By E. B. Preston, E.M., Assistant in the Field.
The Coahuila Valley, in which Salton Lake is situated, is 90 miles long
and from 10 to 30 wide. It separates the San Bernardino range of
mountains from the San Jacinto, and forms a part of the great Colorado
Desert in San Diego County. In its deepest portions it is 275 feet below
the level of the sea; the area of the depressed region is over 1,600
square miles. In its northern portion the valley consists of sandhills
and dunes, the former caused by the accumulation of the sand around the
scrub and bushes growing in the desert. The southern portion is bare
clay. On the borders of the desert palm trees of the fan palm variety
occur in a few scattered clusters. The Southern Pacific Railroad passes
through the valley on its way to Yuma.
In that part of the depression adjacent to the railroad station of
Salton is a salt marsh, where the New Liverpool Salt Company’s works are
located, and where a fine quality of table salt is produced, containing,
according to an analysis made by Thomas Price, of San Francisco:
Chloride of sodium 94.68 per cent.
Calcium sulphate .77 per cent.
Magnesium sulphate 3.12 per cent.
Sodium sulphate .68 per cent.
Water .75 per cent.
100.00 per cent.
The brine from which the salt is obtained shows a density of 27° Baumé.
The company’s works are situated about 3,000 feet west of the railroad
track, and consist of the salt mill and a large storehouse, connected by
track with the main line. This branch line extends into the marsh a
distance of over 10,000 feet. Beyond the end of the track a borehole was
sunk by the company, with the expectation of obtaining water, to a depth
of 300 feet, when the work was interrupted.
The strata passed through during the sinking throw some light on the
past condition of the desert depression, as well as furnishing some
points that may have a bearing on the flooding of this section.
The top covering consists of 6 inches of black mud, resting on a crust
of chlorides of sodium and magnesium, 7 inches in thickness. The drill
on passing through this crust dropped through 22 feet of a black ooze,
containing over 50 per cent of water. Tests of the ooze made at the
State Mining Bureau laboratory showed it to consist largely of chlorides
and carbonates of sodium and magnesium, the soda salts predominating,
besides fine sand, iron oxide, and clay, and a small amount of organic
matter. The ooze was resting on a hard clay bottom, through which the
drill passed for the entire remaining distance, only varied by two or
three streaks of cement. From the shore line of the marsh toward the
adjacent mountain ranges, the soil consists of a fine sand, containing
clay and a notable quantity of mica, and is strewn with well-preserved
shells of _Planorbis ammon_, Gould, _Physa humerosa_, Gould, and
_Amnicola protea_, Gould.
In a few spots near the northwest end of the marsh the accumulated cases
of a species of Caddice worm are found. Northeast of the marsh the
surface slopes gradually upward to where the remains of an ancient sea
beach are to be seen, stretching to the south and east to where the
Colorado River cuts through on its way to the Gulf of California.
Behind the beach extends a mesa to the foot of the San Bernardino range.
Across this mesa are evidences of heavy floods coming down the cañons of
the back range, carrying large quantities of debris with them, mostly
bowlders of mica schists and granitoid rocks, with some quartz
intermixed. On the west side of the marsh the surface has but little
elevation until the granitic bluffs of the San Jacinto range are
reached. These bluffs are coated for a distance of about 25 feet above
the plain with a sponge-shaped incrustation from 2 to 3 inches thick,
consisting largely of carbonate of lime, chloride of sodium, sand, clay,
and oxide of iron; under the glass some of the pores are seen to contain
minute shells of the same varieties as found on the sand of the plain.
Where arroyos have been cut through the sands of the plains to the depth
of several feet, the exposed sections show a stratified arrangement of
the sand, having between the layers a thin division of the same
varieties of shells as found on the surface, the whole resting on a firm
clay bottom. From the position and regularity, as well as the quantity
of these shells, on top of the different strata of sand, while but few
are seen scattered through the sand layers, a periodicity of the
conditions favorable to their existence and growth, as well as a
comparatively sudden cessation of the same, must be inferred. These
conditions mean an occasional flooding of the depression for a period of
time, during which a shallow body of water was maintained, which
evaporated as soon as the water supply was shut off. In what manner this
can occur will be shown later on. The plain supports a scant growth of
desert shrubs, with some mesquite bushes in the vicinity of the few
springs that are found scattered over the desert, most of which are
saline. The presence of the extensive line of sea beach proves that at
some time the gulf has extended much farther inland than at present,
covering the whole of this depression. The Colorado River, in its course
south to the ocean, built up a flood plain on a higher level that
finally shut off the western part from a direct communication with the
sea, and evaporation, with a gradual uplifting of this whole section,
finally laid it bare, although leaving a great part of it still below
the present sea-level. Under these conditions, whenever more water comes
down the river than its banks can contain, the silt-formed shores give
way to the increased pressure and permit the excess of water to flow to,
and gather in, what is termed the river swamp at the back of the flood
plain, from whence it either re-enters the river lower down in its
course, or finds its way into the depression.
The Salton Marsh at present acts as the catchment basin for the waters
draining from the east side of the San Jacinto range, Carizoo Creek, and
a part of the San Bernardino Mountain range, and in times of extreme
high waters as a reservoir for the excess in the Colorado River. In the
month of June, 1891, a steady flow of water entered the depression from
the southeast and continued to the northwest uninterruptedly until an
area 30 miles long and averaging 10 miles in width was covered to a
depth of 6 feet, measured at the end of the Salton Salt Works branch
track. When first examined the water showed a density of 7° Baumé, which
gradually increased to 25° Baumé. The mother liquor used in making salt
at the New Liverpool works usually shows a density of 27° Baumé. This
salt water gave rise to the idea that the waters from the gulf had made
an inroad through some underground channel, and to prove the source and
possible permanency of these waters several investigating parties were
sent out. No such previous incursion had been witnessed by the settlers,
but inquiry proved that a similar lake existed here in 1849. Finally one
of the parties showed that the Colorado River was the source, and then
the question was brought to the attention of the State Mining Bureau to
investigate as to the probability of the permanency of the lake and its
probable effect on the climate of the surrounding country.
The Colorado in its great length accumulates a large amount of sediment,
from 0.1 to 0.4 per cent per gallon, and after this has been deposited
the water on evaporation is found to contain 0.14 grammes solid residue,
consisting largely of sulphate of soda and chloride of sodium. With the
sediment the river is all the time building up its flood plain, and it
may not be out of place to recall the general laws that govern the
actions of streams while depositing the solid matter they carry in
suspension.
Currents bearing sediments deposit a large percentage as soon as their
velocity is checked in the least, but pick up deposited sediments again
as soon as the current is increased. Thus a variable current is
depositing at one point while eroding deposits at other points.
The velocity of a river current varies with the seasons on account of
the different volumes of water carried, but it also varies at different
points of the river, during the different hours of the day, through the
constant action of the laws of erosion and deposition. The banks not
furnishing a constant, even resistance, erode in the looser parts,
causing each time a deflection of the current, as well as a change in
the velocity. This action soon changes a straight stream with an even
current, to a serpentine course with currents of different velocities,
as the outside of a curve has always greater speed than the inside;
consequently the outer edge is eroding while the inner is depositing
sediment. The Colorado River, in the lower part of its course from Yuma
down, overflows its flood plain, and deposits thereon a certain amount
of sediment during every high water. This flood plain extends back for
several miles on the west bank, showing a succession of benches or
levees running parallel with the stream on a higher level than the back
country. The water, when high enough to reach over these benches,
gathers in the back part, where it evaporates, leaving a mud that
greatly enriches the soil. These levees are the result of the law quoted
above, as the water in the regular channel flows swifter than on the
flood plain; consequently on the border of the two a large amount of
sediment is thrown down. As these border lands are only covered during
the highest floods, they maintain a growth of willows and poplars that
gives the banks a greater stability, and preserves them to a certain
extent from the erosive force of the current.
A source of changes in the direction of the main current is the
stranding of logs and tree tops brought down from the upper courses of
the river, as the sediment accumulates behind them and forms islands in
midstream. Several of these are to be seen in the river channel in
passing down the stream.
From soundings made across a section of the river at the railroad bridge
at Yuma, the depth was found to vary from 2½ feet to 21 feet. At the
meter placed in the main channel the indicator showed a depth of 17
feet.
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.
[Illustration: _SECTION OF COLORADO RIVER AT YUMA_]
[Illustration: _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.