Figure 5. Ancient Egyptian plow (Whitney 1925).

      The floodplain of the Nile proved ideal for sustained agriculture. In contrast to Sumerian agriculture's vulnerability to salinization, Egyptian agriculture fed a succession of civilizations for seven thousand years, from the ancient pharaohs through the Roman Empire and into the Arab era. The difference was that the Nile's life-giving flood reliably brought little salt and a lot of fresh silt to fields along the river each year.

      The geography of the river's two great tributaries mixed up the ideal formula to nourish crops. Each year the Blue Nile brought a twentieth of an inch (about a millimeter) of silt eroded from the Abyssinian highlands. The White Nile brought humus from central Africa's swampy jungles. Fresh silt replaced mineral nutrients used by the previous crop and the influx of humus refreshed soil organic matter that decayed rapidly under the desert sun. In addition, the heavy rains that fell during June in the uplands to the south produced a flood that reliably reached the lower Nile in September and subsided in November, just the right time for planting crops. The combination produced abundant harvests year after year.

      Egyptian irrigation exploited a natural process through which overflow channels spread floodwaters across the valley. Irrigating fields did not require elaborate canals; instead the river's natural levees were breached to direct water to particular places on the floodplain. After the annual flood, the water table dropped more than ten feet below the valley bottom, eliminating the threat of salinization. In contrast to the experience of Mesopotamian farmers, Egyptian wheat harvests increased over time. The longevity of Egyptian agriculture reflects a system that took advantage of the natural flood regime with minimal modification.

      Fresh dirt delivered by predictable annual floods meant that fields could be kept in continuous production without compromising soil fertility. But the population was still subject to the whims of the climate. A few bad years, or even a single disastrous one, could be catastrophic. Extended drought severely reduced crop yields; a peasant revolt during one from about 2250 to 1950 BC toppled the Old Kingdom. Still, the generally reliable Nile sustained a remarkably successful agricultural endeavor.

      Unlike in Mesopotamia, regulating the distribution of the river's annual floodwaters remained a local responsibility. There was little impetus for developing a centralized authority. Class distinctions and division of labor developed in Egypt only after the adoption of perennial irrigation to produce cash crops had undermined traditional village communities. The despotic political superstructure of Mesopotamia was not an inevitable result of hydraulic civilization.

      Eventually, however, the agricultural surplus fueled the growth of an administrative and political elite. Egypt coalesced into a unified state about 3000 BC, developing into an ancient superpower that rivaled Mesopotamia. The rise of commercial farming not only allowed the population to grow, it meant that they had to be kept occupied. Some even suggest that the Great Pyramids were public works projects intended to combat unemployment.

      Egyptian agriculture remained remarkably productive for thousands of years until people adopted new approaches out of tune with the river's natural rhythm. Desire to grow cotton for export to Europe brought aggressive year-round irrigation to the Nile in the early nineteenth century. Just as in the scenario that unfolded thousands of years earlier in Mesopotamia, salt began to build up in the soil as the water table rose below overly irrigated fields. By the 1880s British agricultural expert Mackenzie Wallace described irrigated fields covered by white salts “covering the soil and glistening in the sun like untrodden snow.”¹ As dramatic as this spectacle appeared, the adverse effects of irrigation were dwarfed by those of damming the Nile.

      In the past half century, civilization finally acquired the engineering skill to cripple an almost indestructible land. After four years of work, Egyptian president Gamal Abdel Nasser and Soviet premier Nikita Khrushchev watched Soviet engineers divert the Nile in May 1964 to build the Aswan High Dam. Two and a half miles across, and more than seventeen times as massive as the Great Pyramid, the dam impounds a lake 300 miles long and 35 miles wide that can hold twice the river's annual flow.

      The British hydrologists who controlled Egypt's river until the 1952 coup that brought Nasser to power opposed building the dam because evaporation would send too much of the huge new lake back into the sky. Their fears were well founded. Under the desert sun six feet of water evaporates off the top of the lake each year—more than fourteen cubic kilometers of water that used to head down the river. But a greater problem was that the 130 million tons of dirt that the Nile carried off from Ethiopia settled out at the bottom of Lake Nasser.

      After advancing for thousands of years since sea level stabilized, the Nile delta is now eroding, cut off from a supply of silt. Although the dam allows farmers to grow two or three crops a year using artificial irrigation, the water now delivers salt instead of silt. A decade ago salinization had already reduced crop yields from a tenth of the fields on the Nile delta. Taming the Nile disrupted the most stable agricultural environment on Earth.

      As the renowned fertility of the Nile valley began to fall, agricultural output was sustained with chemical fertilizers that peasant farmers could not afford. Modern farmers along the Nile are some of the world's foremost users of chemical fertilizers—conveniently produced in new factories that are among the largest users of power generated by Nasser's dam. Now, for the first time in seven thousand years, Egypt—home of humanity's most durable garden—imports most of its food. Still, the remarkable longevity of Egyptian civilization is a primary exception to the general rise-and-fall of ancient civilizations.

      The history of Chinese agriculture provides another example where, as in Mesopotamia, dryland farmers from the uplands moved down onto floodplains as the population exploded. Unlike the Sumerians who appear to have treated all soils the same, the Yao dynasty (2357—2261 BC) based taxation on a soil survey that recognized nine distinct types of dirt. A later soil classification, dating from 500 BC, codified older ideas based on soil color, texture, moisture, and fertility.

      Today, the Chinese people overwhelmingly live on the alluvial plains where great rivers descending from the Tibetan Plateau deposit much of their load of silt. Flooding has been a problem for thousands of years on the Huanghe, better known in the West as the Yellow River, a name imparted by the color of dirt eroded from the river's deforested headwaters. Before the first levees and dikes were constructed in 340 BC, the river meandered across a broad floodplain. In the second century BC the river's Chinese name changed from Great River to Yellow River when the sediment load increased tenfold as farmers began plowing up the highly erodible silty (loess) soils into the river's headwaters.

      The earliest communities along the Yellow River were situated on elevated terraces along tributaries. Only later, after the area became densely populated, did people crowd onto the floodplain. Extensive levees to protect farmlands and towns along the river kept floodwaters, and the sediment they carried, confined between the levees. Where the river hit the plains, the weakening current began dropping sediment out between the levees instead of across the floodplain. Rebuilding levees ever higher to contain the floodwaters ensured that the riverbed climbed above