K. Bryan — Rock Tanks and Charcos. 203 



tion quickly dries, but because the pore spaces between 

 grains are relatively large, and capillary action is unable 

 to bring the water to the surface, further evaporation can 

 not take place. Though for the same size of cavity the 

 volume of water in sand tanks is less than a fourth that 

 of rock tanks, the water commonly remains for a longer 

 period after a flood than in rock tanks. The use of the 

 water by animals is restricted because it is necessary for 

 them to dig holes down to the water level and throw the 

 sand out of the tank. Coyotes are able to do this with 

 great ease, but horses, burros, and cattle have great diffi- 

 culty in digging in the sand. Many rock tanks, on the 

 other hand, are so accessible to wild animals and stock 

 that within a few days after they are full all of the water 

 has been used. 



For instance, at Black Tanks, tanks 7 and 8 (fig. 6) 

 are commonly filled with sand, and water is obtained by 

 digging. The basins are relatively shallow, and though 

 No. 8 is considered the best, in neither does water last 

 all the year. Tank 4, however, is usually swept clean 

 and makes a large pool. But as Nos. 1, 4, and 5 are 

 accessible to stock the water is rapidly used up under 

 ordinary circumstances. When, however, as sometimes 

 happens, a flood leaves No. 4 tank full of sand, the water 

 in the sand will last throughout the year. Nos. 2, 3, and 

 6 are too small to be of moment. Nos. 9 and 10 are very 

 shallow. Thus, in dry seasons water is more likely to 

 be found in the sand tanks Nos. 7 and 8 than in the clean 

 rock tanks. 



Charcos. 



Charco is the name applied to natural water holes in 

 adobe flats and washes in the Papago Country, but in 

 other parts of the southwest, the same type of water hole 

 is called a mudhole, or mud tank, or is not distinguished 

 from other types of tanks and "tanques." Charco is a 

 Spanish word signifying a pool of standing or stagnant 

 water, and has the advantage of being a distinctive name. 

 The word "tank" may then be reserved for natural 

 reservoirs in rock. 



Charcos are found as single pools or a series of pools 

 along streams which deposit fine-grained material, usually 

 sandy clay or adobe. They vary from shallow pans 18 

 inches wide by 6 feet long, to depressions 5 to 6 feet deep, 

 15 to 30 feet wide, and more than 1,000 feet long;: see 

 fig. 12. B ' 



