Dam Vulnerability

For all their breathtaking immensity, dams are oddly vulnerable things—a vulnerability that is shared and greatly intensified among the millions of people who depend on them. The engineers who have built them have gone to great lengths to make them safe from earthquakes, landslides, and floods. But their ultimate vulnerability, as Berkey wrote, is to silt. Every reservoir eventually silts up—it is only a matter of when. In hard-rock terrain with a lot of forest cover—the Sierra Nevada, the Catskill Mountains—a dam may have a useful life of a thousand years. In some overpopulated nations whose forests are nearly gone and whose farmlands are moving up mountains and whose rivers are therefore thick with silt, reservoirs built after the Second World War may be solid mud before the century is out. The Sanmexia Reservoir in China, an extreme case, was completed in 1960 and already decommissioned by 1964; it had silted up completely. The Tehri Dam in India, the sixth-highest in the world, recently saw its projected useful life reduced from 100 to 30 years due to horrific deforestation in the Himalaya foothills. In the Dominican Republic, the 80,000 kilowatt Tavera Hydroelectric Project, the country’s largest, was completed in 1973; by 1984, silt behind the dam had reached a depth of eighteen meters and storage capacity had been reduced by 40%. In countries suffering from overpopulation, deforestation, which is the primary cause of reservoir siltation, can only be expected to grow worse.

As a matter of principle, any place where vegetation is relatively sparse, where soils are erodable, but where six inches of rain in a day or twenty inches in a month are not unknown is a less than ideal place to situate a dam. Those conditions, however, apply to a large part of the intermountain West—and, since the arrival of intensive agriculture, to a great portion of the Middle West as well. 

The Eel River in California is the most rapidly eroding watershed in North America—partly because the topography is ridden with erodable sediments, partly because of rampant clear-cutting from which the forests may never recover, partly because of stubble grazing by cattle and sheep that is still going on. 

Meanwhile, erosive forces are hard at work in the watersheds of the Missouri River, the Colorado, the Rio Grande, the Platte, the Arkansas, the Brazos, the Colorado of Texas, the Sevier, the Republican, the Pecos, the Willamette, the Gila—rivers on which there are dozens of dams.

Earlier in the century, it was thought by some that irrigation in those watersheds might actually slow the rate of erosion by creating more groundcover to hold the soil in place. In the 1920s, however, no one foresaw interest rates so high that farmers, pushed to the brink, would almost be forced to abandon careful husbandry of the soil for maximum profit. No one foresaw cheap fertilizers that allow land to be plowed year after year, never going fallow. No one foresaw six-ton tractors that tear up the soil and make it more apt to be carried off. No one foresaw a demand for U.S. agricultural exports that makes it profitable to farm Class VI land. As a result of all this—and because it was inevitable anyway—dams are silting up.