Geography Reference
In-Depth Information
Recent work in geomorphology, summarized in figure 5.6, confirms
that an abrupt increase in the sediment load carried by the Yellow R iver
occurred about one thousand years ago, precisely the moment that pas-
ture and forest clearing began to expose the soil there at a higher rate
than ever before.
26
Scientists already assert that the shift resulted from
a rapid increase in human activity on the loess plateau. Cultivation and
deforestation on the loess plateau resulted in erosion, which discharged
more sediment into the river.
27
Historical sources confirm that flooding,
which had been infrequent for eight hundred years, escalated from the
ninth and tenth centuries onward.
28
Early eleventh-century erosion on the loess plateau had severe ef-
fects downriver. In the lower reaches of the river, especially from Henan
province to the sea, the river drops in elevation by only 93.6 meters.
As the river enters the plains, the silt carried from the middle reaches
is deposited in the riverbed. In prehistoric times, the river meandered
through wetlands and shallow lakes, changing its route whenever older
beds filled with sediment. However, in historical times, residents and
a state that had invested in cities and farms wished to prevent the river
from breaching. They built levees higher and higher, until the riverbed
loomed over the surrounding farmland.
29
Flood-prevention activities
were not always effective, and even when they were, the plains become
waterlogged and saline as a result of changes to the whole ecosystem.
Seepage from the dikes increased the surrounding groundwater level
and led to the formation of swampland. Salinization and other forms of
soil deterioration reduced agricultural output.
30
Once it was constrained
by levees, the river was locked into place. W hile population densities
were high, the adjacent creeks and wetlands disappeared, diminishing
opportunities for irrigation and making subsequent floods more violent.
Water shortages and waterlogging were both common as wetlands and
lakes were drained and floods persisted, while silt buildup dictated that
wells had to be dug increasingly deep.
31
The Yellow R iver maintained a stable course in its lower reaches
from 11 to 1048 ce. This was its longest single era of stability in recorded
history. Even before the shock of the eleventh-century silt increase, the
lower course was old and fragile, and the agrarian ecology surrounding
