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The Hydroelectric Dam: A River Ecosystem’s Worst Enemy…

Sean Feature Image

This week’s blog post is written by Sean, a former Drummers student. He explains the negative side to a common “green” energy method. The hydroelectric dam may be beneficial for creating energy but has a significant impact on our aquatic ecosystems. 

American rivers are being overcrowded with large hydroelectric dams, which are destroying native ecosystems in rivers and streams. One way we are negatively affected is that hydroelectric dams can collect large amounts of mercury behind the dam, contaminating the fish and us. Additionally, when the water current is slowed behind the dam, the entire native ecosystem is altered. Certain fish can only survive in certain types of water, whether it’s fast or slow moving. This can also cause sedimentation, which destroys native habitat above and below the dam. Instead of sediments being evenly distributed throughout the course of the river it is dumped in a single location above the dam. Hydroelectric dams block migrating fish from getting to their spawning grounds. Hydroelectric dams damage native river ecosystems by changing the physical, chemical, and biological properties of the river and are therefore not a good solution to our energy crisis.

Napa River

Open, free-flowing rivers are used all over the world by migratory fish coming from the ocean to spawn in freshwater streams and rivers. The number of free flowing rivers has drastically decreased due to dams, especially hydroelectric. These rivers can be virtually useless to the migrating fish, because they can’t reach their spawning grounds. Out of all of the rivers in the United States that are over 600 miles long, only the Yellowstone River is still free flowing. Wild New York Salmon have virtually become extinct because of hydroelectric dams. The construction of two dams on Sacramento River have isolated Salmon from large portions of their spawning grounds. There once were thousands of Salmon that would spawn in the Sacramento River annually, but 95% of the past two generations have died. It is believed that the dams have not only separated the Salmon from their spawning grounds, but also affected the water temperature. The reservoir releases warmer water that’s unsuitable for young Salmon and the their eggs.  The dam operators have tried to alleviate this stress on the Salmon by releasing cooler pockets of water from the reservoir, but when the cool pockets are all used up, warm water once again flows in the river killing the fish. The New York government is attempting to soften the blow by raising Salmon in hatcheries and releasing them into the wild, but this is very expensive to do and is ineffective due to a large die off when released. The same thing has happened in the Snake River in  the Pacific Northwest where the Salmon populations have plummeted from 200,000 to just 175. Another migration route blocked by a hydroelectric dam is in the Yangtze River in China, where the Three Gorges Dam was built. There are an estimated forty fish species which have had their migratory routes blocked by the Three Gorges Dam. This puts one of the top twenty-five biodiverse hotspots in the world at risk.

Some argue that fish ladders are a practical solution to the effect that dams have on migratory fish. However, we don’t have to look far to see that they are not a viable solution. Perhaps one of the most devastated species is the American Shad, which spawn in the Delaware and Susquehanna River. There were once stories of the Shad coming up the Susquehanna making waves in the river several feet tall. Annually there were 17.5 million pounds of Shad caught per year. There were reports of Shad travelling the whole way up the Susquehanna 500 miles to Cooperstown, New York. Now the Shad can only travel ten miles from the mouth of the Susquehanna until it hits a hundred foot wall of concrete, called the Conowingo Dam. Now all the dams on the Susquehanna and Delaware have fish ladders for moving the fish to the top of the dam. However, it is estimated that as low as 32% of the Shad make it past the first two dams on the river, and there is a total of four dams on the Susquehanna River. Fish ladders are simply impractical because they transport so little numbers of fish to the top of the dam. Many become disoriented and confused when they hit the artificially warm and slack water at the top of the dam, and never reach their spawning ground. Shad go back to the ocean when they are done spawning, which means that they have to get down river and past the dams some how. An estimated 30% of fish are killed that go through the turbines of hydroelectric dams.

fish ladder
Fish Ladder

Hydroelectric dam negatively affect streamside ecosystems called Riparian Zones. Riparian zones are low lying areas along rivers that are often flooded. Riparian zones have a huge impact on terrestrial and aquatic organisms. The often flooding of riparian zone is perfect for a some plants which need the constant moisture and soil type. Some of these plants are vital sources of food and habitat for animals. For aquatic organisms riparian zones improve water quality in the river. They create shade along the streams and rivers which helps to keep water temperatures at acceptable levels for the organisms, that are living in the water. They also help filter out sediments in runoff water, which lessens the surplus runoff of sediment being dumped in the river. So riparian zones are vital to the river and the land around it. When hydroelectric dams are built these low-lying riparian zones along the river are flooded above the dam. The riparian zones down from the river are also affected since dams control flooding. Riparian zones rely on flooding to add moisture and nutrients to the soil. So without flooding riparian zones will slowly be destroyed.

Recent research has found that newly flooded dams can contaminate water with dangerous amounts of mercury. In some locations there are high levels of mercury in soil, and when the land behind the dam is flooded, the soil that had previously not been submerged is now underwater. There are certain macroorganisms in water that are able to convert mercury into methylmercury, which is soluble in water. It works its way up the foodchain collecting in greater quantities each time, and eventually into us, when we eat fish contaminated by the mercury. Mercury can cause damage to the brain, kidneys and lungs possibly leading to death. This problem has been seen more in North America than other parts of the world. One of the best examples is the Smallwood Reservoir created by the Churchill Falls Hydropower Project in Labrador, Canada. Local fish studies revealed the elevated mercury levels in the Smallwood Reservoir. Many people in this area live off the land, which means that they are at a very high risk of mercury poisoning.

Hydroelectric dams dump huge amounts of sediments above the dam. When the water is slowed behind the dam in the reservoir, the sediment particles suspended in the water are dropped and pile up on the reservoir floor. This creates a river bed covered in several feet of muck compared to the natural stone river bed. This area can become useless to many native fish species because they can only lay eggs on certain types of riverbeds and when it is covered in feet of sediments it is unusable to them. So this allows nonnative species and invasive species to enter the river and outcompete native fish species. Sedimentation above the dam also greatly affects the river below it. Deltas, barrier islands, and coastal wetlands at mouths of rivers are made from sedimentation, and without sediments in the water, the deltas are washed away into the ocean. Many species of animals depend on deltas for habitat and food. Another interesting fact is that dams cause erosion downstream from the reservoir. Since the water coming out of the reservoir is unsaturated with sediments, it looks to become resaturated. It does this by eroding river banks, beds, and deltas down river until it becomes saturated again, causing massive damage. An example as to just how much sediment is stopped by a hydroelectric dam is the Three Gorges dam in China. Before filling the dam, 500 million tons of sediment washed down the river per year. After filling the dam, 64 million tons of sediment washed down the river per year.

Conowingo Dam

Dams also alter native ecosystems in the river. As previously stated, the sedimentation behind the dam covers the rocks needed for some fish to lay eggs. The covering of the riverbed with sediments also destroys habitat for microorganisms and crustaceans like shellfish, crayfish, and insect larva. The river above the dam is transformed from a free flowing river into a stagnant flat lake. The water in the reservoir is also low in oxygen and has high temperature compared to what the river used to be like. This limits what type of fish can live behind the dam and native species are replaced with fish often found in lakes. Better suited animal populations would explode while others will diminish, creating an imbalance in the river.

Hydroelectric dams damage native river ecosystems by changing the physical, chemical, and biological properties of the river and are therefore not a good solution to our energy crisis. Hydroelectric dams have proven over and over again how destructive they are. Fish ladders alone are not enough to fix the problems of hydroelectric dams. There should be more planning done, when preparing sites for hydroelectric dams. The companies should pick rivers based on minimizing environmental impacts, not how much money they can make. Also, instead of building a dam ten miles from its mouth, build it closer to its source. This would give the river time to recover from sedimentation. It would also give migratory fish greater access to the river that they’ve been using for thousands of years. This would not solve all the problems of hydroelectric dams like mercury poisoning, habitat destruction, and riparian zone destruction, but it would make hydroelectric power a more green source of energy, with minimal impact on the river ecosystem.