We have been building dams across  rivers for thousands of years, as far back as ancient Mesopotamia.

And generally speaking,  they’ve been a great success, powering machinery, controlling  flooding, and storing water. But there’s a catch: dams do  terrible things to the environment.

They transform a river’s  ecosystem practically overnight, leading to algae blooms and  even greenhouse gas emissions. And if you’re a fish, I probably  don’t need to convince you a dam is not what you want to see ahead of you. Luckily, there’s a solution.

If we don’t need a dam anymore,  it’s sometimes possible to get rid of the dam thing. So let’s take a look at  just how much better things can get when a dam comes down. [♪ INTRO] We’ve built a ton of dams. There might be over 16 million  of them around the world.

Not all of these are Hoover-sized megastructures; most are less than 10 meters tall. They do all kinds of jobs, turning water  wheels that power mills or other machinery, or storing water for irrigation or drinking. All good things.

Large modern dams power hydroelectric generators, which make up the largest renewable  energy source currently in use. But many older dams just no longer serve  the purpose they were originally built for. Europe has 150,000 dams just sitting  around, their function no longer needed.

And old dams continue to  cause environmental issues. Building a dam turns a fast-moving  river into a slow-moving lake. That stagnant water behind  a dam traps nutrients that cause algae blooms and starve the water of oxygen.

Organic matter from the flooded land decomposes, emitting greenhouse gasses like  carbon dioxide and methane. And of course, they block fish  swimming up or downstream. Which is a bigger problem than  just for the fish themselves.

Fish like salmon carry nutrients from the ocean far up rivers and fertilize  entire ecosystems on land. Think of the fat bears! They’re snacking on salmon, meaning energy and nutrients are traveling from sea to land.

What this means is that ecosystems  can get a big boost from dam removal. Take the Klamath river that  runs from southern Oregon into the Pacific Ocean in Northern California. This used to be the third  largest salmon-producing river on the west coast of the US.

But for over 100 years, the river has  been blocked by 6 hydroelectric dams. In addition to blocking the salmon’s  migration, the stagnant water grew algae, which used up the oxygen in the  water that the fish depend on. Fish populations in the Klamath fell by over 90%.

So for decades, the Indigenous Yurok people have been calling for the removal of the dams. Dams are often built without  consulting Indigenous peoples. They can displace those people  or destroy their way of life.

The Yurok people basically said:  We’ll be having our river back now. And on the Klamath, thanks to this  advocacy, four of the six dams came out! This is the largest dam removal that  has been conducted anywhere ever.

And the process of restoring the  land has also been Indigenous-led. The newly drained land was hand-planted  with native seeds of plants that will stabilize sediment, provide  food, and hold cultural significance. As we’re filming this, it’s  still too early to fully see the ecological results of these dams coming down.

But forecasts say salmon populations will  recover by 81% over the next 40 years. And these predictions are  based on it working elsewhere. When the Elwha River Dam in Washington  came down in 2012, by just a year later over 4000 Chinook salmon had returned  to waters that were previously blocked.

Scientists looked at the feathers  and blood of fish-eating birds, and found they contained nutrients  that could be traced back to the ocean. Meaning a good chunk of the ecosystem  had started to bounce back right away. We’ll talk more about how those  ecosystems can bounce back, but first, all science needs funding.

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Removing dams can improve water quality too. The Sélune river winds its  way through Normandy, France. But two large dams built in the early 20th  century trapped sediment and nutrients like phosphorus and silica behind them.

This was a feeding frenzy for algae, causing massive blooms and low oxygen in the river water. In 2019, the largest dam  removal project in Europe began, in order to take them both out. Scientists are closely monitoring  the Sélune until at least 2027 to study how the river and  surrounding ecosystem responds.

They’ve seen that nutrient levels quickly rebound on each side of where the dams once stood. Sediment in the downstream water  has increased, which means the water is now actually less clear in many  places than it was with the dams. It’s a bit counterintuitive, but clear water  can be a bad thing because the energy in the water that would have otherwise  gone into transporting sediment, instead ends up eroding the riverbank.

Removing a dam also starts to  release some of the sediment that built up behind it over the decades. This is still happening on  the Sélune, and scientists are closely monitoring the sediment  release to find out what effect it has. This last example we want to tell you about is very close to home for us  here at SciShow… literally.

The Milltown Dam was a  hydroelectric dam completed in 1907, about 10 kilometers upstream of  SciShow’s home in Missoula, Montana. But the dam didn’t stand for long before  a big problem came rushing towards it. In 1908, a massive flood washed mining runoff contaminated with heavy metals like  arsenic down the Clark Fork river.

Remember how dams trap sediment?  Well, the brand new Milltown dam suddenly had millions of cubic meters  of poisoned dirt trapped behind it. It sat there for nearly a century,  the arsenic poisoning fish and leaching into the groundwater. Eventually it contaminated the drinking  water used by Milltown’s residents.

So in 2006, work began to take down  the dam and scoop up all that sediment. This was no easy task. Cleaning up  toxic substances like arsenic is tricky.

In 1973 the Fort Edward Dam in New York came down. But nearby factories had used  toxic PCBs for decades prior, which had stuck to sediments held behind that dam. The chemicals washed downstream,  causing a decades-long cleanup project.

Lesson learned, the order in Milltown went: Divert river first, remove sediment  second, dismantle dam third. Restoration of this site has  been so successful that it’s now the Milltown State Park, which opened in 2018. You can take your kids for a  picnic at the local Superfund site and it’s like, a pleasant afternoon.

These are only a few examples. As of 2019 there have been around 1700 dam removals in the US, and 4900 in Europe. And it’s clear that when done carefully,  dam removal can restore ecosystems, fix the problems the dam  created in the first place, and create some pretty awesome  spaces for people as well.

So even though dams have done  some useful stuff for us, sometimes, the best thing they can do is go. [♪ OUTRO]