All Dams Are Temporary

Summary

Lewis and Clark Lake, located on the border of Nebraska and South Dakota, faces an existential threat due to sedimentation. The dam and reservoir, which provide hydropower, flood control, and recreational opportunities, are slowly filling with sediment. The problem arises from the natural sediment flow of the Missouri River, which has been disrupted since the dam was built. This issue is widespread across the globe, affecting many reservoirs. While dredging and sediment management are options, they come with significant costs and challenges. The video explores this complex issue and the long-term impact on infrastructure, the environment, and water management.

Takeaways

• 😀 Lewis and Clark Lake, located on the border of Nebraska and South Dakota, is facing an existential threat due to sediment buildup.
• 😀 Around 5 million tons of sediment flow down the Missouri River annually, leading to a massive delta in the reservoir.
• 😀 The sediment buildup has already reduced the lake's storage capacity by 30%, with projections that it may be half full of sediment by 2045.
• 😀 Dams disrupt the natural movement of sediment, which is essential for nutrient transport, fish habitat, and stabilizing river ecosystems.
• 😀 Sediment in reservoirs can lead to a variety of problems, such as reduced water storage, clogged gates, and increased risks to dam infrastructure.
• 😀 Dredging is one potential solution to remove sediment, but it’s costly and requires substantial resources with minimal long-term benefits.
• 😀 An alternative solution is to release sediment downstream, but this must be done carefully to avoid pollution and environmental damage.
• 😀 Flushing the reservoir is another option, but it is often impractical due to the need to draw down the water level and the risks of sending sediment downstream.
• 😀 Capturing sediment upstream through techniques like bedload interceptors or soil conservation can help, but it requires significant land management efforts.
• 😀 Managing sediment buildup in reservoirs is a global issue, affecting many dams around the world, and the challenge requires innovative, long-term solutions.
• 😀 The ongoing issue of sedimentation highlights the complex intersection of engineering, environmental impacts, and the long-term sustainability of water infrastructure.

Q & A

What is the primary issue facing Lewis and Clark Lake and its reservoir?

The primary issue is sedimentation. Over time, sediment, mainly sand and silt, accumulates in the reservoir, which threatens to reduce its storage capacity and affect its hydropower, flood control, and recreational functions.

Why is sediment accumulation in a reservoir considered a significant problem?

Sediment accumulation reduces the reservoir's storage capacity, limits its use for recreation, clogs infrastructure like gates and turbines, and can put additional pressure on the dam structure, potentially affecting its stability.

What are the functions of sediment in a natural river system?

Sediment plays vital roles such as transporting nutrients, creating habitats for various species, fertilizing floodplains, stabilizing riverbanks, and forming deltas and beaches that help protect coastal areas from waves and storms.

What does Grady's model demonstrate about sedimentation in reservoirs?

Grady's model demonstrates how sediment, when introduced into a slow-moving reservoir, falls out of suspension and accumulates, forming a delta. The process mimics the real-world buildup of sediment in reservoirs.

What are the challenges associated with dredging as a solution to sedimentation?

Dredging is costly and inefficient for large-scale sediment removal, requiring the removal of large volumes of sediment to restore storage capacity. Additionally, the spoils can be wet, difficult to handle, and sometimes contaminated, requiring special disposal methods.

What alternative does the script propose for managing sediment downstream of dams?

One alternative is to allow sediment to flow through the dam, which can help restore natural river functions. However, this process must be carefully controlled to avoid environmental harm, as releasing too much sediment at once can have negative effects downstream.

What is the concept of a 'dead pool' in reservoir design, and how does it relate to sedimentation?

A 'dead pool' is a reserved volume of water in a reservoir set aside for sediment accumulation. It’s designed to prevent sediment from clogging gates, but it’s only a temporary solution as sediment eventually fills the reservoir.

Why is sedimentation considered an inevitable problem for most reservoirs?

Sedimentation is inevitable because all rivers carry some sediment, and over time, it accumulates in reservoirs. The issue is worsened by factors like soil type, land use, and water-driven erosion upstream.

How does soil conservation play a role in reducing sedimentation in reservoirs?

Soil conservation techniques, like maintaining vegetation and good agricultural practices, help reduce the erosion of soil upstream, thereby limiting the amount of sediment that reaches reservoirs.

What are some of the challenges in managing sedimentation at large reservoirs like Lewis and Clark Reservoir?

Managing sedimentation at large reservoirs is challenging due to the vast size of the watershed, the complexity of long-term land management, and the difficulty in implementing widespread soil conservation measures across such large areas.