Dams are structures built across rivers and streams to generate hydroelectric power, provide water for irrigation, control flooding, and provide recreational opportunities. However, dam construction and operation can have significant impacts on river ecosystems and fish populations.
The construction of dams has various ecological and environmental impacts on river ecosystems and their surrounding habitats. Therefore, it is important to understand the positive and negative impacts of dam construction and removal. Although dams have their benefits, the research indicates direct negative impacts of dams on the biological, chemical, and physical characteristics of rivers and riparian ecosystems, as well as on fish populations. Extinction of fish and aquatic species by preventing migration, destruction of coastal deltas, and disruption of downstream sediment transport essential for habitat maintenance are some of the main impacts of dam construction. Furthermore, dams alter the free-flow upstream ecosystem to a lentic habitat. Changes in temperature and dissolved oxygen levels in the water are some the examples of physical impacts of dams on the river ecosystem.
One of the most important impacts of dam construction on river ecosystems is the alteration of natural flow regimes, which can influence the timing, frequency, and duration of flood events, droughts, and seasonal fluctuations. For example, dams can reduce downstream flows, resulting in less water availability for fish and other aquatic organisms. They can also retain sediment behind the dam, causing erosion downstream and changing the shape and depth of river channels. These changes can negatively affect fish populations by reducing the amount and quality of habitat available for spawning, feeding, and migration.
Dam removal is a process to restore and revitalize the river ecosystem. Dam removal rapidly restores connectivity between upstream and downstream and facilitates migration and spawning. Consequently, populations of fish and other aquatic species increase. In addition, dam removal replaces the lentic ecosystem with a lotic ecosystem, which is the river’s natural ecosystem for aquatic communities. Removal of the dam results in erosion and relocation of sediment from the reservoir to the lower river, providing plants and animals with the former underwater level. However, the fate of the washed sediments depends on their characteristics and the rate of dam removal. Dam removal can have significant impacts on river ecosystems and fish populations. Dam removal can restore natural flow regimes, sediment transport, and channel morphology, which can improve the quantity and quality of fish habitat. For example, the removal of Elwha Dam in Washington State resulted in the restoration of more than 120 kilometers of river habitat and the recovery of several fish species, including salmon and steelhead. Similarly, the removal of Glines Canyon Dam on the Elwha River resulted in the creation of new habitats, such as spawning gravel beds, which increased fish productivity and diversity.
In addition to improving habitat quality, dam removal can improve fish migration, spawning, and population dynamics. For example, the removal of Edwards Dam on the Kennebec River in Maine, USA, resulted in the recovery of populations of American shad, alewife, and blueback herring that had been cut off from their historic spawning grounds for over 160 years. The effects of dam construction and removal on river ecosystems and fish populations are complex and context-dependent. While dam removal can improve habitat quality and strengthen fish populations by restoring natural flow regimes and removing barriers to migration and spawning, it can also cause short-term disturbance and alter sediment transport.
In conclusion, special attention should be paid to reducing biodiversity loss and other ecological and environmental impacts when planning dam construction and removal. The long-term impacts of dam construction on ecosystems and biodiversity are rarely considered in planning. It appears that there is a need to change the approach to the evaluation and siting of new dam construction. Alternative methods of energy generation should be given greater consideration and study. There are several sustainable alternatives for power generation that can replace hydroelectric power generated by the dam. For example, solar power, wind power, geothermal power, etc.
A balance must be struck between harnessing the hydropower potential and preserving the river ecosystem and fish life cycle. Without the proper planning system, dam construction will only lead to species extinction and ecosystem destruction. Decision-makers should work together to create sustainable and compatible solutions that balance the needs of people and the environment.
Written by Ali Khari, Master’s student with the SEG.