Question

Consider the consequences of constructing a dam on a river that has a large stream load, such as the Yellow River. Assume the dam and its reservoir are located about two-thirds of the way down the river. How would stream flow conditions be altered above and below the dam and its reservoir? What would be the implications for erosion, transport, and deposition?

Short answer

A dam alters river flow by slowing and depositing sediments above, increasing erosion below due to clearer, faster flow.

Step-by-step solution

Understand Stream Flow Alteration Above the Dam

Above the dam, the river flow slows down as it approaches the reservoir behind the dam, leading to the formation of a lake-like environment. This decrease in flow velocity causes sediment that was carried by the river to settle and deposit in the reservoir. Over time, this process can lead to sediment buildup in the reservoir, potentially reducing its capacity to hold water and altering the upstream habitat.

Analyze Stream Flow Changes Below the Dam

Below the dam, water is released typically at a controlled rate and much of the sediment is left behind in the reservoir. The absence of sediment reduces the sediment load carried by the river, resulting in a clearer and faster-flowing stream. The reduced sediment can increase the erosive power of the water as it flows downstream, potentially causing erosion of the riverbed and banks.

Examine Erosion Implications

Above the dam, the reduced velocity and increased deposition lead to less erosion. In contrast, below the dam, the accelerated flow and decreased sediment load increase erosion, as the water seeks to pick up sediment from the riverbed and banks to regain its sediment equilibrium.

Consider Transport Effects in the River System

The transport of materials in the river system is greatly altered by the dam. Above the dam, transport is minimal due to reduced flow velocity. Below the dam, transport increases initially due to increased water velocity, but the overall transport capacity decreases over time as the river adjusts to the new equilibrium with less sediment.

Review Deposition Changes Due to the Dam

Deposition predominantly occurs in the reservoir behind the dam due to lower energy conditions. As a result, deposition downstream is reduced, potentially affecting ecosystems that rely on deposited sediments as a nutrient source. This can alter habitats and affect biological communities downstream.

Key concepts

Sediment Deposition

When a dam is constructed on a river with a significant stream load, such as the Yellow River, one major consequence is sediment deposition. As the river approaches the dam, its velocity decreases, creating a calm, lake-like area in the reservoir. This change in speed causes sediments that were once held in suspension by the fast-moving water to settle out.
As a result, these sediments accumulate in the reservoir, reducing its water-holding capacity over time. This can lead to a decrease in the effectiveness of the reservoir to manage water flow and even impact its role in flood control strategies. Additionally, this buildup can alter the habitat within and around the reservoir, potentially impacting aquatic and terrestrial life.

• Sediment settles as stream flow slows.
• Build-up can reduce reservoir capacity.
• Affects local aquatic habitats and ecosystem dynamics.

Erosion Dynamics

Erosion dynamics undergo significant changes due to the presence of a dam. Above the dam, the reduction in river velocity lowers the potential for erosion as there is less energy to carry sediments away. This means that organic matter and nutrients that normally travel downstream tend to deposit upstream, altering the local riverine environment.
Below the dam, however, the impact is noticeably different. The controlled release of water leaves sediment behind in the reservoir, making the downstream water clearer but more erosive. This clear water, now deprived of sediments, possesses enhanced erosive power, trying to return to its natural state by picking up sediment from the riverbed and banks. This can result in increased erosion and the formation of new channels or deepened existing ones.

• Upstream erosion is reduced due to slow flow.
• Downstream erosion increases, impacting river beds and banks.
• Potential for altered river courses over time due to increased erosion.

River Transport

River transport is significantly altered by the construction of a dam. Above the dam, the sediment transport is heavily reduced due to the decrease in flow velocity. The river loses its ability to carry particles, leading to sediment settling in the reservoir.
Below the dam, water flow becomes fast and sediment-poor. Initially, even though water speeds up, the lack of sediment means transport efficiency is compromised. Over time, the river adjusts, seeking equilibrium by eroding materials from its banks and bed. This can result in further downstream effects, where less sediment reaches areas that rely on deposits for ecological balance.

• Transport is minimal above the dam due to slowed flow.
• Below the dam, water is fast but carries less sediment.
• River seeks sediment from local sources, leading to further erosion.

Dam Impact on Ecosystems

The construction of a dam can deeply impact ecosystems both above and below it. In the reservoir, the new lake-like habitat can change the type of species that are supported, often leading to a shift from riverine to more lacustrine ecosystems.
Downstream, the clearer water lacks the nutrients and sediments that many riverine ecosystems rely on. This can lead to reduced fertility of floodplain areas, impacting plant and animal life that depends on regular sediment deposits for nutrients. Additionally, because the flow regime is altered, it can affect fish migration patterns and other biological processes critical for ecosystem health and stability.

• New habitats form in reservoirs, altering local ecosystems.
• Downstream ecosystems may lose nutrients essential for growth.
• Altered flow can disrupt biological processes and species migration.