engineering and innovation. Commonly built across a river to store irrigation water and to produce electricity through hydropower, dams have proven to be very useful to humanity. Dams have had such an impact on the world that more than half of the world’s major rivers had dams constructed on them by the end of the 20th Century¹. However, such alterations of nature have not come without great tolls on the environment, such as the wiping out of entire species, flooding wetlands, forests, and farmlands, and displacing millions of people around the world.
In this article we will examine some of the major ways that dams impact our environment. Realizing the environmental damage that dams have caused, perhaps humanity will one day innovate even further and bring forth the restoration of these natural river ecosystems and meet the challenges of our irrigation and electricity needs in much more sustainable ways.
Many dams do not allow fish to pass through
Unless specifically engineered to allow fish to pass through them, dams present a barrier to fish that need to migrate to spawn and reproduce downstream and upstream along a river. This not only impacts the populations of the fish themselves, but it can negatively impact other species in the food chain that either eat that fish or are preyed upon by that fish. Such alterations in fish populations can also impact other species in a river’s ecosystem such as different plant species that might be eaten by the fish, and even the surrounding ecosystems, where other animals may depend on the fish from that river system.
Flooding and the destruction of surrounding habitat
Dammed rivers create a reservoir upstream from the dam, which spills out into the surrounding environments and floods ecosystems and habitats that once existed there. Such flooding can kill or displace many different organisms, including plants, wildlife, and humans.
Dams produce greenhouse gases
The flooding of surrounding habitat around dams kills trees and other plant life that then decomposes and releases large amounts of carbon into the atmosphere. Because the river is no longer flowing freely, the water becomes stagnant and the bottom of the reservoir becomes becomes depleted of oxygen. This lack of oxygen creates a situation where methane (a very potent greenhouse gas) is produced from the decomposition of the plant materials at the bottom of the reservoir that eventually gets released into the atmosphere, contributing to global climate change.
Sediment builds up behind the dam
Because a dammed river no longer flows freely, the sediment that would have otherwise been deposited naturally downstream begins to build up behind the dam, forming new riverbanks, river deltas, alluvial fans, braided rivers, oxbow lakes, levees and coastal shores. These changes in sedimentation can lead to dramatic alterations in plant life and animal life and how they are distributed.
Downstream sediment erosion
Due to the restrictions in the sediment flow above a dam, the lack of sediment that would have once flowed downstream ultimately leads to a deficiency in sediment load, and therefore, leads to an increase in downstream erosion. This lack of sediment load causes the riverbed to deepen and narrow over time, a compromised water table, the homogenization of the river’s flow, reduced wildlife support, and a reduction in sediment that reaches coasts and deltas.
Negative impacts on local fish populations
Typically, local fish species will not be adapted to the new environment that is present after a dam is built and do not survive, leading to the extirpation of local populations. Many factors impact their survival, including the blockage of migration routes, a disconnection from the river’s flood plain, changes in a river’s flow, changes in temperature, turbidity, dissolved oxygen, and changes in local plant life. The construction of dams is one of the major factors that is contributing to the loss of freshwater fish species worldwide².
Production of methylmercury
The stagnant water in reservoirs creates a situation where the decomposition of organic matter from decaying plants can transform inorganic mercury into methylmercury. Unfortunately, methylmercury tends to bioaccumulate and cause toxic effects in humans and wildlife that eat the fish in reservoirs³.