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 the alluvial plain about a foot every century.
By the 1920s the surface of the river towered thirty feet above the flood-plain during the high-water season. This guaranteed that any flood that breached the levees was devastating. Floodwaters released from the confines of the levees roared down onto the floodplain, submerging farms, towns, and sometimes even whole cities beneath a temporary lake. In 1852 the river jumped its dikes and flowed north, flooding cities and villages and killing millions of people before draining out hundreds of miles to the north. More than two million people drowned or died in the resulting famine when the river breached its southern dike and submerged the province of Henan during the flood of 1887-89. With the river flowing high above its floodplain, levee breaches are always catastrophic.
Soil erosion in northern China grabbed international attention when a withering drought killed half a million people in 1920—21. Some twenty million people were reduced to eating literally anything that grew from the soil. In some areas starving people stripped the landscape down to bare dirt. The ensuing erosion triggered mass migrations when fields blew away. But this was not unusual. A 1920s famine-relief study documented that famine had occurred in some part of China during each of the previous two thousand years.
In 1922 forester and Rhodes scholar Walter Lowdermilk took a job at the University of Nanking to work on famine prevention in China. Touring the country, he deduced how soil abuse had influenced Chinese society. The experience impressed upon him the fact that soil erosion could cripple civilizations. Years later, after traveling widely to study soil erosion in Asia, the Middle East, and Europe, Lowdermilk described his profession as reading “the record which farmers, nations, and civilizations have written in the land.”2
Approaching the site where the Yellow River broke through its dikes in 1852, Lowdermilk described how a huge flat-topped hill rose fifty feet above the alluvial plain to dominate the horizon. Climbing up to this elevated plain inside the river's outer levee, Lowdermilk's party traversed seven miles of raised land before coming to the inner dike and then the river itself. Over thousands of years, millions of farmers armed with baskets full of dirt walled in and gradually raised four hundred miles of the river above its floodplain and delta. Seeing the muddy yellowish water, Lowdermilk realized that the heavy load of silt eroded from the highlands began to settle out when the river's slope dropped to less than one foot per mile. The more silt built up the riverbed, the faster farmers raised the dikes. There was no winning this game.
Determined to find the source of the dirt filling in the river, Lowdermilk traveled upstream to the province of Shansi (Shanxi), the cradle of Chinese civilization. There in northwestern China he found a landscape deeply incised by gullies, where intensive cultivation after clearing of forests from steep, highly erodible slopes was sending the soil downstream. Lowdermilk was convinced that deforestation alone would not cause catastrophic erosion—shrubs and then trees simply grew back too fast. Instead, farmers cultivating steep slopes left the soil vulnerable to erosion during intense summer downpours. “Erosion is only indirectly related to the destruction of the former extensive
