♪ Rivers are vital for life for both people and for nature.

For 2 billion people, their drinking water comes from rivers, and a quarter of the food that we eat depends on them.

♪ Rivers irrigate our crops, give us power, a way to get around, and get rid of our waste... ♪ but our rivers are in deep trouble.

We've dammed and diverted them, polluted their waters, or taken too much out.

We've used them, and we've abused them to the point that they've died or disappeared entirely.

The wildlife that relies upon them is disappearing, too.

Since 1970, populations of freshwater river species have fallen by 84%, twice as fast as land and marine species.

Fisheries that sustain millions of people are collapsing.

There's an urgent need to protect and revive our rivers.

♪ In this, the fourth year of "Changing Planet," where we look at precious ecosystems and the people trying to save them, we are focusing on two of the largest river-restoration projects ever attempted.

I'm on the California-Oregon border, where the largest dam removal project in the world is underway, but it's just one part of a people's ambitious plan to bring life back to a sacred waterway.

[Bird screeches] ♪ Journalist and Paralympian Ade Adepitan is in France, where the Olympics have triggered a huge cleanup.

Adepitan: Here in Paris, the iconic River Seine is undergoing a colossal detox.

A river so dirty, it was declared dead.

It's been illegal to swim here for over 100 years, but now all of that is changing.

Sanjayan, voice-over: These two very different waterways-- one urban, one wild and remote-- face different challenges, but both projects hope to revive these rivers quickly and at scale.

If successful, they'll act as a template for other countries and other communities to follow.

These are stories of ambition and action for our changing planet.

♪ [Birds chirping] ♪ I've returned to the Klamath River in Northern California, where one of the biggest river-restoration projects in the world is underway.

♪ The Klamath runs for 254 miles from its headwaters in Oregon through California before emptying into the Pacific Ocean.

♪ Historically, it had one of the largest salmon runs on the West Coast of America, but from the early 1900s onward, a series of giant dams were built.

These dams blocked the natural flow of the river.

Toxic algae flourished in the warmer waters, and fish migrating from the sea to breed were cut off from 400 miles of spawning grounds.

Numbers of Chinook salmon fell by more than 90%.

Woman: It is a life-and-death situation when there aren't salmon in the river.

Man: This river has sustained Karuk people for over 10,000 years.

There's no way to overstate the importance of this river to the continued existence of our people.

♪ Sanjayan, voice-over: When I was last here 3 years ago, I met members of the Yurok Tribe, who had started to restore the river, but each mile was going to take a year's worth of work.

At that rate, it would take centuries.

The quickest way to heal the Klamath would be to get rid of the dams and allow it to flow freely again, but that felt like a distant dream.

Now the tribe's long, hard-fought battle to tear down the dams has paid off.

Group: Un-dam the Klamath!

Sanjayan, voice-over: We're on the verge of an unprecedented moment in history.

♪ I'm catching up with vice chair of the Yurok Tribe Frankie Myers.

How are you, brother?

Good.

How are you doing, brother?

Good to see you, man.

Good to see you.

You guys are doing it.

We're doing it.

We're doing it.

Last time we talked, we were trying to work on dam removal, but it still felt miles away.

It's the amount of pieces that had to come together, and so wide array of organizations and individuals to make it happen.

It's big.

It's massive, and as big as dam removal is-- like half a billion dollars, you know, going to get done in 12 months-- it's phenomenal.

You know, and it couldn't have come at a moment too soon.

Absolutely.

No.

It's been heartbreaking for our community, and when we were in the middle and not knowing whether we were going to be able to implement dam removal, what was going to happen, those were really, really depressing times.

That's the worst time... That was the worst time.

because you had the idea and you have the community...

Absolutely.

but you don't have the means.

To do it.

I think that is the most depressing time...

It is.

Yeah.

and to have the grit and persistence to get through that to get to the other side, that's actually the most important story about this.

We're not going to have a salmon season this year, but everyone knows there's hope, right?

It's different.

It will be the first run to return with a dam-free river.

Perfect.

There is a bit of excitement about that, and there's a huge amount of excitement, and they'll be able to spawn where they haven't spawned in over 100 years.

They'll come back to a river that's cleaner, that's more natural, and that's completely connected.

Yeah.

It almost feels-- it almost feels right.

I'm coming back for that salmon piece.

That's the one I want to be a part of...

Absolutely, absolutely.

I'm so happy.

Thank you.

Really am.

♪ Sanjayan, voice-over: This is a pioneering project, Indigenous wisdom and cutting-edge science working hand in hand.

♪ We wouldn't be here today but for the activism and the advocacy of tribes and conservation groups.

Sanjayan, voice-over: Mark Bransom is an engineer and CEO of the organization tasked with removing the dams and bringing the river back to life.

Bransom: The dams have essentially reached the end of their useful life.

They've been in place for over 100 years, but at the time the dams were built, there was very little, if any, recognition of the social, cultural, and environmental impacts.

♪ Sanjayan, voice-over: This is Copco 1 Dam.

Built in 1918, it's an amazing feat of engineering, bringing hydroelectric power to this part of California for the very first time, but now it's coming down.

I've met a lot of people who are building dams.

You're the first person I'm speaking to who's actively taking one down.

Well, it's a remarkable project to play a small role in.

It's a huge scope and scale, but we're making great progress, and these dams will be completely gone from the Klamath River within a matter of a few months.

That kind of blows me away because the last time I was here, like 3 years ago with the tribes, they're telling me about this ambition, but then to see it happen.

Has the pace of it just caught you by surprise, too?

Because it feels unbelievably quick for me.

We've got a limited amount of time during these dry months of the year to get these dams out of the river.

We'll see the fall run of Chinook salmon entering the Klamath River sometime in the August- September time frame, depending on weather conditions.

We'd like to welcome them back to a free-flowing river with no sign of any of these dams left.

Sanjayan, voice-over: Mark's team is working hard to meet that deadline.

[Explosion] Copco 1, built of concrete and metal, towers 125 feet above the riverbed.

The vast reservoir it created has already been drained.

Now it's demolition day.

♪ Man on radio: Ammunition's here.

Bransom: Hundreds of pounds of dynamite about to take a big section of it down.

Man on radio: In position and secured.

Different man on radio: Copy.

One-minute warning to blast, one-minute warning to blast, ready to set... [Alarm wails] That osprey looked up.

I think it's heard that sound before.

It certainly has, and I'm a bit surprised that it's still sitting on the nest.

I'm strangely apprehensive.

There's lots to be apprehensive about.

I mean, who doesn't love to see a good explosion?

But at the same time, until it goes off and we have an opportunity to take a look and see what's happened, we just won't know.

Man on radio: [Indistinct] to Y, Pat, are you in position?

Pat, on radio: In position and secure.

Man on radio: Copy.

We're counting down from 5... 4, 3, 2, 1, zero, and fire, fire in the hole.

Here we go.

Woof!

And there it goes.

♪ Wow.

Ha ha ha!

It's amazing, isn't it?

It's amazing.

It's louder than I thought it would be from this distance.

That's a genuine first.

Like, I've never seen that happen, and, honestly, in my life as an environmentalist, to think that I'm watching a dam being blown up and I'm not going to get arrested, I mean, [indistinct].

Success.

Sanjayan, voice-over: This is just the start.

Over the next few months, Copco 1 will be demolished layer by layer until it's completely gone.

Sanjayan: It is interesting because it's a hydro dam, and hydro, in many ways, is a clean source of power.

Does that ever conflict you that, even though it's good for the river, it may have a knock-on consequence for climate change?

Well, while these hydroelectric dams are carbon-free, we now recognize that they have significant other environmental impacts, and that's really what this project has set out to correct.

The electric utility that previously owned and operated these dams has replaced the power that these dams produced many, many times over, including with additional green energy like wind and solar.

Right.

OK. Sanjayan, voice-over: A few miles downstream, Iron Gate Dam is the largest of the 4 dams being removed.

Today, work starts to dismantle this vast structure made of rock and earth, a historic occasion for all involved, especially those who've been campaigning to get rid of the dams for decades.

Man: It's been a long struggle.

This day was always inevitable for us because there was no other answer, and there were many times where, No, I didn't think I would live to see this day.

I was pretty sure I probably wouldn't, but I knew this day was coming, but that I'm still alive to see it, pretty awesome.

♪ Sanjayan, voice-over: This is the culmination of years of activism by local tribes and other people who cared about the health of the river and its wildlife... ♪ and one truly shocking event in 2002 galvanized action.

Woman: I was 7 years old when the 2002 fish kill happened.

I remember getting up early in the morning with my mom and holding her hand and walking on the rocky shoreline on the lower Klamath River by the mouth, and just seeing thousands and thousands of dead salmon lining the shoreline.

For me, as a 7-year-old girl, these salmon are the same size as my own body.

♪ Man, voice-over: I was out on the river as a fisheries technician at the time, collecting carcasses and counting carcasses for days on end.

Probably somewhere in the neighborhood of 70,000 fall-run Chinook died on their migration upstream.

This was due to a couple of diseases.

One is called ich, and one is called columnaris.

The reason that happened was because there wasn't enough flow coming down the river.

The fish weren't able to migrate, so they all got stacked up in the lower river.

They got stressed.

The water was warm.

The disease spread like wildfire.

To Yurok people, salmon are probably our most precious fish.

You know, we evolved along the banks of this river, we believe, along with salmon, and so seeing them suffer like that, seeing their numbers plummet, seeing all that death, it hit in a way that's really hard to explain.

Sanjayan, voice-over: The Chinook salmon population never recovered, and their loss has had a profound impact.

Traditionally, a person living on the Klamath would eat 450 pounds of salmon a year, over a pound a day.

Now it's virtually zero.

With so few salmon in the river, fishing has been banned, and a vital source of free, healthy protein is no longer available.

Thompson: We live in a food desert, where you have to drive at least either two hours, round trip, north in Klamath, California, or 4 hours, round trip, south to get to the nearest grocery store.

♪ You have to buy things with a long shelf life, and those things are nonperishable foods that are cheap, which are the ones that tend to have low nutritional value, and that really affects us in the long term.

There is a increase in heart disease and diabetes rates, which really does lead to people's deaths.

♪ Sanjayan, voice-over: Once the dams are removed and the river flows naturally again, the salmon spawning grounds will be restored.

They'll face less risk of disease, encouraging their population to increase.

Un-dam the Klamath!

Un-dam the Klamath!

Sanjayan, voice-over: Dam removal should bring a turnaround in fortunes not just for fish but also for the people of the Klamath, who have fought so hard for so long to save their river.

Hillman: I used to be pretty hopeless growing up, feeling like you have no voice, but I think our young people know that they have a voice, and they know that their voice is important and it's going to be heard.

Group: Let the river flow... [Cheering] Thompson, voice-over: To have these dams removed, despite being told it was going to fail a thousand times and still persevere, gives me so much hope for the future when it comes to climate change and things we're told right now that, you know, "It's going to be too hard.

It's going to be impossible to do this."

But, no, if you push hard enough and have enough perseverance that we can make a lot of these changes when it comes to climate change and the future of the Earth, and that it's not too late and that we do have the ability as people to organize and make these changes today.

♪ Sanjayan, voice-over: It's not only our wild rivers that need help.

For thousands of years, we've been drawn to rivers, and many of our cities are built on their banks.

♪ In France, Ade is investigating the cleanup of Paris' iconic River Seine not just for wildlife but to make it safe for people to swim in.

Urban rivers get a raw deal.

They're amongst the most polluted bodies of water on Earth.

♪ The Seine has been the lifeblood of Paris for centuries, transporting goods and providing power, but it also became a dumping ground for sewage.

The Seine was so polluted that it was declared biologically dead.

The problem starts below ground.

There are over 1,600 miles of sewers and pipes running under the streets of Paris, carrying more than 300 billion liters of rainwater and wastewater each year... ♪ but this impressive feat of engineering wasn't designed to carry human waste.

♪ Man: When the sewers were built in Paris, they were made to collect rainwater and only rainwater.

Adepitan, voice-over: Human waste was collected and used to fertilize crops and grow food.

It was a treasured resource.

When did the change occur that it went from rainwater to sewage?

So during the 19th century, people started to bring water inside the houses.

Yeah.

Now the flush toilet becomes possible.

Adepitan, voice-over: Mixing human waste with water made it useless as a fertilizer, so rather than using it, Paris started dumping it into the river.

Esculier: The worst moment was the 1960s, 1970s in Paris.

At that time, half of all the wastewater from Paris was going directly into the Seine.

The quality of the river was disastrous.

There was nearly no oxygen at all in the river, and nearly all the fish had disappeared.

Adepitan, voice-over: The Seine was practically dead, and with levels of E. coli over 500 times the safe limit, no one would want to swim in it.

New wastewater treatment plants helped improve water quality, but heavy rain could still overwhelm the aging sewer system, sending human waste into the Seine.

♪ The catalyst for action finally came when Paris won the bid to hold the 2024 Paralympic and Olympic Games.

Key to the bid was the mayor's promise of a radical environmental cleanup, so that swimming events could take place in the Seine, but the city needed to guarantee that athletes wouldn't get ill from contaminated water.

The answer--a $1.5 billion mega engineering project, including a colossal underground tank right in the center of Paris.

If it works, it will hold a quarter of a million bathtubs of stormwater that can then be diverted to treatment plants, so the sewer network isn't overwhelmed.

Keeping raw sewage out of the Seine is a big part of the solution, but Fabien believes we should look to the past for inspiration and recycle some of our human waste.

♪ To see how this could work, we're heading to the European Space Agency... ♪ at the frontier of new science and technology that offers a globally scalable solution by turning urine into agricultural fertilizer.

Esculier: This is a special toilet.

It's a urine-diverting toilet.

Ah.

So there's two holes-- one for the feces, which is flushed, but there's a small hole here to collect separately the urine.

And why is this so important?

Well, because urine is a powerful fertilizer.

If we collect separately the urine, then we can transform it and turn it into a fertilizer.

Adepitan, voice-over: Urine contains lots of nitrogen that boosts plant growth.

When it ends up in rivers, it can cause algal blooms that deplete oxygen and choke rivers to death, but that same nitrogen could be hugely valuable for growing crops.

♪ The European Space Agency has installed over 70 of these ingenious toilets, and down in the basement, they're producing concentrated plant food.

So, well, you can have a smell.

Ooh!

Yeah.

This is quite pungent.

It doesn't smell of urine.

It smells more like soil and dirt.

Adepitan, voice-over: Fabien's team has already been testing out this fertilizer in fields of wheat.

If we collected the whole urine of the whole Greater Paris, then we would have enough fertilizers to produce 25 millions of baguettes every day.

25 million baguettes a day...

Yes.

from the urine of all the people in Paris?

Yes.

It's a huge quantity.

I love that, urine being used as a resource.

It's great.

Exactly.

Adepitan, voice-over: This system is already being incorporated into 600 new homes in Paris.

If it's adopted more widely, urine wouldn't even enter the sewer system, so there'd be no risk of it ending up in the Seine, and Parisians could harness the fertilizer they produce instead of flushing it away, a win-win situation all round.

♪ Sanjayan, voice-over: Here in California, I've traveled 5 hours downriver from where the dams are being demolished... ♪ but even at this distance, they affect the amount of water in the river and the health of the Klamath's aquatic life.

♪ You know, I think about this project as the largest dam-removal project in U.S. history, but in reality, it's bigger than that.

It's the largest salmon-restoration project attempted anywhere at any time.

The Klamath is home to coho and Chinook salmon as well as trout.

All these fish start their life in fresh water before heading downstream, sometimes hundreds of miles, to the ocean.

Here, they spend a few years before instinct draws them back to breed in the same spot where they were hatched.

I'm joining Jamie Holt and Gil Myers from the Yurok Tribal Fisheries Program, who monitor, manage, and conserve fish populations on the Klamath.

Today's job is to take a census of the river.

Only a tiny percentage of the babies spawned here will make it back to this river as adults.

Knowing how many go out to sea allows them to predict future populations and intervene if numbers become worryingly low.

Coming on.

Holt: Come aboard.

What is this?

We're looking at a juvenile rotary screw trap.

It's designed to catch juveniles as they migrate out towards the ocean.

It's important to know their migration timing-- How quickly are they getting through the system?

As they move through the system, are they staying healthy?

Why do you call it a screw trap?

Because it turns just like a rotary screw.

They'll filter in down to the narrow part of the cone into an actual live box underneath us.

I'm skeptical you're going to catch very many.

Should we look?

Absolutely.

♪ All right, what we've all been waiting for.

Oh, my God.

I actually didn't think it was going to be that big a...

Yes.

"container."

I don't see any fish.

Shall I scoop?

Absolutely.

Really?

Scoop, and then you'll lay it right on this board.

Oh, my, there's a fish.

Oh, my God.

It's actually a salmon.

There's a baby salmon at the first scoop.

I'm gonna bring it up, OK?

All right.

Yep.

Yep.

Oh, my goodness.

Holy Toledo.

He's so silvery.

Yes.

If I had to guess, I think you got yourself a baby coho there.

He'll go in the live well, and we'll keep digging.

That's kind of like the most endangered, right?

It is.

The last time I caught a coho was 30 years ago, like, really, off the coast of California, and you're never allowed to catch coho ever, and here I am scooping them up.

Oh, another one.

Yep.

Right?

Yep.

Look at that.

So we'll just run a little deep end.

Ah, it's so beautiful, so beautiful.

We'll see you on the other side.

That looked like a trout.

It did.

That's like a baby trout.

It could have been a steelhead.

Yeah.

These are, like, mythical fish.

Like, catching a steelhead, catching a coho salmon, I mean, they're mythical.

I mean, I know they're small, but it's still kind of cool.

Oh, no, absolutely, right?

Guys, there's a river monster in here, like, a proper river monster.

Where did he go?

You know, I'm happy to catch it.

Ah!

Ha ha ha!

Oh, you do have a Pacific lamprey in there.

Myers: That's a big one.

That's a large down runner.

Yep.

That means he's done his business and headed back.

Stay in.

Mother.

Oh, he went down that way.

Isn't it amazing how hard it is to catch something in kind of a tiny box?

This is really hard work.

Like, I'm just like, "This is really hard work."

Oh, yeah.

Here you go.

Holy bejesus.

Bejesus!

Oh, ho ho!

Slippery.

Oh!

He's slippery.

Put him in the net.

It's all right.

He's back in the box.

No.

That's all right.

You're good.

Ah, he's getting in.

Get the scuba kit out.

Holt: You got him?

Yeah.

Holt: If you'd like to take a feel-- No.

No, no, no.

Get him in.

Get him in, Jamie.

I can't do this one more time.

All right.

He's in.

I think that's all the fish.

All right.

That was way more amazing than I thought it was going to be...

Awesome, yes.

and, like, everything, right?

Like, there's Chinook.

There's coho, suckers... Holt: Steelhead.

steelhead.

♪ Sanjayan, voice-over: We take our haul to the riverbank for a closer look, where some local fishermen have caught a truly fantastic fish--a huge sturgeon.

This is unbelievable.

Holt: No, absolutely.

This is a treat.

Holy cow.

So this is a green sturgeon.

We're thinking it's a female, so we're gonna let her go.

We want these beautiful mamas to keep making babies.

Honestly, like, that's a spirit fish for me.

I mean, the sturgeon is just from the time of the dinosaurs.

The fact that they're still in the Klamath River tells you how unbelievably important this river really is for biodiversity, obviously, for people.

That takes my breath away.

Oh, my goodness.

Go makes some babies, girl.

Holt: Thank you for your service.

Waah.

♪ Sanjayan, voice-over: Back to business, Jamie needs to examine all the fish we caught.

Holt: He's looking pretty good, no scale loss or anything.

He's got some life in him, though.

You can see he's missing his maxillary.

Yes.

Yeah, so he is a clipped coho out of one of the hatcheries up here.

If I had to guess, it would have been Iron Gate.

Sanjayan, voice-over: In recent years, the wild salmon population has fallen so low that their numbers are boosted with salmon raised in hatcheries.

Holt: We'll roll over here to the scale-- 18.3, about 127.

Sanjayan: You know, it's interesting.

You take it at the notch of the tail, not the tip of the tail.

Holt: Yeah.

Yep.

I've been over-measuring my fish all my life as a fisherman.

Ah.

Ha ha ha!

♪ Sanjayan, voice-over: By assessing the growth rate and numbers of fish, Jamie and Gil can gauge how well the river is recovering as the dams come down.

It'll take years for the salmon population to bounce back, but these folks are in it for the long haul.

♪ Adepitan, voice-over: The Seine's fish populations are also closely monitored to keep track of the river's health, but here, local anglers have taken on the task.

♪ Today I'm going to find out if the huge cleanup operation of recent years has allowed fish and other animals to recover.

♪ It's 5:00 in the morning, and most sensible Parisians will be tucked up and asleep.

I'm meeting fish biologist Bill Francois, head of the local angling association that protects aquatic life in Paris.

Bonjour.

Hello.

How are you, Bill?

Good.

How are you?

Yeah, not bad, not bad.

Good meeting you.

Oh, thank you, thank you.

When I think of Paris, I think of a lot of things, but fishing is not one of them.

Yeah, of course, but Paris is a fishing city at its origin.

At its very origin, it was put here because it was on the way of the migration of the salmon back in the days.

Wow.

OK.

Even just under Notre Dame, there are archeological remains of a fishing village from Neolithic time.

So what's the state of health of the Seine?

It has improved a lot over the time.

Like, after the Second World War, there were 3 species of fish left in the Seine... Only 3 species.

because it was so polluted.

It was open-air sewage.

Oh, wow.

But then in the eighties, they started to clean the water from all the houses and industries, and it slowly improved.

Adepitan, voice-over: Bill and 5,000 other regular anglers provide vital data on fish populations from their catches.

♪ Today we're hoping to hook the largest predator in the river-- a wels catfish.

It can grow to nearly 9 feet long and weigh twice as much as me.

This opportunistic hunter is the largest freshwater fish in Europe, at the top of the food chain, and a good indicator of a healthy river.

♪ Oh!

That was a good one.

Hee hee hee!

Yeah, and then when you get too close to the seaweed, you release faster so it doesn't get-- Oh, yeah.

I see what you mean.

so you don't catch salad.

Ha ha ha!

Yeah.

Like that.

No.

I've caught it.

Ha ha ha!

Ha ha ha!

♪ Adepitan, voice-over: Hopefully, Bill's friends further up the riverbank are having better luck than us.

[Rings] Allo?

Oh, yes.

Yes!

Yes.

Let's go and have a look.

♪ Adepitan: Is it still there?

The fish is about as big as you.

As big as me?

Yeah.

It's a good one, huh?

Oh, yeah.

It's huge.

It's huge.

Look at the size of that.

♪ Adepitan: Oh, my days.

Look at this.

Wow.

You can see-- Look at the size.

It's impressive to see something this wild in the urban environment in Paris.

Yeah.

Do you want to put it back in?

Yes, please.

Adepitan, voice-over: Before the catfish is released, Simon and Kaseem take photographs and record its measurements.

From the tail, one meter, 50.

How are you able to monitor the numbers of fish here in the Seine?

Francois: So what we do is, when we fish, we take pictures, and we know with the spots they have, it's like our fingerprints.

It's unique for each fish... Oh, wow.

so if this one, we catch it again, we will know, and we will know how much it grows from one year to another.

It's slowly swimming away, you see.

Merci.

Thank you.

Thank you.

Thank you so much, guys.

No.

That was brilliant, really, really good.

♪ Adepitan, voice-over: Over the past few decades, Paris' anglers have overseen the remarkable comeback of fish in the Seine... ♪ from just 3 species to more than 40, and it's not just aquatic animals that may benefit from the cleaner waters.

♪ In the Gothic heart of the city, bats come out at night and enjoy what the Seine has to offer.

Diane Megias is the environmental outreach officer for the City of Paris and a bat expert.

♪ The water is great also for bats because they use it as a foraging ground, they use it just to drink, and they also use it as a corridor to commute.

Yeah, so it's like their motorway, their highway.

Exactly, yeah.

How much has the cleanup of the Seine benefited the bat population?

The cleaner the water is, the better it is for all the animals because the pollutants that might be in the water will be passed on through the food chain.

Yeah.

The insect might be contaminated, and so the bats that will eat them might be contaminated, as well.

The Seine is the start of the food chain, isn't it?

Yeah, yeah, for lots of animals.

For lots of animals, it's the beginning.

♪ Adepitan, voice-over: Insects like midges and mosquitoes that start life in the water make up a huge part of a bat's diet.

A common pipistrelle can eat up to 3,000 in one night.

♪ As the light finally fades, we're hoping to track down these nocturnal hunters, using a special bit of kit that listens in on the bat's ultrasonic calls.

So we have this detector.

It allows us to basically hear bats, so what it does is sort of translates the echolocation-- so the ultrasounds, the sounds that we, as humans, can't hear-- into audible sounds, but it also shows you a picture of the shape of the sound, and so from this picture, you can identify what bat species is flying right next to you.

[Chirping] Oh, just like that.

I thought I saw one.

Yeah, yeah, yeah... Do you know what?

I saw something fly past... Did you see it?

and I thought it might have been a bat, so I looked down.

You can see it's sort of a drop shape...

Yes.

and we're right on 45 kilohertz, so a common pipistrelle.

Does that mean it was hunting?

It might be hunting.

Here, you can see that the echolocation is getting much closer around here, so that's what we call a feeding buzz, so that's when the bat echolocates really, really fast to get as much information as possible really quickly to catch the prey.

♪ Adepitan, voice-over: Research has shown that a healthy river will attract higher numbers of bats.

After centuries of decline, the Seine is now recovering, clean enough to provide food for both bats and catfish in the heart of this bustling capital city.

[Car horns honk] ♪ Sanjayan, voice-over: Back in California, dam removal is also allowing life to return, and not just to the river itself.

Before demolition work could start, the reservoirs behind Copco 1 and Iron Gate dams, holding over 45 billion gallons of water, had to be drained.

All of this land-- which looks like a moonscape, really hard to walk on-- well, this is brand-new land.

This did not exist.

I mean, if I was here in January, just a few months ago, I'd be under 40 or 50 feet of water.

To restore this barren wasteland, a massive revegetation effort is underway.

I'm meeting the project leader, Joshua Chenoweth.

Sanjayan: I can't even keep my small garden in Virginia, you know, alive.

You're trying to do this scale.

I mean, how is that going?

Yeah.

It's great.

I mean, just for perspective, there's 9 miles of river just in this reservoir alone.

The reservoir rim is 23 miles, so it's big, you know, 900-plus acres.

It definitely feels intimidating.

You guys put this down.

Now I feel kind of a little bit bad walking on it, to be honest.

Sanjayan, voice-over: These seedlings are the product of years of hard work.

Since 2019, Joshua and the rest of the revegetation crew have painstakingly collected millions of seeds from 98 species of native plant.

As soon as the reservoirs were drained, the mud was seeded by hand and, when the going was too treacherous, by helicopter.

♪ Only by acting fast did the team have a hope of restoring this precious ecosystem.

Chenoweth: The surrounding landscape is full of invasive species.

If we did nothing, then some of those species could have been the first ones to fill the gaps.

Also, if there's no vegetation for a while and you get a big rainstorm, then there's just constant flow of sediments into the river, which, you know, if you overload the river can kill salmon and other fish.

Sanjayan, voice-over: To help stabilize the soil and stop it washing into the river, the crew also plant seedlings, like milkweed, a vital food plant for the endangered monarch butterflies.

This landscape is changing fast, and the scale of this river-restoration project is huge.

♪ The best way to monitor its progress is from the air.

♪ The last time I was here, the Yurok were talking about getting a plane just so that they can get a sense of what they're doing, and I actually thought it was going to be, like, some small, little, 30-year-old plane, but they've got a really, really nice plane, and I'm told it's chock full of equipment.

It's even got a new-plane smell.

That's how new it is.

♪ Felicity Cross is a Yurok tribal member and a highly qualified engineer.

Using this kit, she is mapping the river to discover how it changes course and evolves during the dam-removal process.

♪ Cross: So this plane is equipped to do aerial LiDAR surveys and what LiDAR is, it stands for Light Detection and Ranging, and essentially, it uses pulses of light to measure the distance between the object on the ground and the sensor.

Sanjayan, voice-over: LiDAR fires up to 100,000 laser pulses a second, and then software knits together millions of data points to create detailed 3-D maps of the terrain.

Sanjayan: Can you find old river channels where the river used to be before the dam?

With this project specific, the river is actually following, in general, its old river channel that existed before the reservoirs were built.

Sanjayan, voice-over: Using LiDAR to monitor how the landscape is changing in real time also helps the restoration team to assess what work needs to be done.

Cross: We're also able to see the landscape and where sediment has built up in those tributaries, so we're able to use that data to kind of formulate plans-- where do we need to excavate, where do we need to make a meander, where can we put large wood to create fish habitat.

Sanjayan, voice-over: And from up here, I can finally understand the enormity of this project.

Cross: So that is Iron Gate Dam right now, and if you actually look off to the right of that dam, that's called the borrow pit, so the dam was constructed with material that was borrowed from that mountainside, so now as they're deconstructing Iron Gate, they're putting that material back to where it originally came from, so the borrow pit.

That's incredible.

It really is a pinch-me moment, you know, and the fact that the world can move fast at times is really inspiring.

Absolutely, and this project is really special because I've grown up with the talk of dam removal my whole life.

Really?

My friends, my family have protested against this dam for the last 20 years, and I knew once I got into college, I'm like, "I want to be part of that dam-removal effort," and now I'm here in a capacity, and I'm actively involved in the Klamath Dam removal, and it's very amazing.

It's really a dream come true.

♪ Sanjayan, voice-over: You know, the thing I really loved about Felicity, flying around the plane today with her, is that she's a tech geek, and, you know, in any other time, in any other place, I think she would have got an engineering degree, gone to Silicon Valley or gone into some big company and would have been lost from this landscape, maybe forever, but right now-- in this time, this place-- it's like all the stars aligned to bring her back here, and with it comes this cutting-edge Western knowledge.

I mean, she's using LiDAR.

She's using AI.

She's got a whole plane chock full of equipment to solve what is a deep, long-standing cultural challenge that this people have had right here at home.

♪ Adepitan, voice-over: Traditionally, Parisians have also had a deep connection with their river, but pollution kept people out of its waters.

Now a $1.5-billion cleanup project to make the Seine swimmable for the 2024 Paris Games is complete.

If it works, it's not only elite athletes who will benefit.

Ahead of the Olympics, a public open water event is due to start.

Now we're on Canal de l'Ourcq, which breaks off from the main river, and right now, it all looks pretty peaceful and calm, but all of that is going to change later because hundreds of swimmers from all over France are going to arrive to take part in a 5K race.

[Quacking] Adepitan, voice-over: It's been illegal to swim in the Seine since 1923 because its waters were so dirty and dangerous.

The race organizers have got special permission for people to swim here, but it can only go ahead if the water is clean enough.

There's been a lot of tension this week.

There's been much more rain in Paris than expected, which has meant there's been a lot of worry about the quality of the water.

In the past, weather like this would have caused sewers to overflow into the river, increasing the risk of swimmers catching E. coli or other nasty diseases.

This is the first real test of the Austerlitz basin, the gigantic tank built to stop sewage ending up in the Seine, but luckily, it's done its job, and the water has been deemed safe enough to swim in.

I'm meeting veteran wild water swimmer Pierre Malherbaud, whose life has been completely transformed by swimming.

Pierre, there's lots of people queuing up, getting ready.

Are you excited?

Are you feeling the buzz for the race?

Yeah.

I'm feeling the buzz always, always.

Before every race, I'm excited to jump in the water.

What goes through your mind when you are in that water swimming?

When you jump, what goes in your mind is, "Aah!"...

Ha ha ha!

because the shock of the cold, that's the only reaction you can have at that time.

Why do you do this?

For 3 main reasons-- I enjoy it, I like overcoming challenges, and for health.

I lost 60 kilos in 12 years.

60 kilos?

60 kilos.

I'm 58 kilos.

You lost me and a hat.

The main tool to change my life at that point was swimming.

Adepitan, voice-over: At the start line, Pierre is joined by 270 other open water enthusiasts.

♪ It's Pierre.

Allez, Pierre.

Allez.

♪ What a trouper.

♪ You know, if there was ever any doubt about the benefits of cleaning up waterways in cities, look at all of this.

Improvements in water quality are transforming how people interact with the Seine, but there are no easy fixes.

Daily tests of the river keep track of bacteria levels, and at the 2024 Olympics, persistent heavy rain puts racing in the river in jeopardy.

E. coli spikes above safe limits, but the actions of the Austerlitz basin enable the river to return to legal levels far quicker than previously.

Woman: Organizers will now breathe a sigh of relief that all the events that were meant to take place in the Seine have now taken place after so many water concerns.

Adepitan, voice-over: As a legacy of the 2024 Games and the ongoing cleanup, swimming in the Seine will become legal for everyone, reconnecting Parisians with their river, and as someone that has competed in two Paralympic Games and has competed in sport all of my life, it makes me feel really proud to know that it was the power of sport that kickstarted the cleanup of the Seine.

♪ Sanjayan, voice-over: It's clear that restoring rivers doesn't just benefit animals that live in the water.

There's a ripple effect.

Here in California, the restoration of the Klamath River will also help the comeback of the largest landbird in North America just in time to feast on that salmon that will soon be returning.

♪ The California condor has a wingspan of nearly 10 feet.

It can live for more than 60 years and can fly 200 miles in a day.

As scavengers, they're crucial for maintaining a healthy ecosystem, disposing of carcasses, and removing disease from the food chain.

They're also spiritually significant to the tribes here, and they very nearly died out.

♪ So for the California condor, it got very bad.

Actually, they were right on the brink of extinction, with just less than 30 individuals left in the entire world.

Sanjayan, voice-over: To save the species, all the remaining wild birds were trapped and brought into captivity for breeding.

♪ It worked, and now there are nearly 600 condors, of which over half are flying free across several states.

♪ The Yurok Tribe have been reintroducing condors here for the past few years, and Tiana Williams-Claussen, director of the Yurok Condor Project, is taking me to a top-secret location, where the birds are being held before release and where they also return twice a year for health checks.

♪ Head of operations is Chris West.

♪ ♪ ♪ ♪ Sanjayan, voice-over: Over the past few weeks, the free-flying condors have been lured back with food into this enclosure so the team can give them checkups.

It's all good.

Sanjayan, voice-over: The biggest threat to condors is poisoning from lead bullets used by hunters.

This one has been tested and given the all-clear to be released.

♪ To have California condors back in the skies rebuilds a link between the river and the rest of the ecosystem.

♪ Williams-Claussen: Condors traditionally fed on the salmon spawn-outs, which were literally in the hundreds of thousands annually.

But not only that, the salmon will feed the bears, you know, and the bears go upland, and they carry those marine nutrients to the redwoods, and the bears are fed, and then they also eat the berries.

They scatter the berry seeds around.

Everything is connected, and fish is at the heart of it.

These salmon are at the heart of it.

♪ When the Yurok Tribe talks about restoration, we're really talking about at that condor scale, that condor's-eye view, that landscape that is just so broad and so interconnected in a way that we humans just can't comprehend.

I love that-- condor-scale restoration.

That's good.

♪ Sanjayan: You know, you come up here, and you think the story is about the dams, but then you realize it's not.

It's really about the salmon, and then you think, "No.

It's not just about the salmon.

It's about the condors," and at the end of the day, it's all connected, and you realize that to put a landscape back, you can't just do one piece.

You sort of have to do it all, and you have to sort of make it all fit back again like a giant jigsaw puzzle, and that's exactly what they've done here.

♪ What has been achieved on the Klamath in just a few months is astonishing.

By mid-August, Copco 1 Dam is breached and little remains of Iron Gate.

When the fall-run salmon return, they can swim upriver unimpeded for the first time in a century, and if this river can be restored after decades of damage, then there's no reason why it can't be done elsewhere.

♪ When I started on this journey of documenting a changing planet, I expected to be documenting an obituary-- the planet was changing, and it was never going to change for the better-- and then you come here and you meet these communities.

You see what these people are doing, but it's not just the tribes.

It's also the government.

It's also the agencies.

It's also the power companies.

It's the whole thing.

We're talking about something truly epic.

I mean, they are putting back 400 river miles, making it available to salmon, and what blows you away is, when all of that comes together, then you have a planet that is changing for the better, and it can change fast.

♪ ♪