Homework - Environmental Issues with Dams

Last updated 11 months ago

8 questions

Most of the human activities that require water, require freshwater. Of all the water on Earth, freshwater makes up only 3 %. Traditional sources of freshwater include ground water, lakes, rivers, and other surface water. As the human population increases, some natural systems are pushed beyond their limits.

Changing the Flow of Surface Water People worldwide depend on sources of freshwater for their water supplies. Another method to obtain water is through dams. Streams and rivers are often dammed to create reservoirs that store water.

The large concrete structure shown to the right is the Hoover Dam in Nevada. The dam was built to control water flow and flooding along the Colorado River. Notice the large reservoir, Lake Mead, behind the dam. Freshwater from Lake Mead is used for recreational purposes, drinking water, irrigation, and hydroelectric power.

But dams can also have negative consequences on the environment and the biodiversity of ecosystems around the river. Dams can increase the rate of erosion along the banks of the streams. They also act as a geographic barrier for migratory fish. Because of dams, some rivers, such as the Colorado River, are nearly dry before they reach the ocean.

While dams can have positive effects for humans and other organisms living upstream, dams have many negative effects, particularly on ecosystems downstream of the dam.

The world's last free flowing rivers are under threat. What's commonly thought of as a _______  solution to the energy crisis is destroying the planet's last remaining pockets of _______  wilderness and intact ecosystems. 

Today, there are roughly 60,000 large dams around the world. 3,700 more are currently planned or under construction. Over 500 of them will be built in protected areas. At first sight, the concept of hydropower is a simple one. Flowing water is trapped in front of dams, then directed through _______ , which spin to generate electricity. It may seem like a green solution to meeting the world's growing electricity demand, but it's actually a _______  one. Dams emit greenhouse _______ , endanger fish species and can uproot communities.

What is the true cost of hydropower? To understand the environmental impact of hydropower, we first have to understand the value & characteristics of a free flowing river. 

In a wild river, animals, such as _______  fish, can swim up and downstream at will. Groundwater and aquifers naturally refresh the water, silt and other natural materials can move along freely. The water moves out onto the floodplain in harmony with the seasons, _______ important nutrients to wildlife and other habitats, signaling fish to spawn & bringing nutrients back into the river itself. The sad news is that there aren't many wild _______ left, 2/3 of them are already dammed and that number is increasing every day. What started with old fashioned waterwheels has developed into modern hydroelectric plants embedded in dams to generate power. Currently, 22% of the world's _______ is provided by renewable resources, 73% of which is hydropower. But although hydropower is renewable, it isn't _______ . Building a large barrier like a dam destroys the single most important thing about a river: its flow.

Both the reduced downstream flow and the reservoir upstream can destroy wildlife _______  and farmland, either through drying out or flooding. A dam also changes the structure of a river. It _______ the flow of water, which means that sediments like sand or gravel hit the dam and then drop to the riverbed. In a way, dams are thus creating big underwater _______ . And these are bad news for fish, which depend on clean, rocky bottoms. 

The clustered sediments aren't good for the dam itself either, because as the area in front of it is filled up with more and more sediment, it holds less and less _______ . Dams don't just form a barrier for sediment, but also mean a drastic impact on flora and fauna. When the flow of a river is blocked, fish can't reach their spawning grounds and over time, dammed rivers can lead to drastically reduced fish populations. 

Currently, the most common way of solving this migration problem is installing fish _______. Fish ladders are stepped pools which allow fish to swim or jump from one pool to another all the way up and behind the dam. But in most cases, they don't actually work very well. The structures are often too high or too steep, too small to support massive migrations or not able to hold enough water. Even if fish ladders do succeed, they might work for fish like salmon, but not for all types of fish because some, like eels, can't _______ . Blocked migration routes aren't the only problem. 

Reservoir water usually contains an excess of _______ or other aquatic weeds, which can crowd out other animal and plant life. The water in reservoir is typically also low in dissolved _______  and colder than normal river water. When this water is released from time to time, it can have a bad impact on wildlife. As a result of all these negative consequences, worldwide populations of freshwater species have already experienced a staggering 83% decline since 1970. More dams will likely further deplete their numbers. 

Despite all these aspects, there's a popular consent that hydropower is a vital element for the energy industry's shift away from _______ fuels, but one fundamental piece of information is missing from the argument. It might sound counterintuitive at first, but hydropower is not emission _______ . Around 10% of the world's hydropower facilities emit as many greenhouse gases as conventional fossil fueled power plants. Some existing dams in the lowland Amazon have even been shown to be up to _______  times more carbon intensive than coal fired power plants. Generating power by spinning turbines with water doesn't directly emit greenhouse gases. 

So, where do they come from?

When reservoirs cover organic matter like dead plants, trees, fields and farmland, they produce greenhouse gases, especially in _______ climates. The organic material breaks down and releases gases like carbon _______ and methane into the reservoir water. And these are the two major greenhouse gases associated with _______  change. Worldwide, the _______  vegetation in reservoirs represents 1.3% of the total annual global emissions caused by humans. 

This is alarming, but it also shows that there's potential to improve the current situation and here's how. Hydropower needs to be viewed as part of a broader _______ for energy in which the costs and benefits of different sources should be assessed and weighed against each other. A strategic mix of _______ , wind and storage technologies around river basins could be an option. Greater investments need to be made in detailed upfront planning and environmental impact assessments, which should be carried out by independent scientists.

Existing dams should be _______  and retrofitted where possible instead of building new ones. And fish passage systems need to be optimized to make it more friendly for all different kinds of fish. 

Sounds like a lot of work, right? 

But it's finally getting started. In October 2020, several environmental groups and the industry that operates America's hydroelectric dams announced an unusual agreement to work together to _______  the environmental harm caused by dams. They're also going to join forces to remove older dams that are no longer needed. More than 1,000 dams nationwide have already been _______  down in recent decades. 

In many of these places, such as the Elwha River in Washington state, nature has come roaring back, with fish populations increasing. It's expected that if this trend continues, between 4,000 and 36,000 more dams will have been _______  in the U.S. by 2050. The world's free flowing rivers are crucial for the environment and humans alike. And it's now more important than ever that we choose wisely whether we really want to build new dams. 

It's about finding the right balance between hydropower and rivers, but also more generally between energy _______  and ecosystem conservation.

Environmental Issues Associated with Dams
Dams hold the promise of a nearly constant supply of electrical power with no associated emissions of greenhouse gases or toxic contaminants. They have some advantages compared to other ways of generating electricity, such as the ability to rapidly change the amount of electricity being generated, which reduces the amount of energy lost.

Unfortunately, dams have negative effects on the environment. When filling the reservoir any land located in the area where the reservoir will be, such as farm land, houses, or cities can be destroyed. The flooding will also destroy all the existing vegetation and animal habitat in that area.

Dams also interfere with the natural river dynamics, changing the natural flow patterns such as creating a more consistent flow. A dam traps sediment carried by the rivers and streams flowing into the reservoir, and the reservoir will eventually fill up with sediment. This blockage deprives the downstream river of sediment and associated nutrients. This can cause erosion and drastically change the downstream habitat, as clear, cold water from the depths of the reservoir replaces the warmer, muddy water that flowed down the river before the construction of the dam.

Dams are also lethal for migratory fish, such as salmon. Adult fish are blocked from migrating to upstream spawning areas. Juvenile fish die if they go through hydroelectric turbines.