Are we ever safe from the threat!
By Allen C. Reed, March 1951, originally appeared in Arizona Highways
Photos courtesy of the Grand Canyon Museum
Thanks to Mike Quinn!
|It may be just the
place for a dam and then it may not. That's what the U. S. Bureau of Reclamation field
crew is there to discover, but one thing is certain. It was a tough place to get into, in
order to find out. That's Marble Canyon's proposed damsite on the mighty Colorado, upriver
just a few miles from the Grand Canvon.
Investigating a damsite calls for a hardy, experienced field crew and many tons of equipment on the spot. When Marble Canyon was first considered as a likely site an experienced team which had worked together since 1946 on four other investigations was in shape and ready to go. The big problem was in finding a practical way to move them in and provide them with a continuous flow of equipment, supplies, and living essentials for the year or two that such an investigation might take.
Previous sites were accessible either by roadway or riverboat but such means of approach to this project were out of reason. Impetuous snarling rapids barred the waterway while sheer rugged canyon wall steps reared massively half mile high on both sides. To complicate matters eve further, this section of canyon under question was deep in the Navajo Reservation, isolated from highways and other transportation routes.
Never having bothered to include the word "impossible in their vocabulary, the crew applied their usual engineering ingenuity and daring, rolled up their sleeves and went to work. Step by step, first things first, obstacles were methodically eliminated. Soon the crew and equipment was swooping in and out of the canyon on a custom made cable system with the ease of a city dweller riding the elevator to and from his office high atop a Manhattan skyscraper; the main difference being in that their view was not encased within four elevator walls but extended as far as the eve could sweep in a scenic extravaganza of colorful canyon and sky. With this system the job that might have taken years to complete could better be figured in months.
The first step was the construction of 25 miles of roadway through the Navajo Reservation to the canyon rim from Cedar Ridge, an Indian trading post on U. S. Highway 89 about midway between Cameron, Arizona, and Navajo Bridge. Next a base camp was established near the rim and then the problem of canyon access was tackled.
It is very likely that the first thought to enter the minds of the field engineers when they peered into the chasm to see the proposed site some 2,500 feet below was the fundamental theorem; "the shortest distance between two points is a straight line" for they came pretty close to installing just such a line, but they greatly decreased the tension by allowing it to sag in the center approximately 270 feet.
Before the canyon cableway was designed and constructed a search was made for a type in use elsewhere that would be applicable. None of the ski lifts or other cable-ways in the United States or in foreign countries that were checked would meet the requirements of this project. This system called for a single length, specially manufactured cable nearly three quarters of a mile in length anchored only at the two terminal ends ... on the rim and on the canyon floor, as there was no practical way to use intermediate supports.
Specifications for such a system were created on paper and contracts were placed with various manufacturers. When all the parts arrived, like a giant Erector Set, they were assembled and installed by the reclamation crew. It was necessary to first lower a smaller messenger cable down the steep canyon walls to the lower anchor point. Once pulled into position, this temporary cableway served as a guide to lower the one and three-eighth inch locked coil 3,667 foot long permanent track cable. This cable was attached at the upper end to a twenty foot structural steel tower and at the lower end to a forty-foot system to take-up blocks which allow for expansion and contraction adjustments and for tension control. Both the tower and the blocks are anchored by back stays to three-inch U shaped bars embedded four feet in solid rock. The track cable weighs 4.75 pounds per foot, a total weight including terminal attachment sockets of approximately 18,000 pounds. It has a breaking strength of 105 tons and an allowable load capacity of 2,000 pounds including the 500 pound weight of the cage. The raising and lowering of the carriage and its load is accomplished at the rate of 350 feet per minute by an -attached five-eighth inch load-line cable and a power hoist on the canyon rim. A one-way trip takes from twelve to fourteen minutes.
With the cableway completed, a lower camp was set up in the canyon and all the
equipment used there has floated in seemingly out of the sky. This includes such heavy
items as compressors, pumps, drill machines and barges, building supplies, a refrigerator,
stoves etc. Every ten days the crew soars to the rim to spend four days in the upper
A smaller cableway almost identical in its basic design has been constructed from the rim of the inner gorge bench on which the lower camp is located to the river level 350 feet below. This 693 foot seven-eighth inch track cable, in descending, crosses over the river to the exact location the proposed dam where diamond drilling is underway. A series of 25 two inch holes are being drilled 100 to 500 feet deep on both sides of the river and in the bed from anchored barges. The rock cores removed from these holes are packed and catalogued in proper sequence for careful analysis of strata and rock formation at this point before it can be approved as a suitable dam location.
Another of many operations necessary is the drilling and blasting of two 5x7 foot drifts 40 to 50 feet back into the solid rock abutments high above the river for further tests and examination.
Such an investigation does not necessarily mean that a Marble Canyon Dam will be constructed at this site in the near future even if the conditions prove satisfactory for this is but one phase of a reclamation masterplan. The ultimate decisions will be largely governed by many factors such as over-all reclamation developments, alternate locations that might prove more favorable, available appropriations, and the increased power requirements of a growing population and new industry in the West. However, if and when it becomes time to construct a dam on this part of the Colorado River to further harness nature's forces for the good of mankind, the pioneering will have been done. The dam is on paper for Marble Canyon. It would rise approximately 300 feet above the river surface and sink 100 feet below to bedrock. It's potential power output is calculated at two and three quarters billion killo-watt hours a year.
Someday man-made thunder may reverberate throughout Marble Canyon and when the dust and smell of blasting powder has settled powerful trucks may be seen grinding up and down a steep winding roadway serving a full-fledged dam construction operation far below. Or the sun might glisten on the stout multiple strands supporting huge shuttling cablecars as they glide in and out of the canyon loaded with cement and steel.
Perhaps someday a passerby will peer over the rim at Marble Canyon and see a gracefully curved vvhite concrete dam for below. Where the muddy Colorado has carved its way for twelve million years there will be a narrow winding clear blue lake fifty miles long. The traveler will know that a new breed of pioneers has passed this way and that civilization has once more touched the great canyon of the Colorado.
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