A River of Silt Meets Concrete
The Yellow River, or Huang He, is the most sediment-rich major river on Earth. Before the construction of the Sanmenxia Dam, it carried an average of 1.6 billion tons of silt downstream annually. This enormous load comes from the river's passage through the Loess Plateau, a region of fine, wind-blown soil that is easily eroded. When engineers, with assistance from the Soviet Union, designed the first major dam on the river, they underestimated the sheer volume of this sediment.
Construction on the concrete gravity dam began on April 13, 1957, and was completed in 1960. The project was a major success for the new People's Republic of China, even appearing on its banknotes. The initial plan was to operate the dam as a storage reservoir, holding back the muddy water to let the sediment settle before releasing clear water downstream. This strategy proved disastrous almost immediately. In the first 18 months of operation, 1.8 billion metric tons of sediment accumulated in the reservoir, reducing its capacity by 17%. The riverbed upstream began to rise, elevating 4.5 meters at the town of Tongguan and threatening the major city of Xi'an with floods.
Engineering an Escape Route
Faced with a rapidly failing project, engineers had to devise a radical solution. The dam's operational philosophy shifted from "storing water and sediment" to "storing clear water and discharging muddy water." This required significant structural changes. The first reconstruction took place from 1965 to 1968. Two new tunnels, each 11 meters in diameter, were bored through the dam's left abutment. Four of the eight penstocks originally intended for power generation were converted into sediment-flushing pipes.
A second, more extensive reconstruction occurred between 1969 and 1978. During this phase, eight bottom sluices were opened near the original riverbed level to provide a low-level escape route for the dense, sediment-laden currents. With these modifications, the dam could now pass large volumes of muddy floodwater during the wet season, scouring out deposited silt and maintaining a long-term sediment balance. This new operational mode, which began in 1974, involves lowering the reservoir level before the flood season to allow the silt-heavy water to pass through, and then raising it during the drier seasons to store clearer water for power generation and irrigation.