Visitors to the Grand Canyon
invariably marvel at the enormity and sheer beauty of the place while
a few even go so far as to wonder where all the excavated dirt went. The
few that pose that intriguing question seldom realize that they are concerned
with just an infinitesimal portion of a much larger problem with a most
For many years various writers have described the Colorado River as “too
thin to plow and too thick to drink” and it seem to be a widely
accepted aphorism that the river carries off one million tons of silt
every day. It is obvious that there is a lot of behind-the-scene activity,
that much ground is being moved each and every day and that its ultimate
disposal site must be somewhere downriver.
Through the geologic eons, the Colorado River has carried away, down and
through what would eventually enlarge to become the Grand Canyon, several
thousands of vertical feet of rock and soil that once covered an area
of many thousands of square miles of land all west of the present Rocky
Mountains, including a quarter of Colorado, much of Wyoming and New Mexico,
and far into Utah. That is a lot of dirt and it is still being moved just
as it has been for many millennia, moved from where it was originally
located to a large and most interesting depository, an inexorable movement
only slightly delayed by man’s very recent introduction of relatively
minuscule concrete barriers (dams forming small, local, temporary silt
depositories) in the river’s ages-old channel.
Back when all this movement of earth and rock was just getting started,
the place on a modern map we now call Yuma, Arizona, was at the mouth
of a very young river. Yuma-to-be was on a seashore, facing west across
what we now call the Gulf of California (or the Sea of Cortez, if you
prefer) which extended at that time far northward, far past Yuma, at least
as far as present day Palm Springs, California, a long, wide, deep arm
of a prehistoric sea. The new river at Yuma cut down into the land and
ended up trapped in the canyons of its own making, unable to escape, forced
to cut deeper and deeper as the whole land rose beneath it, eventually
excavating the Grand Canyon as we see it today.
For eons, from its very beginning, the Colorado River dumped its ever
present load of silt and soil into the blue waters of the Sea of Cortez.
A typical river delta appeared and grew. It grew until it reached the
far western shore of the Sea of Cortez, eventually to become the wide,
flat, wholly natural dam that exists today blocking off the northern reaches
of that ancient sea from the open water to the south.
Through the years the river continues to transport cubic miles of material,
strengthening the dam, making it permanent.
The river-borne dirt continued to arrive. The new land built out north
and south solid across the now truncated Sea of Cortez, mostly to the
south toward the open ocean, but often to the north into a new landlocked
basin forever isolated from its ocean origin.
When the river flowed north,
a great freshwater lake appeared (modern geologists have named it Ancient
Lake Cahuilla) which overflowed at the ever growing delta’s lowest
point back southward into what remained of the Sea of Cortez. When the
great river flowed south, the abandoned freshwater lake dried into a salty
pond far below sea level on the almost dry bottom of the ancient sea,
leaving an agricultural paradise of rich, fine soil now called Imperial
Valley on the gently north slopes of the giant river delta.
In 1905, the great river which historically flowed southward below Yuma
again switched directions and started to flow northward, an international
disaster of epic proportions in the making, one only averted by the obstinate
determination of E. H. Harriman, the much maligned president of the Southern
Pacific Railroad. But that is another fascinating story.
No vestige remains of the triangular shape of the river delta’s
Greek Alphabet namesake. The built-up silt depository is now best described
as an ill-defined quadrilateral limited on two sides by roughly parallel
mountain ranges approximately fifty miles apart, both canted to conform
to the distinctive northwest to southeast geography of the southern half
of the state of California. The southern limit is the southern shore of
California’s sub-sea level Salton Sea, a distance of, very roughly,
one hundred miles.
This otherwise featureless ex-delta has one distinctive characteristic,
however, in its vertical elevation. Its highest point is at Yuma, Arizona,
elevation 160 feet above sea level where the Colorado River escapes its
confining canyons and the land falls fan-shaped in every possible direction
from there. This typical rive delta pattern allows gravity delivery of
irrigation water from Yuma to Imperial Valley and everywhere else on the
ancient river outfall plain.
The delta soil is rich and the land is essentially dead flat with a slope
of less than five feet per mile. Actually, the land is so flat that a
destructive tidal bore was a regular feature of the Colorado River below
Yuma until the man-made dams upstream limited the river flow and silted
the channels thus putting an end to that exciting hydraulic phenomenon.
The accompanying picture of an early (1905) three-dimensional map of the
area in question, with a much exaggerated vertical scale, distinctly shows
the typical delta construction of almost 5000 square miles of dry land,
the final resting place of an awesome amount of Colorado, Wyoming, Utah
and Arizona dirt.