The desert climate of the Salton Trough is characterized by extreme aridity and high summer temperatures. Average annual precipitation is slightly less than 3 inches on the valley floor and about 40 inches at the crests of the San Jacinto Mountains. Maximum summer temperatures commonly exceed 104 degrees Fahrenheit, and winter minimums are seldom below 32 degrees Fahrenheit.
The Salton Trough is about 130 miles long and as much as 70 miles wide (fig. 46). The trough is a landward extension of the depression that is partially filled by the Gulf of California. The trough and gulf are separated by the broad fan-shaped subaerial delta of the Colorado River. Much of the land surface of the trough is below sea level and, before the delta was formed, the trough may have been part of the Gulf of California. The lowest part of the trough is occupied by the Salton Sea, whose surface is at an elevation of more than 200 feet below sea level. Most of the surface drainage is intermittent and is toward the Salton Sea in the central part of the valley. Surface water moves northwestward to the Salton Sea from the boundary between the United States and Mexico and southeastward from the San Gorgonio Pass.
The Imperial Valley occupies the wider southern part of the Salton Trough. The valley ends at the Salton Sea to the northwest and continues southward into Mexico as the Mexicali Valley. The Chocolate Mountains border the valley to the northeast, and the Peninsular Range of Baja California and southern California borders it to the southwest. The floor of the valley slopes northwestward from about sea level at the international boundary to about 230 feet below sea level at the Salton Sea. The Salton Trough was once occupied by prehistoric Lake Cahuilla. In the eastern and western parts of the trough, several ancient lake shorelines (fig. 47) are at elevations of 42 to 50 feet above sea level.
The central part of the Imperial Valley is a large area of cultivated land entirely within the shorelines of prehistoric Lake Cahuilla. Most of the central Imperial Valley is a monotonous plain dissected by the Alamo and the New Rivers, which have incised trenches as much as 40 feet deep in soft, silty lacustrine deposits. Much of the entrenchment took place during 1905 through 1907 when virtually the entire Colorado River flowed uncontrolled in these channels and established the present-day Salton Sea. Before 1905, the center of the trough had been a playa. The lowest point of the surface of the trough is beneath the southern part of the Salton Sea and is about 275 feet below sea level.
The Coachella Valley in the northwestern part of the Salton Trough (fig. 46) extends from the east end of San Gorgonio Pass southeastward to the Salton Sea. It is bordered on the north and east by the Little San Bernardino Mountains and on the southwest by the San Jacinto and the Santa Rosa Mountains. The Whitewater River is the main drainage in the valley.
The Coachella Valley is affected by the San Andreas Fault system, which is a complex strike-slip fault system that includes the Mission Creek, the Banning, the Garnet Hill, and the Indio Hills Faults and associated folds (fig. 48 below). These faults act as barriers to ground-water flow and, combined with constrictions in the basin width and changes in permeability of the water-yielding units, have compartmentalized the Coachella Valley into the Desert Hot Springs, the Mission Creek, the Garnet Hill, and the Whitewater River ground-water subbasins (fig. 48). The Whitewater River subbasin is the largest of the four and contains the most productive aquifer.
The Gulf of California and its landward extension, the Salton Trough, are structural, as well as topographic, depressions beneath which consolidated rock is thousands to tens of thousands of feet lower than the consolidated rock in the bordering mountains. The Gulf of California and the Salton Trough formed during late Cenozoic time as a result of spreading of the ancient sea floor.
The aquifer system in the Salton Trough ranges from unconfined in the periphery of the trough to confined in the central part. It consists of basin-fill deposits of Quaternary and Tertiary age (fig. 49); these deposits of alluvium are underlain by rocks of pre-Tertiary age that are referred to as the "basement complex." Although the basin fill probably is more than 20,000 feet thick, the water-yielding parts of the basin fill extend only to depths of a few thousand feet. The water at greater depths is too saline for most uses, and the hydraulic connection between the shallow and deep deposits is poor.
Near the margins of the Imperial Valley, the basin-fill deposits were derived from the adjacent mountains and are mostly coarse sand and gravel. Deposits in the central part of the valley consist mostly of fine-grained sand, silt, and clay that were deposited by the Colorado River. In the eastern and western parts of the Imperial Valley, wells that are open to several hundred feet of the basin-fill deposits yield moderate to large volumes of water. Transmissivity values of 20,000 to 30,000 feet squared per day are characteristic of these deposits, and wells that yield 50 gallons per minute or more per foot of drawdown are attainable. In the central part of the Imperial Valley, transmissivity values of 150 to 1,500 feet squared per day were calculated from two aquifer tests of wells completed in the upper 500 feet of the fine-grained deposits.
Aquifer material in the Coachella Valley is mostly coarse-grained sediments, and the aquifer is generally unconfined. These deposits are more than 3,000 feet thick, are moderately to highly permeable, and yield large quantities of water to wells. Transmissivity is greatest in the central part of the valley from Palm Springs to the Salton Sea because of the great thickness of permeable deposits. Maximum transmissivity values of about 25,000 feet squared per day are similar to those reported from the Imperial Valley.
Unlike the other valleys discussed above, the most important source of ground-water recharge to the Imperial and the lower Coachella Valleys is the Colorado River, not runoff from the surrounding mountains. Minor sources of recharge are ground-water inflow from adjacent areas, infiltration of precipitation that falls on the valley floor, and local runoff from the mountains that border the area.
The Colorado River has been a source of recharge to the aquifer system of the Imperial Valley since the river delta built to a height sufficient to separate the Gulf of California from the Salton Trough. As the delta was built, natural levees beside the river channel kept the Colorado River above the land surface altitude in much of the valley. Under natural conditions, water from the river seeped downward through the river bed and then moved laterally to recharge the aquifer in the Imperial Valley. The water also moved into the Imperial Valley from the Mexicali Valley to the south and through a section of alluvium northeast of the Cargo Muchacho Mountains.
Since 1901, recharge to the shallow part of the aquifer system under natural conditions has been augmented by percolation of water imported from the Colorado River beginning in 1901. Originally, water was diverted about 10 miles southwest of Yuma, Ariz., at the confluence of the Colorado River and Mexico's Alamo Canal, and was delivered to the Imperial Valley through the Alamo and the New Rivers (fig. 46). The completion of the All American Canal (fig. 46), which permitted the diversion of Coloraro River water to the Imperial Valley through a canal located entirely in the United States rather than along a route that passed through the Mexicali Valley, greatly increased the opportunity for ground-water recharge. The All American Canal became the sole means for diverting Colorado River water to the Imperial Valley in February 1942. Six years later, the Coachella Canal was completed and thereafter supplied water to the lower part of Coachella Valley.
The canals, which are as much as 200 feet wide, are major sources of recharge because they are unlined, flow across many miles of sandy terrain (especially in the eastern part of the Imperial Valley), and are much higher in altitude than the general ground-water levels along their course. The leakage from the canals almost immediately caused ground-water mounds to form beneath the canals, and, over time, ground-water levels rose to the water level in the canals. The leakage also spread horizontally, thereby causing water levels to rise over large areas. Water levels eventually rose to the point that much of the leakage, especially from the All American Canal, was discharged to drains and areas of natural discharge, rather than continuing to add to the quantity of ground water stored in the aquifer system.
The rise in water levels that resulted from leakage from the easternmost canals between 1939 (before the canals were completed) and 1960 is shown in figure 50 below. The water-level rise along the All American Canal was generally more than 40 feet, and the rise along the Coachella Canal was about 40 feet near the junction of the canal with the Colorado River and gradually increased northward to more than 70 feet. Throughout most of the length of the East Highline Canal, which began operating in 1942, the original water table was shallow, and the water-level rise was small.
Water losses along selected reaches of the All American and the Coachella Canals are shown in figure 51. The annual flows are generally 3,000,000 to 4,000,000 acre-feet in the reach of the All American Canal and are about 500,000 acre-feet in the reach of the Coachella Canal. From 1950 through 1967, the average leakage from the two reaches was about 215,000 acre-feet per year.
The general direction of ground-water movement in the basin-fill aquifer of the Imperial Valley and the lower part of the Coachella Valley is shown by the arrows superimposed on the water-level contour lines in figure 52. The contours were based on water levels in 1965 in wells completed in the main water-yielding zones.
The broad ground-water mound in the southeastern part of the valley is the result of leakage from the All American and the Coachella Canals. Between the canals, the direction of ground-water movement is principally westward, but south of the All American Canal, the movement is toward the Mexican border. Away from the canals, ground water moves generally toward the axis of the valley and then northwestward to the Salton Sea. The principal area of discharge is the central, cultivated part of the valley. Substantial amounts of ground water are discharged to the Alamo River, as indicated by the closely spaced contour lines on the eastern side of the river and the change in direction of the contours which indicates that the ground water flows primarily northward. Ground water also discharges to the New River, but the configuration of the contour lines, which show a relatively wider spacing and moderate upstream displacement, indicates that considerably less ground water moves to the New River than to the Alamo River.
For ground water that moves from the adjacent mountains toward the center of the valley, a wide range of contour spacing can be seen in figure 52 that indicates changes in the hydrogeologic conditions within the aquifer. Some of the seemingly abrupt changes in contour spacing are caused by faults that are barriers to ground-water flow. The closely spaced contours west of the Coachella Canal, near Niland, and west of the Salton Sea, near Salton City, result from ground-water flow through deposits with low permeability.
Ground-water movement in and through the basin-fill aquifer of the upper Coachella Valley is affected by the San Andreas Fault System. Faults that are part of this system, as well as constrictions in basin width and changes in permeability of the water-yielding units, have compartmentalized the valley into four ground-water subbasins (fig. 48).
The general direction of ground-water movement in the upper Coachella Valley was determined from water-level contour maps of the basin for 1936 and 1967 (figs. 53 and 54). Water-level contours during the autumn of 1936 are shown in figure 53. The water-level profile for 1936 (fig. 55) shows the slope of the ground-water surface to be very steep in places and to exceed 50 feet per mile from the San Gorgonio Pass to Windy Point. This steep gradient decreased to less than 10 feet per mile just south of Palm Springs because of the increased width of the basin. From the Thousand Palms area southward, the gradient was about 20 feet per mile.
A water-level profile for 1967 (fig. 55) shows that while water levels declined in most places in the valley, the steep water-level gradient from the San Gorgonio Pass to Windy Point remained the same as in 1936. However, the water-level gradient at the south boundary decreased to about 10 to 15 feet per mile from the 1936 gradient of 20 feet per mile. The lowered water-level altitudes resulted from increased withdrawals within the Whitewater River subbasin. Water levels for 1967 were about the same as those for 1936 in the extreme southern part of the upper valley as a result of leakage from the Coachella Canal.
The general direction of ground-water flow in 1967 is shown in figure 54 by arrows that represent flow lines, which are the shortest possible paths between adjacent water-level contours. Ground-water movement in the Whitewater River subbasin was primarily parallel to the axis of the valley from Windy Point to Indio. The flow lines near the faults that mark the borders of the subbasins indicate that ground water flowed across the faults; exactly how much is unknown.
Ground-water development in the Salton Trough has been primarily in the Coachella Valley. The growth of agriculture and, since the early 1950's, of tourism has drawn heavily on the ground-water resources of the upper Coachella Valley. Ground-water levels have declined as annual withdrawals increased more than tenfold during 1936 to 1967 (fig. 56). In the lower Coachella Valley, concern over the diminishing ground-water supply as a result of agricultural development prompted the construction of the Coachella Canal. Water delivery began in 1949 when large quantities of Colorado River water were brought to the area between Indio and the Salton Sea. However, the upper Coachella Valley received only small quantities of the canal water and, consequently, water levels continued to decline as ground-water use increased in that area.
Water levels began to decline before 1945 (fig. 57) in the southernmost part of the upper Coachella Valley but did not change significantly in the remainder of the upper valley until about 1945 when major withdrawals began (fig. 56). Water levels continued to decline throughout most of the upper valley through 1965 with the exception of an area near the southern boundary where water levels had ceased to decline and began rising. The rise is documented by observation wells 7 and 8 in 1949 and 1954, respectively (fig. 57). The water-level rise in these two wells can be attributed to water from the Coachella Canal leaking downward to the aquifer. The effect of leakage appeared to be moving up the valley in 1955, as indicated by the hydrograph of well 6 (fig. 57), in which water levels ceased to decline, indicating recharge to the aquifer by leakage.
From 1936 to 1967, water-level changes in the upper Coachella Valley (fig. 58) were most prominent in the White-water River subbasin. The Palm Springs area in that subbasin showed the largest water-level decline (nearly 80 feet) because of a concentration of withdrawals in an area with a relatively low aquifer storage capacity located near where the aquifer abuts the nearly impermeable bedrock of the San Jacinto Mountains. Decreases in water levels were probably amplified between 1946 and 1967 because of a prolonged drought in the upper Coachella Valley, as in most of southern California, during those years. A representative plot of the departure from average precipitation in the San Jacinto, the Santa Rosa, and the San Bernardino Mountains (fig. 59) indicates that during the dry period from 1947 through 1964 only four years were wet--1952, 1954, 1957, and 1958. This extended dry period greatly reduced the natural inflow available to the valley. The other three ground-water subbasins showed very little water-level decline because withdrawals in those subbasins were small. A well field for the town of Desert Hot Springs on the east side of the Mission Creek Fault showed some decline because the storage capacity of the aquifer in that area is limited by the juxtaposition of the aquifer with a relatively impermeable fault and the nearly impermeable bedrock of the Little San Bernardino Mountains.