Question

Building a dam is one method of regulating the flow of a river to control flooding. Dams and their reservoirs may also provide recreational opportunities and water for irrigation and hydroelectric power generation. This image, from near Page, Arizona, shows Glen Canyon Dam on the Colorado River upstream from the Grand Canyon and a portion of Lake Powell, the reservoir it created. a. How did the behavior of the stream likely change upstream from Lake Powell? b. Given enough time, how might the reservoir change? c. Speculate on the possible environmental impacts of building a dam such as this one.

Short answer

Upstream, stream flow diminishes and sediment accumulates. Reservoirs fill with sediment over time, affecting capacity. Dams disrupt ecosystems and alter natural water flows.

Step-by-step solution

Analyze Upstream Behavior changes

When a dam like Glen Canyon Dam is built, the stream's behavior upstream changes significantly. The flow of the river is restricted, causing water to back up and form a reservoir. This process reduces the speed and flowrate of the river upstream; sediment that would typically be carried downstream begins to settle at the bottom of the reservoir.

Understand Long-term Changes to the Reservoir

Over time, without enough flow to transport sediment downstream, the reservoir will accumulate sediment. This process can slowly reduce the water capacity of the reservoir, potentially affecting its ability to store water and generate hydroelectric power.

Speculate Environmental Impacts

Building a dam changes natural ecosystems. Fish and other aquatic life can be disrupted by changes in water flow and temperature. Migratory patterns may be altered, and plant and animal life depending on the river's natural flow and floodplains may be negatively impacted. Additionally, downstream flows are reduced, which can impact agricultural and recreational activities.

Key concepts

Hydroelectric Power Generation

Hydroelectric power generation is a sustainable way to produce electricity by using the energy from flowing or falling water. Dams play a central role in this process, as they create reservoirs where water is stored. As water is released from the reservoir, it falls through turbines that convert its kinetic energy into mechanical energy. This mechanical energy is then transformed into electrical energy through generators.

There are several benefits to hydroelectric power generation:

• It is a renewable source of energy that doesn't run out as long as the water cycle continues.
• Produces little to no greenhouse gas emissions during operation.
• Provides a reliable power source that can be adjusted based on demand.

However, it's important to remember that the creation of large reservoirs also comes with challenges. These include changes to local ecosystems and alterations in water flow which can affect the environment downstream.

Environmental Impacts of Dams

Building a dam significantly alters the environment in and around a river system. A variety of ecosystems both upstream and downstream of the dam are impacted. When a dam is constructed, water flow is blocked, creating a large reservoir. This alteration can disrupt native plant and animal species that depend on the natural river conditions. For instance, fish that rely on flowing waters may find it hard to survive.

Furthermore, the change in water flow can alter water temperatures and reduce oxygen levels, impacting aquatic life. For humans and wildlife, decreasing natural flood cycles can affect the availability of resources provided by periodic river flooding, such as nutrient-rich soils.

Other impacts include:

• Reduction of sediment flow, affecting downstream fertility and geomorphology.
• Potential displacement of people and communities living in areas that become part of the reservoir.
• Impacts on recreational and commercial fishing, as well as agriculture and tourism industries downstream.

These effects highlight the intricate balance between benefiting from and protecting river ecosystems.

Sediment Accumulation

Sediment accumulation is a key concern when building and operating dams. Normally, rivers carry sediment downstream, depositing it along their path and at their mouths. This sediment is essential for maintaining healthy river ecosystems and fertile deltas. However, when a dam is built, it interrupts this natural process, trapping sediment in the reservoir it creates.

This can lead to several issues:

• Over time, sediment buildup in the reservoir reduces its storage capacity, hindering its ability to provide water or function efficiently for hydroelectric power generation.
• The lack of sediment downstream can lead to increased erosion and damage to habitats that depend on sediment deposits for survival.
• Changes in sediment levels can affect water quality and increase the cost of water treatment.

Addressing sediment accumulation involves regular monitoring and management, such as dredging or sediment bypasses, to maintain the balance within the ecosystem and the efficacy of the dam.