was then operating a lease on the mining property called the Ajo Consolidated, and the existing wells and cisterns gave an inadequate water supply. A concrete-lined sump 10 by 12 feet and 4 feet deep was constructed in an abandoned prospect drift (fig. 31). This drift had been driven into a side hill from the bed of one of the short gulches leading down from the summit of the Little Ajo Mountains. A small boulder dam diverted the flood water of the gulch into the drift. A screen kept out the larger rubbish, but a large amount of organic matter, leaves, and twigs was carried into the sump. This material decayed and gave the water a vile taste and smell. However, at the end of three or four months putrefaction ceased and the water became clear and wholesome and was an important addition to the water supply.

At the Fortuna mine, 15 miles south of Blaisdell, in Yuma County, there are three water holes which supplement water taken from the

FIGURE 31.-Diagram showing water catch at Old Ajo

main shaft of the mine. All three were dug as prospect holes, and their subsequent use as water catches has arisen during the last 15 years. During this period the mine has been shut down, and the pipe line from Blaisdell has been abandoned. Two of the holes are about 200 yards east of the mine, in the main gulch. One of these is a vertical shaft at one side of the streamway. It fills with flood water, but because rats and other small animals continually fall in and drown the water is not used. The other hole is an inclined shaft parallel to the schistosity of the rocks, which dip into the hillside. It is on the south side of the canyon about 75 feet above the stream bed. Rain water drains to this prospect hole from a hillside. The drainage area is about 200 feet wide and 750 feet long. It is a rocky boulder-strewn slope inclined between 40° and 50° from the horizontal. Rubbish of various kinds, leaves, and twigs are carried into the hole, and putrefaction is active after a rain. Rats seldom drown here, probably because the inclined shaft allows easy entrance and

exit, so that they can come and drink without danger of falling into the water. The water does not become fit to drink for two or three months after a rain. At the time of visit the water was clear but slightly colored and had a pleasant taste. The surface was covered with floating clots of brown algae, mostly collected around the bodies of dead bees. There were also drops of crude oil, which had been put on to prevent the breeding of mosquitoes. The third hole is south west of the mine, among some small hills. It is an inclined shaft 6 by 8 feet and said to be about 50 feet deep. The water drains from a basin having an area of about 25,000 square feet into a hole 4 by 8 by 5 feet deep; thence it flows more slowly through a dry wall about 4 feet thick into the shaft. The wall was originally laid to prevent slumping into the shaft but now forms an admirable screen

FIGURE 32.-Sketch map showing drainage area at water catch near Fortuna mine, south of Blaisdell, Ariz.

for straining out clay and trash. The whole arrangement is shown in the accompanying sketch map (fig. 32) and in Plate XX, B. The drainage area is covered with loose blocks of rock from 4 inches to a foot in diameter, and below them is a scanty soil. A few bushes are scattered at intervals of 5 to 10 feet. The mouth of the old prospect hole is surrounded by a fence, and in Plate XX, B, the drainage area lies to the reader's left of the prospect hole and of the man seated on the hillside. Because dirt and trash are arrested and do not go into the hole the water in it sweetens more rapidly than in the others. At the time of visit it was clear and palatable but had a brown color. If the drainage slope were thoroughly cleared ing plants, all the loose rock stacked in piles, and the thin t up and carried off, this would make an admirable water

CONSTRUCTION OF WATER CATCHES

The construction of a water catch involves the selection and preparation of a site and the construction of a cistern or container. The site selected should have the maximum of bare rock surface and the minimum of soil and vegetation. It is obvious that such requirements can be met most generally by mountain slopes. Excessively large drainage areas should be avoided, because of the expense involved in cleaning and fencing them. Granite and gneiss are most likely to furnish suitable surfaces. Lava is likely to have many cracks, and some lava beds are so porous that the run-off from them in small showers is likely to be almost nothing. Places can be found, however, where the lavas are very thick, uniform, and free from cracks. Certain massive conglomerates, such as those which form the Ajo Peaks, erode with great bare rock surfaces, but they absorb considerable rain, so that on such rock larger drainage areas should be provided.

Where the rock surface does not drain naturally to a single outlet, masonry or concrete walls should be built to direct the water. The cracks in lavas may of course be cemented, or wholly artificial surfaces of concrete may be constructed on hillsides of soft material, but it is thought that in general the expense involved in such work is too great. On ordinary rock surfaces all the bushes should be cut, the soil swept up, and loose rock piled. Free movement of rain water can then take place, and a minimum of dirt and trash will be carried to the cistern. A strong fence, preferably of woven wire, should be provided to prevent contamination of the surface by animals.

The cistern or water container may be built above or below ground. The simplest arangement is a shaft, similar to those at the Fortuna mine, but the difficulty of providing a suitable screen to prevent the entrance of trash with the water and to exclude animals is great. A cistern built partly or wholly above ground has the advantage that water is readily taken from the bottom and suitable screens may be provided. Figure 33 shows such a cistern, 13 feet square and 10 feet high (inside measurements), providing a storage capacity of 12,000 gallons. It requires 30.8 cubic yards of concrete, which, with a mixture of 1 cement to 2 sand and 3 gravel and plastered inside, will require 56 barrels of Portland cement. If reinforced with iron rods spaced 8 inches apart it will require 690 feet of half-inch rods.

AREA OF WATER CATCH REQUIRED

The area of prepared surface required to fill a given cistern is difficult to estimate. It is obvious that the smoother and less absorptive the surface and the greater the slope the more efficiently

will the water catch work. At the Fortuna mine the catchment surfaces are very imperfect, yet the smallest, having a surface of only 25,000 square feet, furnishes enough water to fill the prospect hole, which has a capacity of approximately 15,000 gallons.

SANITARY CONSIDERATIONS

Rain water is practically free from mineral salts, and in sparsely settled regions it lacks disease germs. If conveyed over a bare rock surface to a clean tank or cistern, it remains clean and palatable. At Ajo and the Fortuna mine the water saved by the water catches

FIGURE 33.-Reinforced-concrete cistern for water catch. Designed by A. J. Ellis

contains trash and vegetation of various kinds, which rots or putrefies. This putrefaction is due to bacteria that live in darkness, for in all the examples cited the stored water is protected from direct sunlight. When the available vegetable matter has been consumed by the bacteria no further action can take place. The water is then free from disease germs and only slightly discolored by organic matter. If the water were stored in open pans or reservoirs and not protected from sunlight, other bacteria and many algous plants would grow in the water, die, and in turn decompose. Thus a continuous process would keep the water unfit for drinking. It is therefore essential that light should be excluded from the cistern. Any leaves or twigs that pass the protective screen will be decomposed in a

single period, at the conclusion of which the water will be fit for drinking. The cleaner the water catch the shorter this period will be. With thorough precautions against admitting any organic matter to the cistern, no putrefaction will ensue.

All bodies of water in the desert attract animals, and many watering places are defiled by their dead bodies or excreta.. The smooth, shelving sides of rock tanks are veritable death traps for mountain sheep. As the water gets low the sheep take greater and greater risks in drinking, and some of them fall in and drown. Rats, mice, and rabbits are similarly trapped. Efficient screening is the best protection against contamination by animals, because by using a sufficiently fine mesh bees may also be excluded. Bees are common in the desert, hiving in the caverns and crannies of the rock. At most watering places they drown in large numbers, and their dead bodies. give water an unpleasant taste and odor. In a cistern of the design recommended above the same screen that excludes trash serves to exclude animals also.

The chance of introduction of disease germs into cisterns is much reduced in a desert region because of the sparse population. Even crude devices such as those at the Fortuna mine (p. 137) are not likely to be contaminated by disease germs. Travelers may protect themselves if water from such a supply looks particularly foul by boiling it before use. Boiling will probably make the danger from disease germs negligible but will hardly increase the palatability of really foul water.

RESERVOIRS

PURPOSES

In southwestern Arizona many small reservoirs for the storage of flood water are built by individuals and small companies, largely without expert advice. The number of such enterprises could be increased with great benefit to the region. The following paragraphs review the conditions under which dams must be built and point out the best practice. Large irrigation and power projects are not likely to be considered for most of the region, because of the absence of great rivers. Moreover, such enterprises present many special problems in engineering and finance, the correct solution of which can be arrived at only by large organizations with a competent technical staff.

The problems involved in the construction of the smaller reservoirs vary somewhat according to the purposes for which they are built. Four general purposes are common-supplies for mines, stock-watering places, domestic supplies, and irrigation.

Mines require comparatively large quantities of water for milling operations and the use of the people employed. Shallow wells in