Cadillac Desert_ The American West and Its Disappearing Water - Marc Reisner [307]
For the first time, after reading this paper, the long-range significance of the suffocating effect produced by accumulating silt in all these reservoirs was borne down on the writer. He had been so much taken with the fine things being done that he had not fully appreciated the fact that the program carried elements of destruction sure to bring some kind of ending. It was always evident, of course, that there were severe limitations, but it was too easy to overlook or belittle this element of damage from within.
The experience of founding, in difficult surroundings, settlements which finally grew into influence and power is not new; and neither is their decline, and even their ending. In the past, however, none of them carried, along with the agents that built them up, such relentless elements of destruction as in the present reclamation of arid lands. The astonishing thing is that the life of these relief works promises to be so short. One could forget it if the time vista were indefinite, or if there were promise of a thousand years. In that time most human subsistence and economic lines take new turns and become adjusted; but in some of these projects, typical of the average more or less, the beginnings of decline loom already and will certainly grow into a serious problem in three or four generations. One wonders how many settlers gathering around these projects appreciate what it means.
Of course, if one is able to divorce his interest from the future, there is nothing to worry about. In this generation, and the next and the next, an upgrade can be maintained. One can claim (and it is true) that much has been added to the world; but the longer-range view in this field, as in many others, is threatened by apparently incurable ailments and this one of slowly choking to death with silt is the most stubborn of all. There are no permanent cures.
The conference Berkey and Stevens had attended, “The Future of Lake Mead and Elephant Butte Reservoir,” was, more precisely, a summit meeting on the subject of mud. Before Hoover and Elephant Butte dams were built, the Rio Grande and the Colorado River ran chocolate-brown in the spring and anytime a cloudburst occurred somewhere in the watershed. Now, the water emanating from the penstocks and spillways below the dams was an opalescent blue-green, colored only by the minerals and algae in it. Each year, millions of cubic yards of silt were coming to a dead halt behind both dams.
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 one hundred to thirty years due to horrific deforestation in the Himalaya foothills. In the Dominican Republic, the eighty-thousand-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 percent. In countries suffering