WATER LINES: An engineering wonder, Morrow Point Dam turns 50
Fifty years ago this June, down in the Gunnison River canyons above the national park, the Bureau of Reclamation set two construction companies to work on a project that had not been tried before, at least on the scale proposed: A thin-arch double-curvature high dam.
Most of the dams that the Bureau of Reclamation was building in that dam-building era were “gravity dams” — big masses of either concrete or dirt, clay and rock, piled in the path of a river to store its waters for later uses. Dams tend to be overbuilt for obvious good reasons. Hoover and Glen Canyon dams farther downstream on the Colorado River — each of which can store more than 8 trillion gallons of water — both have huge safety factors built into their mass.
But the dam the bureau began on the Gunnison River 50 years ago this summer, just above its confluence with the Cimarron River, was, by comparison, going to be like wedging an eggshell in the canyon. It would be the elegant crown jewel of the massive Colorado River Storage Project.
The theory of the arch dam is that the horizontal push of the weight of water in storage will exert a compressing force on the upstream curve of the dam, actually strengthening the structure through that compression, and translating much of the water’s force into the canyon walls, into which the dam is securely wedged in keyways.
A “thin-arch” dam is one in which the crest thickness of the dam is less than one-fifth of the dam’s height. And the “double-curvature” reflects the fact that not only is the dam curved upstream in its horizontal cross-section; it is also slightly curved upstream in its vertical cross-section, increasing the compression effect and directing pressure down into the dam’s bedrock foundation.
The Morrow Point Dam would be 468 feet high, only 52 feet thick at its base and 12 feet thick at the crest — a very thin-arch dam, with a crest width only 1/40th of the dam’s height. This was feasible, however, according to the engineers, because the dam would only be backing up some 38 billion gallons of water, due to the narrowness of the canyon above the dam. Power generation, not storage, was the purpose of this dam; its “storage battery” would be the much larger reservoir behind Blue Mesa Dam 12 miles upstream.
Even there, the bureau was trying something new with this project: The entire 120 megawatt power plant was going to be underground in a vast man-made James-Bondish cavern, 230 feet by 60 feet and up to 130 feet high. And there would be a third feature new for the bureau: Rather than a safety spillway tunneled into the rock walls, the spillway would be four gates high in the face of the dam, creating, when opened, a “waterfall” twice as high as Niagara Falls.
The contract for construction of the dam and power plant was awarded in May 1963, to a joint venture between Al Johnson Construction of Minneapolis and Morrison-Knudsen Inc. of Boise — one of the “Six Companies” that built the seminal Hoover Dam in the early 1930s, and on June 13 the bureau gave them the go-ahead to start work. The river was diverted out of its bed into a tunnel in May 1964 — its second tunnel in 15 miles, since Blue Mesa Dam was still under construction upstream. “Keyways” for anchoring the dam were cut into the granite cliffs in 1964 while the riverbed was being excavated to bedrock; mining out the cavernous power plant also began that year. Storage of aggregate and concrete began in the summer of 1965 while a big concrete mixing plant was being built, and concrete placement for the dam began that fall. It only took 15 months to place the concrete for the dam — an advantage of thin-arch construction. The dam was completed by October 1968, and the power plant and related facilities by early 1971.
Travelers on U.S. 50 between Montrose and Gunnison can make a five-minute detour from the highway down the narrow Cimarron Canyon to the viewing area for Morrow Point Dam (a side trip that also has interesting railroad exhibits). One has to thoroughly hate even the idea of dams to not be taken by the beauty of this one — the engineer’s elegant smooth white curves integrated into the rough red-brown of the canyon.
But an even better place from which to view Morrow Point Dam is from up on the Black Mesa Highway to the North Fork valley (Colorado 92). From there, the dam looks unbelievably fragile, with the ephemeral poignancy of most beautiful things: Dams don’t last forever nor do canyons under the persistence of water and weather. But for now, it is a fingernail, a lens wedged into the canyon making the water stand in and push rather than cut and run. An idea made concrete — but the concrete does not obscure the idea.
This is part of a series of articles coordinated by the Water Center at Colorado Mesa University in cooperation with the Colorado and Gunnison Basin Roundtables to raise awareness about water needs, uses and policies in our region. To learn more about the basin roundtables and statewide water planning, and to let the roundtables know what you think, go to http://www.coloradomesa.edu/WaterCenter.
Reach Sibley at email@example.com.
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