At ordinary high water, caused by the annual rising of the Gila River, or of the Colorado in its upper reaches, which events usually occur at different seasons of the year, the meter has not been known to show a depth of over 28 feet, but in consequence of a co-equal rise in both streams during the past season, the meter registered over 33 feet. This excess filled the river swamp on the right bank, where the erosive force of the current was most effective.
The course of the river below Yuma for several miles is to the west, until the neighborhood of El Rio station is reached, where sandstone bluffs on the right bank deflect the current back to its general south course. A little back from the river below this point is a small, isolated range of hills, known as Pilot Knob, formed of granite, fissured with seams of dioritic rock. The granite shows lamination, and the surface of the rocks is polished by the constant attrition of the drifting sands. The beach near Pilot Knob is cemented into a calcareous conglomerate, underlaid by sand in a loose state. After resuming its southern course the river for several miles follows a straight reach, until below Hanlon’s Ferry. The tendency of the current to bear more to the right bank shows itself now in the formation of sloughs and inlets that absorb a large proportion of the water and form several islands, shallowing that part of the river that passes down the proper channel.
SECTION OF COLORADO RIVER AT YUMA
DETAIL OF BREAK
IN
COLORADO RIVER
The accompanying sketch shows the different channels in detail. Formerly the water from these outside channels returned through the bend marked “sand” to the main stream, but at the present time it passes through a crevasse in the bend, marked Tapscott and Carter Rivers, furnishing the largest portion of the water that ultimately found its way into the depression of Salton Lake. During the extreme high water the western bank from El Rio down, which is formed entirely of silt, was broken through in numerous places, and the mostly shallow channels either entered the river again below, or accumulated their waters on the back part of the flood plain, and by following the natural hollows and gullies gradually wound their way in the direction of the old Yuma road. The great force of the current soon cut down below the level of the river bed and relieved in part the congested condition. The grade being gradually to the west, the water cut through the sediment in that direction, partly to the Yuma road, partly farther south parallel to the Rio El Medio, which, judging from analogy, owes its origin to some former similar condition of the river.
Tapscott River may have been formed prior to Carter River, as it has a better defined channel; the latter has no regular channel, but seems to have been caused by the excess of water that could not enter the former, following depressions and sloughs which have no direct connection with one another. At the time these observations were made, the estimated volume of water in the river at the Yuma bridge was about 3,700 cubic feet per second, and at the crevasse forming Tapscott and Carter Rivers, two fifths of the entire volume of water in the river was passing through, but none of it was reaching Salton Lake. Of this two fifths not more than 20 cubic feet per second was running through Carter River.
Beyond Alamo Mocho, which is on the old Yuma road, at the time of the flood, the water flowed into a number of lagoons, and thence into New River, following the old channel that was cut at the time of the 1849 overflow, and thus reaching and forming Salton Lake. All of the water leaving the main river at present finds its way down through the Hardy Colorado back to the Colorado proper, or evaporates.
In the beginning of this article mention was made of a borehole that was sunk by the new Liverpool Salt Company in the salt marsh, and which revealed under a thin crust of mud and salt a depression filled with ooze, composed largely of magnesium and sodium salts. This ooze is probably the final resultant of the evaporations of the former sea water, and which, from the well-known avidity of the magnesian salts for moisture, is kept in this condition of ooze through the natural drainage and seepage waters. This depression may extend beyond the borders of the present marsh toward the gulf, covered over by the sand, and may have some relation with the small so-called mud volcanoes southeast of the lake near the station of Volcano, as well as with some openings in the ground mentioned by one of the gentlemen who investigated these regions between the Colorado River and the lake, and of which he stated that any amount of water running into them did not seem to fill them. Should such a connection exist, the water pouring in from the river would have dissolved the thin salt crust where exposed, and opened a channel, permitting a rapid filling in of the water which formed the lake, for otherwise a body of water running over this light sand and under such a dry, hot temperature would hardly have filled up the depression as rapidly as was the case. Besides, this would explain the reason for the fresh waters of the Colorado River attaining so rapidly a density of over 20° Baumé. Should this supposition prove correct, and it could easily be ascertained by a series of shallow boreholes, it might be expected that every large overflow in the Colorado River in this direction would result in a repetition of Salton Lake.