Construction of the Dujiangyan irrigation project was begun by Li
Bing, the king of western Sichuan, in 256 BCE; his son, Er Lang,
completed the project. Workers using only hand tools cut a trunk
canal, called the "Mouth of the Precious Jar," through a towering
mountain to feed an extensive system of canals on the plains. A
diversion dam was constructed upstream (right), at the "Fish
Mouth," that splits the Minjiang into "Outer" Flow and "Inner"
Flow that is directed to the irrigation system. Another
longitudinal weir, located just above the Mouth of the Precious
Jar, controlsthe amount of water fed into the system. Flood
waters top this lower weir and about 70% of the total flow
returns to the Outer Flow of the Minjiang. The Dujiangyan system
has continued in uninterrupted use since its initial
construction. In 1991, China issued these stamps showing the Dujiangyan
irrigation project. The leftmost stamp shows the same area as the
photograph above, but from a slightly different perspective.
We drive up the deep valley of the Minjiang into the
Longmen Shan (left). The Longmen Shan are a
northeast-southwest-trending orogen, a nappe belt separating the
Songban-Ganzi terrane of the Tibetan Plateau from the South China
plate, on which Chengdu is located. Nappes are large folded
sheets of rocks that have been moved along low-angle thrust
faults. The rocks of the Longmen Shan were deposited upon the
southern margin of the Eurasian Plate in a continental shelf
environment, reminiscent of the present-day eastern coast of the
United States. These include Silurian to Devonian metasedimentary
rocks, late Paleozoic platform carbonates, and Triassic
continental rocks, including coal seams. The lowlands of the
South China Plate to the southeast are covered by alluvial
sediments carried by the Minjiang and other rivers. These were
deposited in the Chengdu Trough, a foreland basin produced when
the weight of the Longmen Shan produced a depressed region in
front of the mountains. NASA's on-line geomorphology book
describes the geology of the
northwestern Sichuan Basin, which shows dramatic folding and
thrusting. 
The Longmen Shan have experienced repeated
deformations during their history as the series of microplates
that make up the Tibetan Plateau collided with the southern
margin of Eurasia. Each collision reactivated previous
structures, causing movement to reoccur along previous faults and
zones of weakness, pushing the mountains southward. The latest
deformational event is Himalayan, associated with the impact of
the Indian subcontinent and Eurasia.
However, the area does not have high seismic activity; for
now, orogenesis has stopped in the Longmen Shan. Recent Global
Positioning Satellite (GPS) studies have revealed that the
triangular block carrying the Longmen Shan is no longer moving
with respect to the South China Block. Instead, deformation is
concentrated in the crust, which is moving independently from the
underlying lithospheric mantle, along major strike-slip faults
north and south of the range (left). The extension of the Kunlun
Fault to the north and the Xianshuihe Fault to the south are very
active. These fault are slipping at rates of about 1 centimeter
per year and have produced six major (magnitude 7+) earthquakes
in this century.
The road improves slightly, with stretches
of intact asphalt appearing at increasingly greater intervals.
However, landslides have covered the roadway in places, forming a
rough veneer of mud and rocks. Recent studfies have revealed that
landslides are the dominant erosional mechanism in the
northwestern Himalayas; the results probably also apply to the
Longmen Shan. Shovel-wielding road crews remove the smaller
boulders and dirt, while a bulldozer pushes the largest blocks
from the roadway into the raging torrent below.
