and away from stream 

 channels. The latter variety are basins formed by the 



1 Published by permission of the Director of the U. S. Geological Survey. 



2 Bryan, Kirk : Guide to desert watering places in the Papago Country, 

 Ariz., U. S. Geol. Survey, Water-Supply Paper 490-D, in preparation. 



3 Bryan, Kirk : The Papago Country, Ariz., U. S. Geol. Survey Water 

 Supply Paper in preparation. 



