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The famous Tinajas Altas (Pl. XVIII, A) is the only watering place that can be definitely ascribed to this cause, but it is thought that a number of other tanks, including Tule Tank (Pl. XVII, B) and Heart Tank, were probably formed in the same way.

Schrader 20 has noted a number of rock tanks on the east flank of the Sierra Estrella at altitudes of 2,000 to 3,000 feet. The topographic map of the Maricopa quadrangle shows that in a number of these canyons the stream grade between altitudes of 1,500 and 3,000

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FIGURE 28.-Sketch map of Tinajas Altas. From plane-table sheet by Kirk Bryan and sketch by C. G. Puffer

feet is so steep as to give rise to almost continuous falls. Above 3,000 feet the grade is flat and the canyon is wide, with gentle slopes. There is thus good evidence that the Sierra Estrella has been rejuvenated by block faulting, and that the tanks are plunge pools and potholes associated with falls due to the dissection of the fault

scarp.

The Tinajas Altas (see pp. 413, 421) consist of eight tanks, distributed for 500 feet along a stream course so steep that it may be considered an almost continuous series of falls. The map in Figure 28

20 Schrader, F. C., report on the Gila River Indian Reservation, Ariz., as to the mineral or nonmineral character of its lands, 1918 (unpublished manuscript in files of U. S. Geol. Survey).

shows that the falls occur in an indentation of the mountain front and that a considerable drainage area lies above the falls. This upland valley, as shown in Plate XVIII, B, has relatively gentle slopes, and the grade of the stream through it is in marked contrast to the grade in the falls section, which is approximately 1 to 1. The rock of this part of the mountains is a coarse pegmatitic granite. Most of the surfaces are stained brown from the deposition of limonite, especially in the cove of Tinajas Altas. In such places there are many caves, reentrants, and pinnacles, which add much to the pictur

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FIGURE 29.-Diagrammatic profile of falls of Tinajas Altas

esqueness of the mountain slopes. The details of the falls and the form and location of the plunge pools are largely affected by the joints of the granite. The master joints strike N. 2° E. and dip 65° NE., and they divide the granite into great slabs from 2 to 10 feet thick. A less perfect system has a strike N. 80° E. and an almost vertical dip. The profile of Figure 29 is made from the plane-table location of the tanks and an aneroid determination of the altitude of the top of the falls by C. G. Puffer. It shows that in general the falls are parallel to the dip of the joints and that the plunge pools occur at places where the joints are closely spaced. In plan, also, the course of the stream is controlled by the joints, as

shown in Figure 30. The steepest grades are parallel to the minor joint system, and the flattest grades to the master joint system. The plunge pools are on the steps developed at the intersections of the joints. The lower tank (No. 1 in fig. 28; see also Pl. XVIII, C) fills with sand after a flood, and therefore the water lasts longer than in the others. It seems likely also that a considerable quantity of water is fed to this tank by seepage from the joint cracks in the granite, as shown in Plate XVIII, C. The second tank is accessible by climbing along a cable fastened to the rock about 75 feet south of the channel. The position of this tank is marked by the round boulder shown in Plate XVIII, A. From the second tank the third is easily reached. The fourth and higher tanks are very inaccessible and are little used except by mountain sheep, many of which fall into the water and drown.

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FIGURE 30.-Idealized diagram to show effect of joint system on course of stream and position of tanks at Tinajas Altas

SAND TANKS

Sand tanks are a variety of rock tanks, formed in stream channels in the several ways above outlined and differing from other rock tanks only in being filled with sand. They are produced when the tail end of a flood carries sufficient sand to fill the cavity; when the later part of the flood is relatively clear the rock tank contains little sand. The sand thus deposited is saturated with water. The upper portion quickly dries, but because the pore spaces between the grains are relatively large and capillary action is unable to bring the water to the surface, further evaporation does not take place. Though for the same size of cavity the volume of water in a sand tank is less than a fourth that in a rock tank, the water commonly remains in it for a longer period after a flood. The use of the water by animals is stricted by the necessity of digging boles down to the water level

and throwing the sand out of the tank. Coyotes are able to do this with great ease, but horses, burros, and cattle have great difficulty 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 the water has been used.

For instance, at Black Tanks tanks 7 and 8 (fig. 26; see also Pl. XVII, 4) are commonly filled with sand, and water is obtained by digging. The basins are relatively shallow, and though No. 8 is considered the better of the two, in neither does water last all the year. Tank 4, however, is usually swept clean by a flood and makes a large pool. But as Nos. 1, 4, and 5 are accessible to stock the water that collects in them 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.

CHARACTERISTICS OF WATER IN TANKS

The water of tanks, on being replenished after rains, is at first muddy, but it rapidly clears, and then by the growth of algae and the decay of organic matter it becomes foul. Sand tanks are likely to have somewhat better water than others, because the sand protects the water from sunlight and contamination. The two analyses given in the table below are probably a fair indication of the chemical characteristics of tank waters. Both samples were obtained in October, 1917. Coyote Water is a place in the axial stream of the Lechuguilla Desert about 8 miles east of Tinajas Altas, where water can be obtained by digging about 4 feet into the sand. The sample from Tinajas Altas (tank No. 1) was collected in October, 1917, after a long dry spell, when the water was particularly foul. In September, 1920, however, the water was sweet and pleasant to the taste.

Analyses of water from water holes

[Analyzed by A. A. Chambers and C. H. Kidwell. Parts per million]

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DIRECT UTILIZATION OF RAIN WATER

RAIN WATER SHED FROM ROOFS

The saving of rain water by various devices has long been practiced in localities where other supplies are inadequate or of poor quality or taste. Many prospectors' tents are equipped with gutters that direct rain water shed from the tent roof into cans and other receptacles.

In and around Ajo during the long period of development of this camp miners and prospectors have contended against the disadvantage of inadequate water supply. Not only was the amount of water small, but many of the wells furnished water containing salts of copper. During the period of construction of the plant of the New Cornelia Copper Co. in 1914 and 1915 the population of the camp was about 5,000, and drinking water was sold by the bucket in stands such as that shown in Plate XIX, A, and peddled on the streets by hucksters. To make up the deficiency in well water, nearly every house was equipped with gutters and tanks to save rain water. A common form of apparatus is shown in Plate XIX, B, which is a photograph of a house in the part of the town now called Old Ajo. Cistern water of this kind was used for drinking and cooking only and thus made to last for a considerable length of time.

Wherever the need is great enough, roofs for the sole purpose of collecting rain water might be erected. Such structures have been used successfully on roads in the deserts of Australia.21 In Sandoval County, N. Mex., a shed with a roof 14 by 20 feet equipped with gutters and a 500-gallon tank supplied water for two men and a saddle horse.

WATER CATCHES

"Water catch" is a term in use in Bermuda, India, and other British colonies for a natural or artificial surface constructed solely for the collection of rain water.22 Such a system of obtaining water has many advantages, and before considering details of construction some examples will be described.

USE OF WATER CATCHES

The water catches that have been in use in southwestern Arizona are all preexisting structures that have been adapted to special use during times of stress. In 1911 and 1912 and part of 1913 a water catch was used for a part of the water supply at Ajo. Samuel Clark

21 Gregory, H. E., Australia, the lonely continent: Nat. Geog. Mag., vol. 30, p. 554, 1916; also personal communication.

Gregory, H. E., The Navajo country: U. S. Geol. Survey Water-Supply Paper 380, p. 120, 1916. See also Mehren, E. J., Engineering impressions of Bermuda: Eng. NewsRecord, vol. 92, pp. 523-525, 1924.

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