More than half of the world’s large rivers have been dammed, regulating and flooding approximately 400,000 square kilometres of land worldwide. These diversions have an effect on diverse ecosystems and habitats around the globe, replacing them with uniform structures and reservoirs and ultimately changing the way otherwise balanced, stable ecosystems function.

Go with the flow
The life of a river is closely tied to its stream flow, which constantly fluctuates. Damming a river and altering its flow pattern generates a number of physical and biological impacts. The disruption of a river’s flow obstructs its’ natural current and affects the water’s habitat.

One of the largest impacts a lack of current has on a river is the sediment flow, which is normally carried down the river by the current. When trapped by a dam, the sediment is held in the reservoir and settles to the bottom while clear water containing very little sediment is released down the river.

Over time, the easily erodable material from the riverbed is carried away with no sediment being deposited to replace it. This leaves a rocky streambed, resulting in a poorer habitat for aquatic fauna.

When flow regimes and sediment loads are altered, the width and depth of the channel are also affected. A river’s natural meanders, pooling areas and riffles – light rapids where water flows across a shallow section – are changed by damming and the modified transportation and distribution of gravel affects fish’ spawning habitats. The lack of current also exposes young migrating fish to predators for a longer period of time in the slower moving water.

The surrounding ecosystem is also affected as new floodplains submerge the vegetation, causing it to decompose. Sometimes wetlands are created, completely altering the ecosystem.

Migration frustration
The most visible and obvious effect of dams is that they fragment rivers and make migration difficult for fish and other aquatic life. Species, such as salmon and eels that migrate to spawn, may not make it to their destination or may suffer injury or death while travelling through turbines or over spillways. Fish that do make it through are often disoriented and become more susceptible to predators.

Some dams are equipped with fish passage structures, or fish ladders, to attempt to accommodate the migration of a river’s aquatic life. Questions have been raised as to whether fish ladders are actually too stressful for an adult fish and that its’ chances for successful spawning is reduced.

Toxic transformation
When water is held in the reservoir of a dam, the quality of water is affected in several ways, the extent of which depending on how long it is held there.

The initial creation of a reservoir as a floodplain submerges the existing vegetation and soil, causing it to decompose and deplete oxygen from the water supply. Mercury, which exists in a harmless inorganic form in soil, may be transformed by bacteria into methyl mercury once the soil is flooded. The toxic methyl mercury is lethal to the fish downstream and can be absorbed and passed up the food chain.

A river’s aquatic community is also sensitive to the temperature of the water. Storing water in the reservoir of a dam for extended periods of time creates stratification, or layers of water that are different temperatures. This can affect the nutrient levels, dissolved oxygen levels and the productivity of heavy metals in the water.

Dam displacement
Millions of people have been displaced to make way for the construction of dams around the world. Communities that have been uprooted are often dispersed, greatly altering their livelihood, way of life and social network.

The community is also often cut off from the operation of the dam because the sophisticated technology of modern dams requires technical expertise that is often taken over by the government or corporations. The community, therefore, loses any control over the water they once depended on.

External links:
Canadian Dam Association

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