The silt behind the dam

A dam is designed around water. The thing that eventually defeats many dams is silt.

Every river carries sediment. When the current slows behind a wall, the sediment drops and stays. The reservoir therefore fills, slowly, from the bottom up, with material that cannot be used and is extremely expensive to remove. Design lives are usually calculated with an allowance for this, and the allowance is frequently optimistic, because deforestation upstream increases the sediment load in ways nobody modelled when the dam was approved.

The second consequence is downstream and is easier to overlook. A delta is built from sediment. It exists because a river has been depositing material faster than the sea removes it, for thousands of years. Trap that sediment behind a wall and the arithmetic reverses: the sea continues to take, the river no longer replaces, and the delta begins to retreat. This is happening to several of the world's largest deltas at once, and the cause is distributed across hundreds of structures, none of which is individually responsible.

Engineers have answers. Sluice gates at the base can flush sediment through during high flow. Reservoirs can be operated to pass sediment rather than store it. Both reduce the power the dam generates, which is the reason they are often designed in and then not used.

The pattern is familiar from other systems. A structure optimised for one quantity quietly consumes another that nobody was measuring, and the second bill arrives decades after the people who approved the design have retired.