So Where Did All The Dirt Go?

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 interesting solution.
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.
John Southworth