Cadillac Desert_ The American West and Its Disappearing Water - Marc Reisner [297]
Like DDT in pelican egg shells, the salts in the oceans are testimony to the effects of concentration. As the evaporative cycle is repeated, day after day, year after year, millennium after millennium, eon after eon, the oceans grow saltier all the time. On March 24, 1992, the dissolved salt content in ocean water off San Francisco was about thirty-five thousand parts per million, perhaps a fraction of a ppm higher than it was ten thousand years ago. The process is so incredibly slow and immense that, for once, no act of man seems capable of affecting it by the tiniest measurable iota. What is changing—what has changed drastically in the very recent past—is the concentration of salts in some of the world’s rivers, and in some of its preeminent agricultural land.
Explaining the collapse of ancient civilizations is a cottage industry within the anthropological and archaeological professions, like the riddle of the dinosaurs. The explanations vary considerably. Some blame their demise on chronic human failings: degeneracy, conflict, war. The decline of Rome, according to some, was the result of the Romans’ use of lead in their eating and drinking utensils; since lead causes irreversible brain damage if eaten or ingested in fairly small amounts, the theory offers a tempting explanation for the obviously demented behavior of certain Roman leaders. (It does not, however, explain why most Romans were demonstrably sane, or why there was so much genius about.) Because most of the great civilizations rose in deserts or semideserts, a popular explanation has always been drought—a drought beyond any that modern mankind has known, perhaps caused by aberrant sunspot cycles or some huge volcanic eruptions that changed the climate.
The most fruitful of the ancient cultures grew up at the southeastern end of the Fertile Crescent, the broad valley formed by the Tigris and Euphrates rivers in what is now Iraq. From there civilization appears to have spread eastward into Persia, and on to Afghanistan, Pakistan, India, and China. Later, it spread to the west. Most of the Romans’ fabled feats of hydrologic engineering were borrowed from the Assyrians, who borrowed them from their predecessors, the Sumerians. In the seventh century B.C., the Assyrians, under Sennacherib, built an inverted siphon into the Nineveh Aqueduct, a feat of hydrologic engineering which was not really improved upon until New York City built a pressurized siphon into its second Croton Aqueduct in the 1860s. For all its precocious brilliance and innovation, however, the southern part of the Fertile Crescent went into eclipse around the year 2000 B.C. When Babylon rose in the eighteenth century B.C., many impressive Sumerian cities lay in ruins around it, as Babylon itself would lie desolated centuries later.
The story was repeated nearly everywhere, even in the New World, where a number of remarkable civilizations arose and prospered independently. One of the most impressive was the Hohokam civilization, in central Arizona, which left as its legacy some seven hundred miles of irrigation canals. Sometime around the fourteenth century, however, the Hohokam vanished—reason unknown. The Inca, Aztec, and Maya used irrigation, too, though they didn’t rely on it as absolutely as the Hohokam. Their fate was sealed by European invaders, so it is perhaps idle speculation whether they would ultimately have gone the route of their predeceessors in Mesopotamia and elsewhere. Whatever the answer, it appears that only one civilization completely dependent on irrigation managed to survive uninterruptedly for thousands of years. That civilization was Egypt—but Egypt was fundamentally different from the others in one way.
The survival of a civilization depends mainly on sufficient food. But what makes a civilization great? Traveling across the United States, Lewis and Clark saw few fat Indians until they had arrived at the mouth of the Columbia River, where the Chinook were gorging themselves on salmon,