Dams: Water Pollution Or Conservation?

Dams are constructed across rivers to store water, generate power, and provide a benefit to people. However, they can have a detrimental impact on the environment, including water pollution. This pollution can be caused by a variety of factors, such as the disruption of natural river processes, the alteration of water temperature and oxygen levels, and the blocking of sediment and organic material transport. These changes can lead to increased stress and death among aquatic organisms and negatively impact the surrounding ecosystem. The decision to build or remove dams involves weighing the benefits against the environmental harm they may cause.

Do Dams Pollute Water?

Dams can disrupt the natural flow of water, causing higher temperatures and lower oxygen levels

Dams are often constructed to block rivers and streams, altering the natural flow of water. While they can benefit society, they also cause considerable harm to rivers. Dams have depleted fisheries, degraded river ecosystems, and altered recreational opportunities. They can also disrupt the natural seasonal flow variations that trigger growth and reproduction cycles in many species.

The water held in reservoirs tends to heat up, increasing the temperature of the river. This can negatively impact fish species, such as salmon and steelhead, which prefer cooler waters. The increased temperature can also lead to a decrease in dissolved oxygen levels, causing stress and even death for fishes, mussels, stream insects, and other aquatic organisms. This can create ""dead zones" incapable of supporting river life.

Additionally, dams can trap sediment, substrate, and large pieces of wood, preventing them from being carried downstream. This can result in a loss of habitat for juvenile and adult salmon, which rely on these materials for spawning and creating complex habitats. The lack of sedimentation in the water flowing over the dam can also deprive downstream ecosystems of essential nutrients.

The presence of a dam can also change the way predators and prey interact. For example, fish delayed by passing a dam may become targets for predators, and changes in habitat can allow non-native species to invade and compete with or prey upon native species.

They can block fish passage, harming migratory fish like salmon

Dams are a significant barrier to fish migration, and this has a particularly detrimental effect on migratory fish like salmon. There are over 90,000 large dams in the United States alone, and tens of thousands of smaller ones, all blocking fish passage to some extent.

Salmon are adapted to living in rivers, and their life cycle is negatively impacted when their habitat is changed to a lake. Dams can block the downstream movement of juvenile salmon, preventing them from reaching the ocean, where they will spend their adult lives. They also block access to the upstream habitat where salmon breed and grow, preventing them from reaching their spawning grounds. This has a devastating effect on salmon populations, as when fish can't reach their habitat, they can't reproduce and build their numbers. Salmon runs, once numbering over a million, are now only around three percent of that.

In addition to physically blocking fish passage, dams also alter the natural flow of water in rivers. They can trap sediment, burying rocky riverbeds where fish spawn, and can also trap gravel, logs, and other important habitat features, reducing the available habitat for juvenile and adult salmon. The water behind a dam can also heat up, increasing the temperature of the river, which can be harmful to salmon, who prefer cooler water.

The impact of dams on salmon populations has been recognised for a long time, and various measures have been attempted to mitigate it, with varying levels of success. Fish ladders and water-filled fish elevators have been built to help adult salmon migrate upstream, but they have not been very effective. In the 1970s, an experiment was conducted where salmon were flown over dams in an airplane, but this was short-lived. In some cases, fish hatcheries have been built downstream of dams to compensate for the loss of spawning habitat. While dams can provide benefits to society, such as hydropower and irrigation, it is important to consider their negative impact on fish migration and take steps to mitigate these effects.

Dams can alter the timing of flows, destroying natural seasonal variations that trigger growth and reproduction cycles

Dams are often constructed across rivers to store water that would naturally flow downstream. While they can benefit society, they also cause harm to rivers and the surrounding environment. Dams disrupt the natural balance of rivers, affecting the animal and plant life in and around them. One of the main ways they do this is by altering the timing of flows, which can destroy natural seasonal variations that trigger growth and reproduction cycles.

Hydropower dams, for example, may withhold and then release water to meet peak power demand. These irregular releases can disrupt the natural seasonal flow variations that many species rely on for their growth and reproduction cycles. This can have a significant impact on fish populations, particularly migratory fish like salmon, which are born upstream and may struggle to complete the round trip to spawn. The wild salmon numbers in the Pacific Northwest’s Columbia River basin, for instance, have declined by 85% since dams were introduced there 50 years ago.

Dams can also affect the natural transport of sediment, which is crucial for maintaining healthy organic riparian channels. Rocks, wood, sand, and other materials build up at the mouth of the reservoir instead of being dispersed through the river channel. This can result in a loss of spawning gravel for fish and a reduction in the creation of complex habitats downstream. The change in river composition can also interfere with the chemical signals that guide species through their biological processes, such as spawning and rearing.

In addition, the water held in reservoirs tends to heat up, increasing the temperature of the river when it is released. This can be detrimental to fish species that prefer cooler waters, such as salmon and steelhead. Warmer temperatures and reduced oxygen levels caused by dams can lead to increased stress and even death for fish, mussels, stream insects, and other aquatic organisms. These changes in water temperature and oxygen levels can persist for several kilometres downstream, affecting a significant portion of the river ecosystem.

The construction of dams can have far-reaching consequences, altering the natural flow patterns of rivers and impacting the species that depend on them. The disruption of seasonal variations and natural cycles can have long-term effects on the health and diversity of aquatic ecosystems.

They can trap sediment, burying rock riverbeds and harming fish habitats

Dams are a human invention that has been used for flood protection, energy supply, water security, and food production. In the past 100 years, the United States has led the world in dam construction, with tens of thousands of large and small dams blocking rivers and streams. While they can be beneficial, dams also cause considerable harm to rivers and the ecosystems that depend on them.

One of the primary ways in which dams harm river ecosystems is by trapping sediment and other substrate materials, such as rocks, gravel, and wood, behind the dam. This sediment buildup can bury rock riverbeds and spawning grounds, harming fish habitats and interrupting the life cycles of fish species. For example, salmon are adapted to living in rivers, and changing their habitat to a lake can negatively impact their spawning and overall life cycle.

Trapped sediment can also release pollutants into the water. For instance, sediments in reservoirs often contain toxic substances such as pesticides or heavy metals. If these sediments are disturbed, they can release these toxins into the water, endangering aquatic life. Additionally, reservoirs tend to produce large amounts of algae and other plants, leading to higher nutrient levels downstream.

The buildup of sediment and substrate materials behind dams can also alter river dynamics and morphology. This includes changes to riverine connectivity, hydrological patterns, thermal regimes, and channel morphology. These alterations can negatively impact the quality and quantity of fish habitats downstream, reducing the available habitat for juvenile and adult fish.

Furthermore, dams can disrupt the natural seasonal flow variations that trigger growth and reproduction cycles in many fish species. When dams withhold and then release water for power generation, they create irregular flow patterns that can strand fish and impact their migration and spawning behaviors. These disruptions have been implicated in the decline and extinction of several fish species, such as North American salmonids and the Chinese paddlefish.

The flooding and inundation of land caused by reservoirs can negatively impact people, wildlife and the environment

The flooding and inundation of land caused by reservoirs can have a detrimental impact on people, wildlife, and the environment. Firstly, the creation of reservoirs can necessitate the relocation of large human populations, particularly if they are in residential areas. For instance, the Three Gorges dam in China led to the displacement of over a million people. This can have a profound impact on people's livelihoods, as they may need to leave their homes, workplaces, and communities.

The flooding of land can also negatively affect wildlife and the environment. The inundation of land can force animals to abandon their natural habitats, and it can destroy eggs and young animals, impacting species' survival and reproduction. Additionally, flooding can lead to habitat destruction in forests and wetlands, threatening rare species and causing long-term damage to ecosystems. This can further impact wildlife by altering the availability of food sources and access to these sources. For example, the Lake Manatali reservoir in Mali, West Africa, disrupted the migration routes of nomadic pastoralists and reduced grazing land availability.

Furthermore, the flooding of land can cause changes in sediment deposition and river flow patterns. Rivers naturally carry sediment downstream, forming depositional features such as river deltas and alluvial fans. However, the construction of a dam can block this process, leading to erosion and reduced sediment delivery to coastal plains and deltas. This alteration in sediment flow can negatively impact aquatic habitats, land elevation maintenance, and channel morphology.

The management of reservoir water levels can also have unintended consequences. The sudden release of water or reduction in flow rates can cause river levels below the dam to fluctuate, potentially stranding fish. Additionally, water held in reservoirs tends to heat up, increasing the river temperature, which can be detrimental to coldwater fish species such as salmon and steelhead. Warmer temperatures and reduced oxygen levels in the water can stress and kill fish, mussels, stream insects, and other aquatic organisms.

Frequently asked questions