NTS Field Trip
Our field trip in autumn of 1995 was to the Nevada Test Site. This crater-scarred wasteland in south-central Nevada was once home to the United States' nuclear weapons testing program. On our trip we studied the geology of crater formation, and also the general geology of the region.
Click here for our Nevada Test Site field trip home page, including the history of the Nuclear Test Site, and images of explosion craters (and us in our radiation suits).
On our way back from NTS on 30 September, we crossed the Colorado River at Hoover Dam, which was the subject of my field trip report:
Hoover Dam and the Colorado River
The Formation of Lake Mead
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| Black Canyon |
For millions of years the mighty river now known as the Colorado flowed powerfully through the mountains and plateaus of the West, cutting great canyons and splendiferously deep gorges on its way out to the Gulf of California. Then one day humans came along and decided that this was not at all acceptable; the river would have to be stopped. "Dam this river!" they said, and work was begun on a project of monumental proportions: Boulder Dam.
The reasons advanced for building a dam were threefold: (1) flood control, (2) control of water supply, and (3) electric power generation. In early attempts to provide irrigation for southern California's Imperial Valley, canals had been built to carry water westward from the lower Colorado. For political and financial reasons, one canal was constructed south of the Mexican border, but a headgate was not built for it. In early 1905, the unruly Colorado flooded, sending its entire water flow down this canal, into the Alamo River, and toward the Salton Sink.(1,2) Once the northern part of the Gulf of California, this sub-sea level depression had been cut off from the sea by the accumulated silt of the Colorado river delta. In an ironic reversal, the very river that had isolated the Salton Sink was now filling it with water. For two years the entire Colorado flowed into southern California, defying several major attempts to fix it, until finally returned to its "natural" course out into the Gulf in February 1907. Realizing that the puddle it had left behind was much too large to mop up, local residents renamed the Salton Sink the Salton Sea and tried to pretend that it was a good thing.
Still, something had to be done about the incorrigible river. After years of debate, seven States (WY, CO, NM, NV, CA, UT, and AZ) entered into the Colorado River Compact, an agreement (which only Arizona failed to ratify) to dam the Colorado and divide its waters. Two sites below the confluence with the Virgin River were considered for the dam. The first choice, Boulder Canyon, had bedrock of granite, but it was found to be seriously jointed and faulted.(1) Farther downstream was Black Canyon, a lovely yet inhospitable little canyon inconveniently separating millions of gamblers in Arizona from Las Vegas. Black Canyon offered a foundation of andesite breccia, and would create a larger reservoir, and so the choice was made. The dam was to be built by Six Companies, and entirely funded by the U.S. Government, to be paid off over 50 years by selling the electricity it produced.
Construction was started in 1931. To begin with, four diversionary tunnels 16 meters in diameter had to be drilled 1.2 km through the walls of the canyon to carry the flow of the river around the construction site. The Colorado was diverted into the tunnels in November 1932, and two large cofferdams were built, uptsream and downstream of the riverbed construction site, isolating it from the river.
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| Hoover Dam |
Next the dam site had to be excavated, removing the sand and silt, and the loose boulders and gravel of the river bed so that the dam would rest upon a solid foundation of volcanic bedrock. The first concrete was poured for the base of the huge arch-gravity dam in June 1933, and pouring continued 24 hours per day for the next two years (about 6 billion kg). The base, 200 m thick, was situated between two transverse fault lines in the canyon floor, exerting a maximum compressive stress of ~ 3.8 MPa.(1) The dam tapers down with increasing height to about 14 m thick at the crest, 221 m above the foundation. In February 1935, as the dam neared completion, the diversion tunnels were closed, the Colorado was penned, and the new reservoir began filling with water. Finally after four years of construction, during which 112 men lost their lives * in various, often boneheaded accidents, Boulder Dam was dedicated by President Roosevelt in late 1935 and turned over to the U.S. Government the following March. The two powerhouses (one on the Arizona side, one in Nevada) were completed not long after. Two of the original diversion tunnels became penstocks feeding the powerhouses from the four 120 m tall intake towers; the other two tunnels were connected to spillways on either side of the canyon.
The most obvious effect of the dam was the creation of the world's largest human-made reservoir, Lake Mead (also America's first National Recreation Area). As the water level rose 180 m over the next six years, the Boulder basin was flooded, then the Virgin basin and the lower 50 km of the Virgin river (forming the Overton Arm), and Gregg basin (fully 185 km up river). The tremendous new mass of water (~3.5 x 10e13 kg over ~590 sq. km) caused Earth's crust to deform slightly, resulting in frequent tremors throughout the area while the reservoir was filling. The cool, refreshing lake is much flatter and much more pleasing to the eye than the bumpy, rocky gorges that are now submerged, but no doubt less interesting to wacky geologist types.
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| Figure 1 |
The dam not only regulates the water flow of the Colorado, it also causes the river to release its load of silt as it slows down upon entering Lake Mead. The result is the gradual filling of the reservoir with silt (Fig. 1). In addition, the water that comes out below the dam is relatively clean, with a great capacity to pick up new material. This causes faster cutting of its channel for many miles downstream of the dam, reducing its slope until a new equilibrium is achieved.(4)
Boulder dam (renamed Hoover Dam in 1947) was heralded as a "conquest of nature" by Harold Ickes, Roosevelt's Interior Secretary. To others it was "one of the greatest monuments to man's ingenuity and conceit."(1) The dam has fulfilled its purposes of flood control, water regulation, and power generation well (and even provides a route of passage across the Colorado for college students traveling to Nevada). However, in 1983 a major flood reminded us that nature has not been completely tamed. After massive snowmelting far upstream, the spillways in the dam had to be opened, letting the floodwaters through, and causing millions of dollars worth of damage downstream. The Colorado still runs wild at times.
References:
1 J.E.Stevens, Hoover Dam, An American Adventure, 1988
2 The Colorado River Commission, Colorado River and Boulder Canyon Project, 1931
3 A.Dunar & D.McBride, Building Hoover Dam, An Oral History of the Great Depression, 1993
4 C.R.Longwell & R.F.Flint, Introduction to Physical Geology, 1955
* Click here for the Top Ten Causes of Death in the Construction of Hoover Dam.