Dams and Lake Ecosystems: Ecological Changes and Biodiversity Loss

How Do Dams Alter the Natural Flow of Rivers?

The primary way that dams alter the natural flow of rivers is by serving as a barrier to the natural movement of water. A catchment, or in some cases a reservoir, will form behind the dam, reducing the surface area and flow of the river downstream.

According to the Vermont Department of Environmental Conservation, the body of water that develops “behind the dam loses many of its riverine characteristics,” while the “loss of turbulent flow may reduce dissolved oxygen concentration.”

In other words, building a dam makes a river less like a river, altering its characteristics and impacting the types of fish and other species that can survive there.

How Do Dams Affect Downstream Ecosystems?

Large dams affect downstream ecosystems in several ways. Because the dam traps sediment and other debris, the downstream river may have fewer nutrients to support riverine plants and healthy fisheries. The water quality and temperature will also be affected, and lower water levels may mean that fish can’t navigate as easily.

A report by the U.S. Geological Survey explains that the downstream effects can vary from one river to the next, giving examples such as “sand building up in one channel, vegetation crowding into another, and extensive bank erosion in another.”

It also points out that water flow will be dictated less by natural cycles than by human activity, for example, the release of water for irrigation or hydropower.

What Are the Ecological Impacts of Dam Construction?

A report from the University of Pennsylvania cites evidence from the Department of the Interior (DOI) and other sources to highlight three key impacts of dam construction on free-flowing rivers and surrounding ecosystems:

• Effects on river flow, including the formation of catchment basins
• Changes to the “physical characteristics of river channels and floodplains”
• Impacts on connectivity, or the links between river and lake ecosystems

Even hydropower dams may not be as good for the environment as they seem, since large-scale reservoirs release methane gasses that contribute to climate change.

How Do Dams Contribute to Biodiversity Loss in Freshwater Environments?

Dams don’t necessarily result in a decrease in overall biomass. In fact, in some cases, they may increase it. One report in the Journal of Cleaner Production found that dams “decreased the water fungal biomass … in reservoirs and downstream reaches, but increased the amount of soil microorganisms in downstream lake wetlands.”

However, they tend to result in a decrease in freshwater biodiversity, or the range of life forms that exist in lake and river ecosystems. Instead of supporting a variety of riparian plants and freshwater fish species, dams may lead to an increase in microorganisms and invertebrates, such as invasive mussels, that crowd out native species.

Since freshwater ecosystems — especially large rivers like the Mekong in Asia, the Amazon in South America, and the Congo in Africa — account for up to 40% of the Earth’s fish biodiversity, this can have a major impact on global biodiversity.

What Are the Effects of Dams on Fish Migration and Populations?

Dams can affect fish populations in multiple ways, some of which are more visible than others. The most obvious is that they physically impede migratory fish populations, such as salmon that need to swim upstream in order to spawn. This is why dams in the U.S. and Canada often use fish ladders and other systems to facilitate fish passage.

Another impact is fragmentation, which is when populations of a species are separated and are no longer able to spread out or interbreed. This can result in reduced genetic diversity and population crashes in areas which can no longer support them.

Dams can also result in increased water pollution, since reduced flows may result in higher concentrations of agricultural runoff into lakes, tributaries, or other bodies of water in the watershed or river basin.

The North Atlantic Salmon Conservation Organization (NASCO) points out that water pollution is “a major cause of the decline in stocks of Atlantic salmon,” in addition to invasive species, migration barriers, and hydrological changes to their habitat.

How Do Changes in Water Temperature and Sediment Transport Caused by Dams Affect Aquatic Habitats?

The accumulation of water behind dams, or impoundments, impacts water quality in a number of ways, including water temperature and its sediment content. For one, as sediment builds up behind the obstruction, less of it flows downstream, depriving downstream rivers and lakes of important mineral and nutrient content.

As much as “12% of the global river phosphorus load was retained in dam reservoirs,” according to the National Academy of Sciences, contributing to eutrophication, or the growth of microorganisms in a body of water at the expense of other lifeforms.

Increased variability in water temperature is itself a major threat to aquatic life, since some species can only survive in a narrow temperature range, or rely on predictable temperature changes to cue spawning or migration.

Reservoirs tend to contain colder water than free-flowing rivers, and when water is released, it can result in early or late migrations or reproductive cycles.

What Are the Long-Term Ecological Consequences of Sediment Buildup Behind Dams?

In practical terms, the amount of sediment that accumulates behind a dam reduces the amount of water that its reservoir can hold. Eos magazine estimates that sedimentation has “reduced the absolute water storage capacity of U.S. reservoirs by 10%–35%.”

As water scarcity continues to be an issue in the face of climate change, this fact alone should make us think about our reliance on dams for drinking water.

But sedimentation has a secondary effect: Sediment that accumulates behind the dam doesn’t make it downstream, reducing the amount of minerals and nutrients that reach downstream habitats. And since the water that flows out of the reservoir contains less sediment, it’s more likely to erode those habits along the way.

How Do Dams Impact Wetland Areas and Their Associated Biodiversity?

Wetlands are highly dependent on seasonal flooding and the inflow of sediment from rivers. Reduced flooding means that wetlands may not get enough water to sustain plant, fish, and bird life. In some cases, they may shrink or disappear altogether.

A study of the Nenjiang River in Northeast China found that many “riparian wetlands have become separated from the river channel and have changed to marsh.” In other parts of the world, farmers rely on seasonal flooding to irrigate their crops and graze livestock — this kind of land use can be severely curtailed by dam construction and farmers may have no choice but to use groundwater resources instead.

As for biodiversity, BirdLife International reports that many migratory species “time their arrival with the onset of the wet-season … The number of waterbirds that the delta can support is directly related to the extent of flooding during this period.”

What Are the Potential Effects of Dam Removal on Local Ecosystems?

If dams on the whole have a negative effect on river and lake ecosystems, does dam removal reverse these ecological changes and biodiversity loss? In the long run, dam removal can return rivers and wetlands to their natural state — but it can cause some side effects in the short-term that need to be addressed.

First of all, the sediment that has accumulated behind the dam has to go somewhere — and allowing it to flow freely down the river all at once can impact water quality. It may be preferable to dredge the sediment from the catchment, filter it out, or release the water gradually to reduce downstream impacts.

Another impact is on plants that have established themselves downstream while the dam was in place — which could be hundreds of years in some cases. As water flows back into riverbeds, some habitats may be flooded, and it will take time before more suitable plants and animals reestablish themselves in the ecosystem.

Still, dam removal is on the agenda for many American and European rivers, since the long-term benefits outweigh the short-term consequences.

How Do Dams Influence Nutrient Cycling Within Lake Ecosystems?

Lake ecosystems naturally accumulate nutrients over time, particularly phosphorus and nitrogen, which are necessary to support plant and animal life. When dead plants and animals decay, they release these nutrients back into the ecosystem.

Too many of these nutrients, however, can contribute to the growing concern of harmful algal blooms in our ecosystems, in a process called eutrophication. Dams contribute to eutrophication by disrupting the natural flow of nutrients within the ecosystem.

What Role Do Dams Play in the Introduction and Spread of Invasive Species?

Dams can contribute to the introduction and spread of invasive species by serving as a “stepping stone” from which they can infiltrate the surrounding ecosystem. Reservoirs provide a stable habitat for species that can’t survive in a free-flowing river, and that benefit from a lack of competition from native species.

Because a single river may have multiple dams, invasive species can move from one catchment to another or be carried between river systems by recreational boaters.

Dams and Lake Ecosystems: Are They Compatible?

Dams present many threats to river and lake ecosystems that can result in ecological changes and biodiversity loss. From trapping sediments to impeding migration, dams disrupt the natural flow of the river and cause eutrophication of lakes and wetlands.

Dam removal can return ecosystems to their natural state, but in some regions, dams may be necessary for flood mitigation or to produce hydroelectric power.

With effective water conservation and smart water management, we can reduce our reliance on dams and protect our natural resources from overuse and pollution.

Learn more about our water conservation and restoration projects here.