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Gradual disappearance -

Lewis and Clark Lake, nestled between Nebraska and South Dakota in the United States, is at risk of losing its identity as a lake. Over time, sediment has steadily filled its basin, threatening hydropower, recreation, flood control, and the reservoir’s ability to serve local communities.

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Sediment buildup -

Although sediment seems like just dirt, it’s the core threat to the reservoir’s future. Around five million tons enter the lake annually, depositing at the river's entrance and steadily pushing toward the dam itself.

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Swallowing the reservoir -

Since the construction of the dam at Lewis and Clark Lake in the 1950s, sand and silt have formed a large delta at the river’s entry point. This buildup has already reduced the reservoir’s storage by 30%, and projections suggest it will be half full of sediment by 2045.

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A seemingly small problem -

While the problem appears minor, it is actually a symptom of a slow-moving environmental and infrastructure crisis. Its effects ripple far beyond this one lake, and pose challenges for countless reservoirs worldwide.

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Interference -

The construction of dams has consequences upstream and downstream. Not only do they cause sediment to collect in reservoirs, but they also prevent necessary sediment from flowing downstream, which disrupts river ecosystems and alters natural habitats.

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Rivers transport sediment -

Rivers aren't only channels of flowing water; they are also powerful conveyors of sediment. After storms, they move soil, silt, and organic materials across the landscape to nourish and shape environments.

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The unsung hero -

Without sediment, riverbeds lose their structure and life. It creates breeding grounds for fish, insects, amphibians, and birds. It also fertilizes floodplains and maintains coastlines by replenishing beaches and deltas, offering natural protection from erosion and storms.

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Starving rivers of sediment -

When dams trap sediment and water, downstream regions are deprived of vital nutrients and geological renewal. This starvation disrupts food chains, shrinks habitats, and leaves floodplains and coasts vulnerable to degradation and drought.

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The river's flow -

When rivers flow quickly, sediment remains suspended and is typically carried by the current. But as soon as that flow slows in the reservoir zone, it drops out of suspension. This creates a delta that builds over time, with new sediment adding to the growing mass.

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A different design -

While some dams are meant to trap sediment for strategic reasons, the majority were built for water storage, not soil accumulation. As sediment displaces the water, the reservoir becomes less useful for energy production, irrigation, or flood mitigation.

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Recreation -

As sediment fills up a reservoir, it reduces navigable areas for boating and shrinks available swimming space. It also raises water temperatures and fosters harmful algal blooms, which can negatively affect both human activities and aquatic ecosystems.

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Mechanical systems -

Accumulated sediment doesn't just clog outlets—it also damages turbines and gates, increases wear on mechanical systems, and makes operation more difficult. In many cases, the sediment adds pressure and strain that the original dam designs didn’t fully anticipate.

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A matter of weight -

Though often overlooked, the weight of sediment imposes additional stress on dam structures. This mass not only accelerates mechanical degradation, but also introduces physical forces that may have been underestimated during the dam's initial design and engineering.

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All rivers carry sediment -

Regardless of location or size, every river transports some form of sediment. While the rate varies depending on soil types and land use, this natural behavior makes sedimentation in reservoirs an inevitable phenomenon.

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The upstream landscape -

The nature of the land that feeds into a reservoir determines how fast sediment accumulates. Sandy soils erode quicker than clays. Deforested or fire-damaged areas contribute more sediment, while forests and grasslands provide essential soil stability and erosion resistance.

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Mid-century dams -

Most dams around the world were constructed from the 1930s to the 1970s. At the time, sedimentation was largely deferred as a future problem. Now, decades later, society must face the consequences of long-ignored sedimentation as these aging dams reach critical thresholds.

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Planned sediment -

Modern reservoir designs often include a lower basin toward the base of the dam meant to contain sediment. But this only delays the inevitable. As the dead pool fills, sediment starts rising into critical zones, impairing functionality and posing maintenance challenges.

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Dredging -

Removing sediment via dredging has existed for centuries. Methods include excavators on barges or giant suction systems. But despite its reliability, dredging is expensive and labor-intensive, often costing more than the benefits it provides for large reservoirs.

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A logistical nightmare -

Even when sediment is dredged from reservoirs, the removed sediment is typically wet and voluminous, and requires specialized handling and drying before disposal. This dewatering process is oftentimes slow and adds considerable effort and cost.

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Contamination -

Sediment can also potentially be contaminated and hazardous. Runoff can carry pollutants like heavy metals or agricultural chemicals into sediment. When this happens, the spoils cannot be casually discarded. Strict environmental guidelines often mandate controlled, costly disposal processes to prevent secondary contamination.

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Restoring natural balance -

An alternative approach is to let sediment flow downstream through dams. Doing so mimics the natural river system, helping replenish riverbanks, rebuild deltas, and maintain downstream habitats—all while reducing sediment accumulation in the reservoir itself.

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Sediment behavior -

Even the best computer models can't fully capture sediment dynamics. There are many variables influenced by factors like water velocity, grain size, reservoir shape, and even seasonal weather patterns, which makes predictions inherently difficult.

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Reservoir flushing -

Opening gates to flush sediment can move some material downstream, but it often requires lowering water levels significantly. This not only wastes stored water but also leaves much of the sediment behind.

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Overwhelming ecosystems -

Although sediment is natural, releasing it all at once can be disastrous. Such plumes can damage aquatic life, violate environmental standards, and even cause potential flooding. In the US, sediment is legally treated as a pollutant, which makes these events heavily regulated and controversial.

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Trapping sediment upstream -

Preventing sediment from reaching the reservoir is one proactive option. Tools like bedload interceptors (pictured) or upstream check dams catch sediment before it enters the main body, though this simply relocates the problem and still requires eventual maintenance or removal.

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Soil conservation -

Since the 1930s, nations have invested in soil conservation through reforestation, fire prevention, and sustainable agriculture. These practices stabilize the land, reduce runoff, and lower the amount of sediment that enters waterways over time.

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A global issue -

Sedimentation affects virtually every dam worldwide. Reservoirs on every continent are quietly filling up, and the need for scalable, effective sediment management solutions is truly universal.

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Solutions -

While no silver bullet has emerged, the engineering community is not without tools. A range of methods (some technical, others regulatory) are being developed, tested, and applied to reduce, manage, and mitigate sediment accumulation with greater precision than ever before.

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Water infrastructure -

Solving sedimentation issues involves more than engineering. Political, economic, environmental, and societal factors all intersect. Real-world case studies oftentimes reveal how difficult and important it is to manage shared water resources wisely.

Sources: (Nebula) (Dam-It-Dams) (ResearchGate) (Britannica) (Practical Engineering)

See also: The world's worst industrial disasters

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