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great velocity, accompanied by eddies and back currents, is present at the foot of the fall. The erosive effect at the foot of the fall increases with the discharge of the stream in flood and the quantity and character of the sediment carried. Streams of the size common in southwestern Arizona are competent to erode pools about 10 to 20 feet in diameter and 3 to 10 feet deep. The shape of the pool depends on the character of the rock and the amount of modification due to joint-block plucking and pothole formation. Plunge pools form the largest type of channel irregularity and consequently hold the largest pools of water. Most of the larger rock tanks are of this class.

PHYSIOGRAPHIC RELATIONS OF ROCK TANKS

From the foregoing discussion it is obvious that the largest rock tanks are plunge pools at the foot of falls, and that even channel depressions of the other type are likely to be larger in the parts of a stream near falls and rapids. The factors governing the occurrence of falls are thus of importance. Falls are found in southwestern Arizona at localities of at least three types-where there are marked differences in the ability of adjacent parts of the rock to resist erosion; where dissection of a mountain pediment on a new grade produces headwater falls; and where renewed uplift of fault-block mountains produces falls on a stream that crosses the fault line.

FALLS DUE TO DIFFERING RESISTANCE OF ROCKS TO EROSION

Falls due wholly to an unusually resistant rock were found in only one locality in the Papago country and on a minor scale; but the site of falls due to other causes may be determined by a resistant bed, as in the lower falls of Baker Tanks, southeast of Wellton, Yuma County. The variety of circumstances surrounding tanks and the association of tanks of different types is well illustrated by this watering place. The tanks are in a streamway that leads in a northwesterly direction on the southwest flank of the Baker Peaks. Below the tanks the stream channel lies in a small canyon 150 feet wide and about 25 to 30 feet deep. Above the tanks the channel is but 2 to 3 feet below the adjacent plains. Between these two parts of the stream is a stretch of falls and rapids about 1,000 feet long, which is shown in Figure 23. The rock in which these falls occur is a coarse arkosic conglomerate, which strikes N. 77° W. and dips 65° SW. The bulk of the material is a granitic débris with pebbles one-eighth to one-half inch in diameter. The rock is banded and cross-bedded and contains many large boulders, singly or in groups and lenses. The boulders are from 3 inches to 3 feet in diameter and are more or less rounded joint fragments of granite and gneiss. The

bedding is massive, but there are numerous joints, some of which are slickensided.

Since its deposition the conglomerate has been tilted and eroded so that its cut edges now form part of the pediment of the adjacent. Baker Peaks. As in other parts of southwestern Arizona, the pediment is dissected. Headwater cutting of a new canyon by the stream is the cause of the falls. As shown in Figure 23, there is a narrow gorge in the conglomerate beginning 150 feet southeast of the windmill. In this gorge are falls and rapids. From the windmill west for 150 feet is another narrow gorge from 10 to 20 feet wide with walls 10 feet high, the bed of which is a plunge pool. This is filled with coarse sand to a depth said to be more than 20 feet.

The windmill

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Banks of stream are rock covered by thin layer of alluvium

FIGURE 23.-Sketch map of Baker Tanks, Ariz.

pumps water from a pipe sunk in the sand. At the lower end of the little gorge, at the points marked A and B in Figure 23, are potholes, about 3 feet in diameter, developed along joint planes. Below the gorge is a flat about 500 feet long, which ends in a second fall. The flat has an irregular rock floor, the hollows in which are covered with sand. These hollows are due to irregular erosion. At the point marked C there is a hollow somewhat deeper than the others in which water may usually be found by digging away the sand.

The second or lower fall is about 6 feet high and is caused by two beds of very coarse conglomerate that resist erosion somewhat better than the rest of the rock. The downstream bed of coarse conglomerate has been largely cut through; the upstream bed has been cut at only one point, near the abandoned pump. At this place two very perfect potholes about 10 feet in diameter have been formed at one

side of a narrow gully in the rock. Each holds about 4 feet of water, though the total depth of each is about 10 feet. Below the fall is a plunge pool covered with sand, in which water can be found. by digging.

The influence of a notably soft rock in the production of falls is shown at Tabaseca Tank,19 which lies in an arroyo that drains the northern flank of the Chocolate Mountains in sec. 31, T. 7 S., R. 14 E., San Bernardino base and meridian, Riverside County, Calif. The arroyo lies in a narrow gully about 10 feet below the level of the surrounding plain, which is part of a slope cut on rock and extending along the front of the mountains. The plain is underlain by rhyolite, which, in the vicinity of the tank, is broken along a fault plane that crosses the stream and dips upstream about 25° (fig. 24). Shearing and decomposition by water percolating along the fault plane have changed the originally hard rhyolite to soft, claylike gouge about 1 foot thick. The gouge of the fault in its slight resistance to stream

[blocks in formation]

FIGURE 24.-Section along stream channel at Tabaseca Tank, Riverside County, Calif.

action is similar to any other soft rock. As the stream cuts down its bed the triangular block on the upstream side is constantly undermined by the ease with which the clay gouge is eroded. With the waterfall once initiated, the formation of a plunge pool is made easy by the presence of the soft layer, and the channel is excavated below grade. The pool is about 20 feet in diameter and is filled with gravel and boulders. Water is found in the gravel throughout the year unless campers have drawn too heavily on it.

FALLS DUE TO CHANGES IN STREAM GRADE

The pediments of this region, described on pages 93-97, are generally dissected. Straight-walled narrow canyons have been cut by most of the streams issuing from the mountains. (See pp. 99-101.) These little canyons increase in length by headward erosion. At the head of each little canyon is a fall or rapids which marks the separa

19 Brown, J. S., The Salton Sea region, Calif.: U. S. Geol. Survey Water-Supply Paper 497, pp. 248-249, 1923.

tion between the old grade and the new. These relations are brought out in the accompanying diagram (fig. 25).

As each stream that heads in the mountains suffers such a change in grade there are many falls. Though not every fall produces a plunge pool large enough to make an effective watering place, the prevalence of falls due to the dissection of the mountain pediment

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FIGURE 25.-Diagram showing the production of falls and tanks by erosion of mountain pediment on a new grade

is the principal reason for the large number of rock tanks in the desert region. The local conditions at each fall determine the size and effectiveness of the plunge pool and associated potholes as watering places. The variety of these conditions can best be illustrated by an example.

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FIGURE 26.-Sketch map of the Black Tanks, Crater Mountains, Ariz.

Black Tanks (Plate XVII, A) is a well-known watering-place in the Crater Mountains. The tanks consist of a series of plunge pools, numbered from 1 to 8 in Figure 26, with two very temporary water holes, marked 9 and 10. The stream has a sandy bed about 2 feet below its flood plain in the open valley above and west of the tanks. Below the falls section, with its eight tanks, the stream flows in a steep-walled gully with banks from 10 to 20 feet high. The falls and rapids extend about 500 feet, with a total drop of about 50 feet between the upper and lower channels of the stream.

Throughout this stretch the stream runs in a rocky gorge of variable width. The individual falls are determined by the resistance of the horizontal layers of lava that form the bedrock. The larger plunge pools are from 4 to 5 feet deep and 10 to 15 feet wide. On either side of the channel is a rock bench that ranges in width from 50 feet to several hundred feet. This bench slopes gently upward to the foot of the steep slopes of the hills (fig. 26, cross section). On the north side of the stream below the tanks the bench is covered with 2 to 3 feet of cemented gravel. The rock benches extend around the hills and are continuous with similar rock benches that connect with the mountain pediment. It is evident that Black Tanks are the headwater falls of the new canyon that is working headward in an old valley with gentle slopes, which was formed at the same time as the neighboring sloping plain or mountain pediment and forms part of it. Baker Tanks (p. 127) and many others belong to this type.

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FIGURE 27.-Diagram showing production of falls and tanks by renewed uplift of eroded fault-block mountains

FALLS DUE TO RENEWED UPLIFT

Most of the mountains of southwestern Arizona are narrow uplifted strips of the earth's crust bounded by faults on one or both sides. After each uplift streams cut canyons and established smooth grades from the crest of the mountains to the adjacent valleys. In certain ranges renewed uplift took place on one side of the mountains only, probably along the same fault plane on which the original uplift occurred. This uplift resulted in flattening the old stream grades in the higher part of the mountains and produced a cliff or fault scarp across the stream channels on that side of the range. Streams immediately began to cut headward through this fault scarp, and it has generally been removed. However, in certain ranges, the Sierra Estrella and the Tinajas Altas Mountains in particular, the new grade has not yet reached the crest of the range and is separated from the old by falls, as illustrated in Figure 27.

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