The larger the blocks blasted down the more successful the project is likely to be. Such a rock-fill dam can be blasted down in a short time and with only a few men. On this account the project could be undertaken at distant and inaccessible spots, where the cost of transportation of water and supplies is great. The reservoir once filled with débris is stable unless the rocks of the dam are loosened during floods, and at each flood the débris will be saturated with water. The water will seep out at the toe of the dam as a spring for a time depending on the volume of the reservoir and the porosity of the material that fills it. The size of the reservoir must be regulated by local conditions, but it should be as large as possible. The porosity of the material will be somewhat less than 25 per cent, and the size of the pore spaces will depend on the size and arrangement of the rock particles. These conditions can be controlled in part during the process of filling. However, it is impossible to predict what size of pore space will be large enough to absorb water rapidly and yet allow the same water to flow out with sufficient slowness to produce a perennial spring. There is no question that the structure described above will produce a spring. The risk in constructing it lies in the possibility that the spring may have so large a flow as to exhaust the reservoir within a month or two after each flood. GROUND WATER The Papago country is bordered by two regions in which wells obtain water at shallow depths and in relatively large quantities. Along the northern border the valley of Gila River has large supplies of ground water, which have been described by Lee 31 for the area from Florence to Salt River, and by Ross 32 from this point west to Yuma. Along the eastern border wells obtain water at shallow depths in the grassy flats of Nogales Wash and in the flood plain of Santa. Cruz River from Calabasas to the north end of the Tucson Mountains. Ground water has been utilized extensively for irrigation on the Canoa ranch by the Continental Rubber Co. and near Tucson by the Tucson Irrigated Farms Corporation and private individuals. North of the Tucson Mountains the Santa Cruz flows in a broad plain which in the vicinity of Casa Grande is usually called the Casa Grande Valley. In this area many wells are being sunk to obtain water for irrigating long-staple cotton. The ground-water and irrigation possibilities of the Santa Cruz Valley from Calabasas 1 Lee, W. T., The underground waters of Gila Valley, Ariz. U. S. Geol. Survey WaterSupply Paper 104, 1904. Ross, C. P., The lower Gila region, Arizona: U. S. Geol. Survey Water-Supply Paper 498, 1923. to Gila River have for a number of years been under investigation by Prof. G. E. P. Smith, of the University of Arizona. The results of his work will be published as a bulletin of the Agricultural Experiment Station of the university. The great area south of Gila River and west of Santa Cruz River has few wells. Moreover, the wells that reach water at shallow depths yield but small amounts, and in those of large yield the water lies so far below the surface that irrigation on a large scale is impracticable. Sufficient water can be obtained for use in mining, however, and for the number of cattle the region will support. The cattle industry has languished primarily because of the small amount of forage, which, except for the short-lived grasses that spring up after the rains, consists of browse. Consequently only a few cattle can be permanently supported at any single watering place. The present investigation, which is summarized below, has shown that a very large increase in the number of wells is possible, and that these wells will yield enough water to supply as many cattle as can be maintained on the available forage. The high prices for cattle prevailing from 1915 to 1919 and the use of better cattle ranges for dry farming accelerated the movement of stock raisers into the poorer forage areas. In spite of the temporary depression of 1920 and the following years this movement will eventually establish in the Papago country, outside of the areas set aside for the Indians, a permanent though sparse stock-raising population. In the following discussion the tables include only those wells for which rather complete data are available. Within the Indian reservations there are a large number of shallow dug wells used by the Indians, for which depth and location are known. Most of them lie off the main routes of travel and are of interest principally because they furnish additional proof that a great increase in the number of wells can be made in other parts of the region, particularly in the pediment areas at or within the borders of the mountains. SOURCE AND AMOUNT OF GROUND WATER The greater part of the water circulating in the ground within a depth that makes it recoverable by man falls on the surface as rain or snow, sinks into the ground either directly or after running a short distance in a stream, and circulates in the ground under hydrostatic pressure. In a few places, one of which is noted below, the deep-seated waters of the earth's crust come to the surface. It is, however, much easier to observe and study the work of the deepseated water on the rocks than to demonstrate that any particular spring or well has water of such an origin. The amount of ground water in a region is governed by the climate, the character of the rocks, and the topography. The rocks merely form reservoirs to which water may be added or taken away. It is evident that a region in which precipitation, either as snow or rain, is small will have only small annual additions to its groundwater reservoirs, and it is only these annual additions which are available to man. In the Papago country the mean annual rainfall ranges from about 3 inches at Yuma to 12 inches at Tucson, an amount very small in comparison with the precipitation of more favored regions. The prevailing high temperatures lead to excessive evaporation from free water surfaces, which according to published records of observations made at Yuma by the United States Weather Bureau amounted to an average of 75 inches a year for a period of six years. The rain, to be conserved as ground water, must sink into the ground rapidly, or it will be lost by evaporation in the streams. All but one of the basins of the Papago country have watercourses that drain to the sea, but the loss of water from the region through these streams is probably small. Most of the water that falls as rain wets the upper soil and immediately evaporates; but a part reaches deeper soil zones, or the water table. A large share of the water that does not evaporate immediately is lost through transpiration from plants. No tests on the rate of transpiration have been made, but the devices for protection from excessive transpiration exhibited by all the desert plants indicate that such losses of water are much heavier than in other regions. Each plant, then, whose roots lie in the belt of soil moisture or reach ground water helps to reduce the annual supply of available ground water. The Papago country, like much of the rest of the western United States, consists of valleys filled with alluvium lying between ranges of mountains, but it is unlike many other areas in that the mountains receive almost as little rain as the intervening valleys. The mountains are, however, composed of almost impervious rocks that can not absorb any large part of a rain, and therefore they contribute water to the valleys in the form of floods or ephemeral streams. The reservoirs of porous alluvium in the valleys receive that part of the rain which is not lost by direct evaporation, evaporation from streams, evaporation from the soil, transpiration from plants, or runoff in streams that terminate outside of the area. The total of this annual increment is small, and thus the possibilities of obtaining large supplies of water for irrigation are also small. Many more wells than now exist can be dug, however, and eventually wells may be located close enough together to make all the land available for grazing, but the development of large tracts irrigated by ground water is impracticable. VEGETATION AS AN INDICATOR OF GROUND WATER It is a matter of common observation that certain plants require more water than others and that certain plants tend to grow in lines fringing streams or in colonies and bands where water is close to the surface. The use of plants as indicators of the existence or depth of ground water is, however, beset with pitfalls. Most species have a considerable range in water requirements, and it is difficult to fix the limits of range. Again, other factors, such as unsuitable soil, floods, or fire, may cause the absence of a species from a localty in which the water requirements are met. Salt grass grows only in moist soil, and its presence may usually be considered a reliable indication that the ground is saturated within 10 feet of the surface. It occurs sparingly along Santa Cruz River but is abundant in the ciénaga near Gila Crossing, along Gila River, and near a number of springs. The absence of salt grass in the central parts of the interior valleys of the region is due to the greater depth to permanent water in those areas. 33 Mesquite has in some parts of Arizona great value as an indicator of shallow water. Along Gila and Santa Cruz rivers the mesquite groves undoubtedly draw a large part of their supply of water through their roots from the underlying water table. In the interior valleys, however, mesquite trees and even large groves of mesquite exist where the ground water is from 60 to several hundred feet deep and thus seem to be dependent wholly on rain and flood water. It is noticeable, however, that the mesquite in these localities is more erect and more treelike than the mesquite of the areas of shallow ground water. This contrast is evident in the photographs reproduced in Plate XXIII. The tree shown in A is typical of the mesquite groves south of the Casa Grande Ruins, where the depth to water ranges between 30 and 40 feet. The sprawling habit is perhaps more common in this locality than near Tucson but is characteristic of most of the trees in Gila Valley west of Gila Bend. The tree shown in B is one of a group at the abandoned Fresnal ranch, where the ground water is more than 60 feet below the surface, and has the upright habit typical of trees in the interior valleys of the region, though most of them are somewhat smaller. Other desert plants are thought to be indicative of shallow ground water. Of these, batamote (Baccharis glutinosa), desert willow (Chilopsis linearis), and the cherioni or wild china tree (Sapindus marginatus) are the best known, and their virtues as water indicators are extolled by the old settlers of the region, largely on the basis of supposed Papago wisdom in regard to water. It seems likely that 23 Meinzer, O. E., and Kelton, F. C., Geology and water resources of Sulphur Spring Valley, Ariz. U. S. Geol. Survey Water-Supply Paper 320, p. 183, 1913. these plants require somewhat more water than mesquite, catsclaw, palo verde, and palo fierro, but this need can be supplied by small amounts of water stored in joints and fractures in the rocks or held above a bed of clay or other compact material in alluvium, which would be insufficient to supply a well. Along the border of the Baboquivari and Tumacacori mountains the cumaru or hackberry, especially the large species Celtis occidentalis, which grows to a height of 50 feet, the Arizona sycamore (Plantanus wrightii), and the leatherleaf ash (Fraxinus velutina) are found near springs and seeps and may be taken as reliable indicators of ground water. The cottonwood (Populus fremontii) is found only in the flood plains of Nogales Wash, Santa Cruz River, Arivaca Creek, Sonoita River, and Gila River. Another species (Populus macdougalii) which is indistinguishable by the layman from the common cottonwood, is found on Colorado River below Yuma. The cottonwood is a reliable indicator of shallow ground water, but it requires so much moisture that its absence is not a sign that water is lacking. In locating wells the evidence of the presence of water furnished by the native vegetation should be given due weight, but to locate a well at a certain place on the evidence of a single cherioni tree, for example, may lead to failure. The presence of the tree may, however, confirm a judgment arrived at by a consideration of the conditions under which water occurs. These physical conditions, most of which depend on the character and structure of the rocks, are reviewed on pages 167–188. During the progress of field work samples of water were collected for analysis from 2 springs, 1 stream, 2 water holes, and 25 wells. The analyses were made in the water-resources laboratory of the United States Geological Survey by A. A. Chambers and C. H. Kidwell. The analyses of the two spring waters and the only stream water, Sonoita River, are given on pages 163 and 167. The analysis of water from the Blankenship Well is reprinted in the section in which the analyses of waters from shallow wells in alluvium are discussed. METHODS OF ANALYSIS The analyses given in the tables (pp. 163, 167, 172, 176, 184, 185) were made substantially according to the methods outlined by Dole.35 In the preparation of this section the author has benefited by the complete revision of his original draft by W. D. Collins. Dole, R. B., The quality of surface waters in the United States east of the one hundredth meridian: U. S. Geol. Survey Water-Supply Paper 236, 1909. |