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By Root 1751 0

into permanent eclipse or, as in the case of the Hohokam, cause a whole civilization simply to vanish off the face of the earth. There had to be another enemy—something subtle, unseen, subversive. It was likely to be something they could do little or nothing about, something which they may not even have understood, and thus might have been inclined to ascribe to vengeance from gods. Contemplating the list of enemies, natural and man-made, that might fit such a description, more and more anthropologists and archaeologists are concluding that the one that fits it best is salt.

Irrigation is a profoundly unnatural act. It hardly occurs in nature, and that which does occur is mostly along the rare desert rivers, like the Nile, that produce a reliable seasonal flood. In Africa and a few other places, there are natural depressions where runoff collects during rainy seasons, greening the land when it recedes. For every one of those, however, there are dozens of dead saline lakes or lake beds where the same thing used to happen and where, today, nothing can grow. They are common in Nevada—Groom Lake, Newark Lake, Goshute Lake, Winnemucca Lake, China Lake, Searles Lake, Cuddleback Lake—big saucers of salt left over from shallow Pleistocene seas, when the climate of Nevada was more like Szechuan. The waters that filled those lakes came down from ranges a short distance away, but in that brief intimacy with soil and rock had already accumulated enough salts to spell death for the basin below.

Man-made irrigation faces the same problem. In the West, many soils are classified as saline or alkaline. Irrigation water percolates through them, then returns to the river. It is diverted downstream, used again, and returned to the river. On rivers like the Colorado and the Platte, the same water may be used eighteen times over. It also spends a good deal of its time in reservoirs which, in desert country, may lose eight to twelve feet off their surface to the sun every year. The process continues—salts are picked up, fresh water evaporates, more salts are picked up, more fresh water evaporates. The hydrologist Arthur Pillsbury, writing in Scientific American in July of 1981, estimated that of the 120 million acre-feet of water applied to irrigated American crops the previous year, ninety million acre-feet were lost to evaporation and transpiration by plants. The remaining thirty million acre-feet contained virtually all of the salts.

Above a heavily irrigated strip of land along the Pecos River in New Mexico, water taken from the river has a measured salinity level of about 720 parts per million. Thirty miles beyond, salinity levels have shot up to 2,020 parts per million, almost entirely because of irrigation; 2,020 parts per million spells death for many crops. Near its headwaters in the Colorado Rockies, the Arkansas River shows only a trace of salts. A hundred and twenty miles downriver, it contains 2,200 parts per million. The Colorado, a river whose importance is absurdly disproportionate to its size, has the worst problem with salt of any American river. There are small tributaries flowing out of the salt-ridden Piceance Basin with measured concentrations of as much as ninety thousand parts per million—three tablespoons in a cup—so it is plagued by natural sources to begin with. In the Grand Valley of Colorado, irrigation water runs through sedimentary salt formations on its subterranean return to the river, reaching saline levels thirty times higher than at the diversion point. Below there are two huge reservoirs, Powell and Mead, evaporating a million and a half acre-feet of pure water each year—at least a tenth of the river’s flow. It should come as no surprise, then, that by the time the Colorado River has entered Mexico, its waters are almost illegal.

Behind Jan van Schilfgaarde’s desk in his office at the Department of Agriculture’s Salinity Control Laboratory, in 1982, is a plaque proclaiming him a member of the Drainage Hall of Fame. Drainage seems like a pedestrian business, and van Schilfgaarde is an uncommonly