Dams have a lifetime just like everything else, and the dam-building mania of the early 20th century has left us with several damning problems dotted across the landscape.
Last year, I stood on top of the Alder Dam in the state of Washington, USA. Morning had shifted to afternoon, and sun rays beamed through clouds and lit up evergreen slopes. To our left, a quiescent lake sprawled as far as the eye could see, while to our right, past a chain-link fence and concrete barrier, white water plummeted 100 metres down into a narrow rocky gorge.
Alongside other students, I was visiting this picturesque scene to see, hear, and touch what we think of as a typical dam — but later, we would learn of the danger looming beneath the calm surface. Our professor told us the story of dams: how his parents’ generation built dams like this one, how his generation reaped the benefits, and how our generation must now tear them down.
Building dams is a global phenomenon, but not all countries hopped on the bandwagon at the same time or with the same fervour. Today, the number of major dams in the United States has capped out at nearly 100,000. Compare that to the 820 major dams in the driest inhabited continent, Australia, which claims the highest storage capacity per capita of any country in the world. Other regions, including Asia, Africa, and South America, are catching up in numbers by building huge dams that harness electricity. Take China, for example: in 1950, the country was home to less than ten dams. Now there are almost 20,000, including the world’s largest hydroelectric dam.
Debates about dams have raged in the United States for the last two centuries. Opponents point to stories of catastrophe. In one such story, dams in Maine have almost completely cut off all routes for fish to leave the ocean and travel upstream to spawn in freshwater lakes and streams, leading to the near-collapse of many fish populations. But some concrete behemoths have proved their worth. The Hoover Dam stores enough water to sustainably support two million acres of surrounding land and generates enough hydropower to serve 1.3 million people every year. These marvels of engineering inspire pride, awe, and curiosity.
While agencies, organisations and bystanders have been arguing over the pros and cons of building dams, the dams themselves have grown old and decrepit on the sidelines. Their average expected lifetime is just 56 years. Those built in the 1930s should be long gone by now — so why are they still here? Because removing them costs money. The bigger the dam, the bigger the hit to the wallet. Previous dam removals have run up bills of between US$2000 and over US$300 million.
Why dams can't function forever
While standing atop the Alder Dam, which was now over 70 years old, I struggled to imagine it was steadily approaching failure from sediment build up below the lake’s surface. Because of course rivers don’t just carry water. They also carry everything that happens to get washed in — mud, sand, twigs, pollutants and an occasional lone flip flop all course downriver until they become stuck behind a dam. Water pools to form the calm reservoirs that we see, while everything else sinks to the bottom, out of sight. Over decades, all that sinking gunk and trash builds up.
On an even greater scale, sediment from the slopes of far-off glaciers has been eroded over centuries and washed downriver, only to be halted by dams. Sediment has already filled in about 15% of the Alder Dam’s reservoir. Every day brings more and more sediment, decreasing the available space to store water and adding more and more pressure to the back of the dam. This is a massive problem, pushing dams to breaking point.
Many senior dams, which were built to decades-old design standards, are no longer expected to survive upgraded predictions of inevitable storms and earthquakes. As our own backs grow weaker with physical stress and age, so do the backs of dams. Eventually, if we do nothing, one of those tiny specks of sand will be the grain that breaks the dam’s back, unleashing a flood of water and sediment downstream with potentially devastating consequences.
So, how do you remove a dam?
After leaving this first dam, we drove to the Elwha River to see the place where two dams once stood before their successful removal. Built in the early 1900s, the construction plans didn’t include fish ladders: structures around barriers like dams that still allow fish to migrate upriver. For almost a century, the dams effectively removed 90% of the previously available salmon habitat by disconnecting the upper and lower river. Spawning salmon tried in vain to swim upriver but were blocked — along with everything else that flowed downriver. The dams became perfect candidates for removal when the cost of destroying salmon habitats finally outweighed the benefit of controlling floods and drawing hydropower.
The removal process was not easy. In 1992, the Elwha River Ecosystem and Fisheries Restoration Act was passed by the US Congress, kickstarting the project. It was predicted to take 20 years and cost up to US$203 million. No one had ever attempted to remove a dam as large as those on the Elwha River, and the daunting unknowns required years of research and preparation. How could the dams be removed without blasting tons of concrete into the environment? How would wildlife like salmon respond to the unleashed river? Would nearby communities be smothered by sediment and flooded? What would happen to everything else trapped behind the dam?
Sediment posed the most significant environmental challenge. With the concrete gone, the avalanche of sediment that had built up behind the dams would be unleashed: all 19 million cubic metres of it. That’s enough to fill 8000 Olympic-size swimming pools or almost 13 Melbourne Cricket Grounds. People living in Port Angeles, a small city downdrift of the river mouth, hoped the sediment would rapidly flow downriver and end up on their sediment-starved beaches. People living on tribal land near the mouth of the river worried it would come too quickly and smother their land. Both communities were asking how much would come, and when.
On top of these science and engineering hurdles, the timber industry and some local communities opposed the idea of dam removal, leading to federal funding being blocked by a local senator for almost two decades after the restoration act was passed. Finally, an agreement was reached: the dams would be taken down in stages so the water and sediment would be released gradually. In 2011, workers began to chip away at the top of the Elwha Dam. Piece by piece, slowly and precisely, the reservoir was set free. Three years later, and over three decades after the original restoration act set the removal process in motion, the last piece of removable concrete was hauled away from the Elwha River.
Emily Eidam has spent the last six years earning her PhD in oceanography from the University of Washington by tracking mud and sand released from the Elwha. According to Eidam “there was a lot of uncertainty about how the river would respond to the dramatic increase in available sediment”. She explains that researchers had used lab tests and scaled models to predict how fast the sediment would be carried away, but “ultimately the river exceeded all expectations”.
Sediment didn’t flow out of the reservoir at a steady and predictable pace. Instead, it slowly slid down the river like a giant slug until forceful storms came along and sporadically swept pulses of sediment downriver.
During removal, one of the water treatment plants downriver of the dams became clogged with sediment and had to be temporarily shut down. Then, in the first big winter storm after the dam was removed, the muddy floods came. The river channels were already choked with the slug of sediment, which left little room to accommodate all the extra water raining down onto the Elwha valley. As water and sediment stampeded over the banks of the river, roads leading into the national park were devoured and two campgrounds were swept away.
To venture into the park and get a good look at the dam sites, you have to strap on some hiking boots and prepare for the long haul. That’s just what our class did when we visited, and the extra effort was worth it for the view of the enormous valley that used to be a reservoir. The valley was filled with a layer of cobbles, sand, and silt tens of metres thick, and the valley floor was steeply terraced. Each terrace was created at a different stage in the water level as the reservoir slowly drained, evidence of the step-by-step dam removal process.
About two thirds of the sediment in the reservoir had been washed away in the five years since the dam removal process began, yet so much remained, just waiting for the next big storm. And when that storm comes, scientists will be waiting to track its course.
So far, the problems caused by dam removal have been inconvenient, but minor and well worth the benefit of habitat restoration. As the river roars back to life, salmon have returned to what used to be the reservoir for the first time in a century and given a boost to the food chain — more salmon means fatter, healthier birds. Trees have also been replanted to reduce erosion and restore native ecosystems. Eidam’s favourite lesson from the process is the resiliency of nature. “The Earth is a lot bigger than us,” she says, “and while we have found innovative ways to utilise and modify natural systems, ultimately we're visitors here, and earth systems will continue operating with or without us.”
The Elwha isn’t the only river to bounce back. Another study that looked at the impact of over 1000 dam removals in the United States found that most rivers respond surprising quickly after dam removal. Depending on their size, they take just months or years to recover. The Elwha River dams are the largest removed to date, but they probably won’t hold that title for long.
So what's next?
The Elwha River dams are an ideal natural laboratory to study the effects of dam removal. In a sense, the Elwha Dam removal project was easy. The dams were providing little benefit to the community, they hadn’t accumulated a significant amount of pollutants, and the region was sparsely populated. Many other dams aren’t as pristine. There are thousands of dams — plus many more still being built around the world — looming over heavily-populated communities that rely on them for hydropower, flood control, water storage and more.
Not only is it necessary to consider the socioeconomic impact of dam removal on these communities, but we also must consider the health risk to humans and wildlife. In 1973, a dam in New York was removed and resulted in tonnes of sediment loaded with PCBs, a toxin released into the river by General Electric, washed downstream. Even though recent regulations have significantly cut back on toxins entering dammed reservoirs today, the pollutants that were previously released are still there, just waiting beneath the water’s surface.
Dams downstream of nuclear power plants have had plenty of time to accumulate radioactive waste, and dams in agricultural areas have become the final resting place for an alarming amount of pesticides and fertilisers. Cleaning out such pollutants before a dam is removed isn’t a realistic option — it is expensive and the question of how to transport the pollutants and where to dispose of them still remains.
To tackle the problem of dam removal, we need more than just engineers and construction workers. We also need economists, health scientists, chemists, geologists — the list goes on and on. But because dam removal research is still in its infancy, there are limited case studies to learn from, and so each new project will present a new unique set of interdisciplinary concerns.
My generation has inherited a dam problem. When I was first presented with this challenge, while standing on top of the first dam of our trip, I looked at the unconcerned faces of the students around me. At the end of the trip to the Elwha Dams, those same faces displayed anxiety — and motivation. We had seen the picturesque juxtaposed with the inevitable. Beth Fancher, one of the students, says that she had heard about the Elwha Dam removal project before the field trip, but “actually seeing physical effects [of dam removal] on the natural landscape is shocking.”
The complexity that binds the issues of dam removal are not going to go away, but the dams still must be dealt with.
Edited by Lauren Fuge and Tessa Evans