Dams and Rivers – Maybe not a Good Fit

The Swamp Stomp

Volume 19, Issue 4

Rivers and streams are an integral part of the hydrologic cycle of water that occurs throughout the world, transporting rainwater from river basins upstream, to locations downstream and ultimately to the oceans. Along the way, they support fish and wildlife habitats, provide us with drinking water and irrigation, and help provide recreation and other useful functions. When a dam is built on one of them, for whatever reason, the equation changes. There are many obvious and but also sometimes subtle changes to ecosystems. For example, the increased transmission of malaria has been directly linked to dam construction in reservoirs in Southeast Asia and Africa.

Through 2015, dams have disrupted the flow of water to more than half of Earth’s major rivers with approximately 57,000 large dams being built. Millions of people worldwide have been displaced by the construction of dams, in the name of flood mitigation, hydroelectric power, water storage, and recreation.

The loss of forests, wetlands, and wildlife through inundation is one obvious effect of dams. Another effect is that by eliminating the natural flooding of an area we are affecting its ecological balance and this can cause major shifts in species diversity or even the possible loss of a species as in the case of the Tellico Dam project in Tennessee and the endangered Snail Darter.

A contamination problem that is pervasive in reservoirs is the accumulation of high levels of mercury in fish. Mercury is harmless in its organic form, occurring naturally in soils, but decomposing organic matter can transform this mercury into a toxic form called methylmercury. Methylmercury passes up the food chain and becomes dangerously concentrated. Levels of methylmercury in large fish at the top of the food chain can be high, and human consumption of these fish can cause central nervous system poisoning.

Dams affect the deposition of sediment downstream and within the reservoir. The sediment that would normally flow downriver now gets piled behind the dam. The disruption of the natural flow and deposition of sediment downstream leads to increased erosion of the riverbanks and streambeds for hundreds of kilometers downstream from the dam.  Silt from floodwaters deters erosion of delta wetlands and is instrumental in the dispersal of organic nutrients from the outflow of rivers. Without the floodwaters making their way to these natural landforms, the salinity can increase downstream. This has a severe impact on delicate eco-structures of estuarine and coastal wetland ecosystems.

Large temperature changes within a dam reservoir can affect many species of aquatic plankton, invertebrates, mollusks and fish that are extremely sensitive to even mild thermal changes. The water temperature regime of these large reservoirs is altered from its natural state behind the dams. Water channels downstream are also affected as water is released from the dam. Sensitive organisms must either adapt, relocate or die.

An additional concern with the impact of dams on the environment is degraded water quality.  Organics that would normally get washed downstream get built up behind the structure and consume large amounts of oxygen when they decompose. This can result in algal blooms. Rivers that are dammed don’t have the natural transport of sediment that is critical to having a healthy organic riverine channel.

Fish migration depends on a steady flow of a river to guide them to their spawning grounds. Dams can increase the time it takes for migration. While fish ladders and elevators have been installed in some dam structures, getting to them can be devastatingly tedious.

Dams transform the upstream, free-flowing river ecosystem to an artificial, stagnant pond in the reservoir. The changes in temperature, chemical composition, dissolved oxygen levels, and physical properties are not viable to the plants and animals that originally evolved with the river system. Reservoirs host non-native, and invasive species as a result.

Today, many dams that were once at the epicenter of a community’s livelihood that is now old, unsafe or no longer serving their intended purposes and is being removed to restore ecological balance. Trying to weigh the need for developing additional water resources while conserving the environment will continue to impact future generations.


1. “Environmental Impact of Dams”, International Rivers https://www.internationalrivers.org/environmental-impacts-of-dams 

2. “Problems and Benefits of Building a Dam,” Education Center Online. 2019 http://www.educationcenteronline.org/articles/Engineering-Careers/Problems-and-Benfits-of-Building-a-Dam.html

3. “The Downside of Dams: Is the Environmental Price of Hydroelectric Power Too High?”   Scientific American. https://www.scientificamerican.com/article/how-do-dams-hurt rivers/

4. “How Dams Impact Rivers,” American Whitewater, https://www.americanwhitewater.org/content/Wiki/stewardship:dam_impacts

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