Original subtitles

This vast stretch of water

is the mouth of our mightiest river,

the Tay.

Its flow is greater than the Thames and the Severn combined.

The wildest and most unspoiled of all our great rivers.

Its waters give life to countless curious creatures...

..from birds that swim...

..to fish that fly.

I'm a writer and naturalist

and, ever since I was tiny,

I have been obsessed with the natural world.

I've always wanted to follow a river from its source to sea...

..to explore the incredible connections that a running ribbon

of water has to the wider world.

From its birth in Scotland's Western Highlands,

the Tay flows for 120 miles through some of our most remote

and stunning landscapes...

..until its final dissolution in the cold North Sea.

Unlocking the mysteries of this vast river, I'll explore what makes

these threads of wildness so vital for our world.

Travelling from spring onward to winter, new technology

will unlock incredible perspectives on the river.

From the vantage of a soaring hawk

to microscopic marvels...

..I'll meet the people drawn to the river's silvery waters...

..the Tay guardians striving to uncover why its legendary

salmon are disappearing...

..and the scientists searching for new knowledge hidden

in the beauty of the natural world.

This is making me feel quite emotional.

Four seasons,

wonderful wildlife,

and the answers to some of the biggest questions of our time.

All connected by the living thread

of one extraordinary river.

I'm starting with the search

for the source of Britain's biggest river.

Unlike some other great rivers,

there's no signpost at the Tay's beginning.

Not even a path.

To reach a river's source, you have to follow its longest

stream to its very end.

It sounds an easy task, but this boggy, misty place

is full of uncertainties.

The Tay's source lies somewhere on Ben Lui.

It's one of Britain's tallest mountains

and one of our wettest places.

All around me, water is rushing down hillsides.

Quite a lot of the future River Tay is in the sky above me

and falling onto me.

I'm only ten miles from Atlantic waters to the West but these slopes

send the Tay on an eastward course,

flowing for 120 miles until it meets the North Sea.

I've been hiking uphill for about two hours now

in all weathers.

There's been sun and rain and hail

but I'm nearly there.

It's almost impossible to pinpoint

exactly which trickling rill of meltwater

extends furthest up the hill.

But one of these thawing patches of snow marks

the furthest the Tay reaches from the sea.

And there's so much water pouring down the hill here

that I kind of...

It makes me wonder why it is that, when a river's beginnings

so clearly is multiple, we really want it to have this single

point where it starts.

And, I guess, you know, we're human.

We're born, we live, we die.

And we, kind of, can't help but see rivers

through that lens of our own lives.

A river doesn't care where it begins.

What makes the Tay our greatest river isn't a single dot on the map

marking its source, it's the sheer profusion

of trickling rivulets that feed into its waters.

Thousands of running, silver threads

joining together as gravity pulls them downhill.

These streams merge into larger tributaries that collectively drain

2,500 square miles of soil.

The Tay is not just a single ribbon of moving water,

it's a whole geographic region.

One vast, living ecosystem.

The first streams that flow from this barren landscape

are little more than rain and melting snow.

Fast-flowing, cold and pure,

they support very little life.

With one famous exception.

The Tay system's most iconic resident,

the legendary king of fish.

Salmons' lives both begin and end in mountain streams.

But their extraordinary life story

carries them into faraway oceans.

And this vast domain begins just two miles below the source.

This is The Hole In The Wall waterfall.

In the 18th century, it was marked on maps as the source of the Tay.

For salmon, it is the beginning of the Tay.

They can't actually get any higher up the river than the bottom

of that waterfall.

They can't jump that kind of length of water.

Every autumn, adult Atlantic salmon lay eggs in riverbed gravel.

All winter they lie buried, waiting for their moment.

Spring is the season of awakenings and,

even in a barren mountain stream,

the changing year sparks new life.

Warmer water triggers the hatching of tiny salmon fry.

These fish will spend the next two years in these upland streams.

Yet they'll never grow much larger than minnows.

To become the king of fish, they need to move

to richer waters to feed.

Those few creatures that can thrive here are specially equipped

for this hostile territory.

Like this mayfly nymph.

It's a very different shape from its downstream cousins.

Its dirty front legs are shaped like racing car aerofoils.

So, instead of washing it away, the rushing water

pins it harder to the riverbed.

Another animal built for fast-flowing water

is the world's only swimming songbird.

BIRD TWEETS

Bright, bold and decidedly odd, it's one of my favourite birds -

the dipper.

Dippers dive into the icy water to hunt.

Young mayfly and tiny salmon are among their favourite snacks.

Thanks to denser bones than other birds,

they sink rather than float.

This pair built their nest right at the edge of a 20-metre waterfall.

And the first chicks are just emerging onto the riverside.

Soon, the chicks will hunt these turbulent waters on their own.

Those that can adapt can thrive here.

But, in recent decades,

the Tay's most famous resident

has been disappearing,

here and right across the ecosystem.

As in many UK rivers,

numbers of once-abundant salmon have dropped by 70%

over the past 30 years.

Because their life cycle covers such a vast area,

it's been hard to pinpoint an exact cause.

But on one of the Tay's upland tributaries, the River Garry,

a team are trying to unravel the mystery.

The Tay Fisheries Board have placed this trap here to gently gather

juvenile salmon, known as smolts.

Spring's floods and the rising water temperature are triggering them

to move downstream in search of food.

Mike, what have you got in the bucket here?

We have five, wild Atlantic salmon smolts.

These little fish are about

two years old and they're starting

their migration to sea. Ah, they're marvellous.

In preparation for the ocean, the young fishes' bodies

are undergoing an incredible transformation

with a wonderful name - smoltification.

The body becomes elongated, its head changes shape,

its eyes get bigger because this fish has to hunt

in salt water.

And also there are cells within the body

that alter because a fish travelling

from a freshwater lifestyle to a saltwater lifestyle

is a big change for a fish.

This team hope hi-tech surveillance can help pinpoint

where fish are disappearing.

We take a length and a weight before

we tag them. So, this one's...

Under anaesthetic, Mike places

a tiny transponder tag harmlessly

into the body cavity of each smolt.

Just going to make a very small... Tiny incision.

..tiny incision in their belly.

Amazing.

And done with such care and skill.

It's really wonderful.

Then you put the fish into the recovery bucket. Wonderful.

The project basically involves us tagging migrating salmon smolts.

They'll obviously migrate to sea with the other smolts

and then we can record these fish

as adults as they return into freshwater.

The tagging project has only been running for two years.

Too soon to tell whether the cause is more predators

catching young salmon

or other changes affecting adults downstream or at sea.

But the proportion of tagged fish

that eventually return will help

pinpoint the most likely problem.

We've tagged 2,300 salmon this year.

Wow. So, we should see a fairly vast number of these

returning next year and the year after.

Mike is determined to bring back the fish whose abundance here

was once taken for granted.

How does it feel to know that the fish with your transponder in

is swimming around the ocean somewhere? Do you think about them?

Yes, I do. I do think about them, yeah. What they're up to?

I'm hoping that they come back.

Adult salmon are rightly renowned for their heroic upstream journeys.

But the first migration of these tiny fish downstream,

moving up to 100 miles in a fortnight...

..is perhaps even more moving and miraculous.

Salmon are one of the Tay's great treasures

but the river has other gifts,

if you know where to look.

Spring's melting snow is unleashing

torrents of fast-flowing water.

It draws weekend hunters to these upland streams

looking for flakes of pure gold.

Four wee bits.

Worth crashing that up. Yeah.

I haven't got a pocket.

I haven't got one either.

It's in the quartz veins and then, over millions of years,

the rocks rolling down the river erode the quartz

and then that brings the gold up.

And then the force of the water washes the gold

and, then, cos it's heavy,

it settles down into the bedrock

and stays there until...

..until it's found.

Panning for commercial gain is not allowed here.

The few flakes they sift from the cold water are strictly

for passion projects.

I'm due to get married in less then two weeks to my fiancee, Charlotte.

Instead of going to a goldsmith's to buy our wedding bands,

I'm actually going to use

the gold that I've collected.

Currently, Charlotte's ring is looking like this and I've got

less than two weeks to get that into a wedding band for her,

which looks like this.

And she's quite happy with that because I'm actually

using my gold instead of it being sat there in my collection.

To preserve the river's most precious creatures,

restrictions ensure panners do not

disturb streams where salmon eggs

might hide in the gravel.

It's late April and, just a few miles from the source,

the warming air and melting snow

are unleashing all sorts of hidden wonders.

There's something about the smell of this place that reminds me

of every other spring I've ever had.

It's a very pungent, earthy smell that's so evocative.

It takes me back to all the springs I've ever known.

In this high, wet country there are few obvious signs

of the changing season.

No bright daffodils to catch the eye.

Spring's awakening is more subtle,

where mosses and lichens carpet

the river bank rocks and trees.

We tend to see beauty in the landscape in a, kind of, big scale.

We look at hills and waterfalls and rock formations.

But I think, if you get your eye in, the really true, exquisite beauty

in these particular landscapes is very small.

So, if you look at this tree here,

you'll see it's covered

in mosses, in wonderful lichens.

These ones are foliose, they're branching lichens.

These are woodrush, these are ground-dwelling species

that are growing on the tree.

We've got so many different species of mosses.

We could just spend ages!

This is a lovely one, here.

It's called feather moss.

Exquisite branching.

It's magical!

And there are marvels here at an even smaller scale.

Responding to spring's warmth,

networks of microscopic life

are branching and spreading through the wet river bank.

Professor Paul Hoskisson's lab at the University of Strathclyde

studies a soil-dwelling bacteria

that's given us astonishing gifts.

Wow! What am I seeing?

So, you're seeing the streptomyces magnified about 1,000 times.

That's the main growing part

of the organism that forages

in soil for nutrients.

And then you can see, stained in red, the spores as they start

to make their fruiting bodies.

Exquisitely beautiful.

Paul's team are interested in

the chemical streptomyces bacteria release as they grow.

These organisms are competing with lots of other bacteria,

fungi, worms, all within the soil,

and they need to make chemicals in order to give them an advantage

in that environment.

So, the antibiotics that they make kill bacteria,

they make antifungals to kill the fungal competitors.

Antibiotics, such as streptomycin,

are a by-product of this chemical warfare

and have become life-savers to us.

But one of the other chemicals streptomyces makes

transports me straight back to the riverside.

Streptomyces for you.

Oh, my goodness, that waft of smell.

It smells like spring in the incubator. That's astonishing.

Yes, that's a molecule that streptomyces makes called geosmin.

It's the smell of wet, spring soil.

They're so beautiful.

Little silvery lines of colonies, here.

Geosmin is one of these chemicals that we don't really know

the full function of.

But we believe it's there to repel insects that may come and eat

the streptomyces spores.

I remember many years ago being in a desert

in one of the Gulf states and it rained very heavily after

a lot of hot weather and I smelt this smell.

It was almost overpowering.

Yeah, the human nose is very sensitive to the smell of geosmin

and lots of animals are very sensitive to it.

I mean, it's been hypothesised that it's one of the ways

that animals like camels can find water in the desert,

is by following the smell of geosmin.

I love this theory that, deep in our evolutionary past,

we developed the ability to sense water,

we can, kind of, smell it,

even when it's not in sight, and be drawn towards it.

There's something so magnetic about rivers.

They're the only moving things in the landscape.

This one looks like a, kind of, river of mercury,

it's reflecting the sky.

There's white noise all around me.

I could watch this for hours.

It's just magic!

The transformation from winter to spring is now complete.

But the water's flow is constant.

And, two centuries ago, on a Tay tributary called the Tilt,

the river's ceaseless flow helped spark an idea that shook our world

to its core - deep time.

This is a beautiful little river.

And I've come here to find a spot where the power of running water

helped fundamentally change

our understanding of the age of the Earth.

In 1785, an Edinburgh man of science named James Hutton arrived

here on horseback.

He was pondering one of the big questions of his time -

how old is the Earth?

Back then, the consensus pretty much was that the Earth's rocks formed

from the worldwide sea left behind after Noah's flood.

All the rocks crystallised and were left behind as the sea drained away.

But, for Hutton, the idea that all Earth's rocks were created

in one single, biblical moment didn't stack up.

He suspected new rock could form inside the Earth.

But he needed evidence.

Here, on the River Tilt, he found it.

Here, the river's worn through layers of rock,

giving us a glimpse back through time.

It looks like I'm standing amidst just rocks,

but there is actually evidence of an unimaginably

violent, ancient event.

This pale-coloured granite was once a finger of molten lava

flooding its way through the cracks in the layered bedrock.

You can see where it's met weak points,

fissures in that rock, it's prising them apart.

This could only be the aftermath of an eruption.

Hot liquid rock prising apart cold, old bedrock.

Hutton had seen back through time and found

the smoking gun he needed.

The Earth's landscape wasn't created in a single event.

New rocks are constantly being created, then worn away.

What we see in our brief lives is just a moment in a constant cycle

of destruction and renewal.

Hutton's revelation in this place introduced a new concept for us.

A new concept of time.

Deep time.

The understanding that the Earth was way more ancient

than the 6,000 years of biblical time.

Hutton radically shifted our comprehension of our place

in the world.

He's known now as the grandfather of geology

and I think you can put him amongst that select group of legendary

figures who changed the way we see ourselves in relation

to the universe.

Like Copernicus, who moved the Earth from the centre of all things.

Or Darwin, who placed humans among other animals on the tree of life.

But Hutton? Something special.

These rocks and this river, he gave us a new understanding

of the unimaginably vast sweep of history.

As the Tay's many upland tributaries tumble on downstream,

they merge, one by one, to create an ever larger river.

But, just as these waters gather power,

there's an unexpected transformation.

15 miles long and 150 metres deep,

this is Loch Tay,

the tenth largest volume of freshwater in the UK.

Lying only 23 miles from the source,

the pure mountain streams that feed

into the loch carry very few nutrients.

So, it may look like an oasis but it's really more

of a wet desert.

Life is sparse in this vast volume of water.

Salmon smolts race through here en route to the sea's riches.

And the loch's beautiful, silent waters exert a pull that I find

hard to resist.

There's something about standing next to a body of water

like this that does something very strange to the soul.

And it's not just me.

These waters have drawn people for millennia.

2,500 years ago, our Iron Age ancestors constructed huge wooden

buildings over the water.

This is a reconstruction of one of 18 crannogs

that once lined the loch.

What motivated people to create these huge feats of engineering

remains deeply mysterious.

Perhaps they were compelled by the water's spiritual significance?

There's something about water, something about rivers and lakes,

that draws us deeply to them.

And, if you look at nearly all the world's religions,

they all seem to have a spring or a river or a lake at their heart.

But, looking out at the loch here, this implacable grey surface

and a very low misty cloud, I certainly feel a, kind of, stirring.

At the loch's easterly edge, the water gathers speed again.

For the first time this river is now officially named the Tay.

And it's packed with power and energy.

12 miles from the crannogs lie Grandtully Rapids.

The draw here is pure thrill.

The noise and force of whitewater.

It's one of the last stretches of fast running water on the Tay

and marks the beginnings of a new kind of river

where life's challenges are very different,

especially at this time of year.

It's late June.

Summer means longer, warmer days.

A few months for the river's life, in all its forms, to flourish.

This river is far from the raging torrents of the mountain burns.

It's moving here, this water, through a much calmer,

gentler landscape.

These are gentle waters to inhabit.

Here, summertime really is easy living.

Life thrives in the long sunny days.

The salmon smolts that set out two months ago are already

far out at sea.

But these wide waters teem with trout, pike,

perch and adult salmon now returning from their fattening trips

to the sea.

And the ospreys high above know it.

Their pale, keen eyes scan for a sliver of shadow moving

against the riverbed.

It's a food chain connecting water and sky

and fuelling all this abundance is one simple ingredient...

..summer sun.

Light streaming into the river is feeding a great gathering

of green creatures.

Some float free, others cling to the rocks.

Along with bacteria, algae are the foundation

of all the river's food.

Miniature predators

suck them into their jaws.

These then become meals for larger river dwellers

and so on.

But algae aren't just underwater fodder.

They're among Earth's oldest life forms

and these tiny creatures are helping to solve one

of the great mysteries of science -

how cells first joined together to form bodies.

Many are just a single cell

but in some species cells link up

into cooperating colonies.

And it's this ability that fascinates scientists.

This is an algae called volvox

found in ponds, puddles and streams all over the world.

Each of these spheres is a colony of around 1,000 cells.

At the University of Cambridge, Professor Ray Goldstein

leads a team that studies them.

We've got volvox on the microscope live, for you to see.

And there they are. And they're really beautiful.

You can see the spinning motion.

Beautiful.

I've seen still photographs of them before

but they've always looked a little bit like, sort of,

expensive lampshades. But these, in motion, are astonishing!

They look like little transparent planets.

Each cell that makes up volvox's surface is identical.

What's amazing is that, even though there is no brain or nervous system

in overall control, the entire sphere can move

in a purposeful way.

Here, hundreds of volvox colonies

are all swimming towards a light.

To find out how a single colony of cells can swim with purpose,

Ray's team are studying

the parts of volvox's cells

that generate movement.

So, each of the cells on the surface

has two hair-like appendages called

flagella, that beat roughly like this,

and they're organised on the surface in a very regular

pattern so that the beating is basically from the North Pole

to the South pole.

The flagella beat very fast to make the colony move.

But, slowing down footage of a single cell,

you can see them in action.

When two cells are positioned at a distance apart,

their flagella beat independently.

But, when they're moved closer together,

something magical happens.

The neighbouring cell's flagella reacts to the movement

of the fluid around them.

Like a team of rowers, the cells beat in synchrony.

When we look at these organisms in enough detail to see

their individual flagella, we see that they are

synchronised with each other.

So, that seems to be the basic underlying mechanism

by which they talk to each other.

They're actually talking to each other through the fluid

in which they are immersed.

From the combined reactions of 1,000

perfectly arranged individual cells,

the entire colony moves as one.

And, once colonies of linked cells found ways to move,

it's possible to imagine how cells could start to combine

into ever larger and more complicated bodies.

And, over vast expanses of time,

evolve into creatures like us

with 37 trillion cooperating cells.

You know, the more I look at the tiniest kinds of life,

the more I feel incredibly humbled about how little I know

and how complex and beautiful all those diverse creatures are.

It's quite astonishing.

It's early July.

I am now more than halfway along the river.

The calm, deep waters are ideal resting grounds for mature salmon

returning from the sea.

And there's another species waiting for them.

Anglers are drawn from all over the world to the Tay's waters.

And, at nearby Glendelvine,

nearly 100 years ago a record was set.

Salmon fishing history was made right here on the Tay.

On the 7th of October 1922 local angler

Georgina Ballantyne sat with her father in a boat.

It was getting late and she made one last cast before going home.

Just behind that rock in the rapids.

And she felt a tug on the line.

She'd hooked a fish.

And what happened was, in her own words,

a Homeric battle.

The fish raced 300 yards downstream, closely followed

by Georgina and her father in their boat,

and then what happened was two hours and five minutes of nerve-racking

anxiety, thrilling excitement and stiff work.

It was completely dark by the time Georgina hauled her prize

into the boat.

It was an astonishing fish.

When she hooked it she must've had no idea that she would break

a record that still stands.

That fish was 64lb in weight, nearly five feet long

and 28 inches in girth.

It's still the biggest salmon

ever landed in British freshwater...

..despite nearly a century of anglers

trying to beat Georgina's record.

A little way downstream, another female angler,

Claire Mercer Nairne, fishes at Meikleour.

She's passionate about encouraging

more women into angling.

Like many fly-fishers, she ties her own flies.

So, what are you making on that particular fly?

What kind of fly are you making? A Munro Killer.

A Munro Killer.

It's an extraordinarily violent name!

Fishing for Atlantic salmon is especially challenging for one

very big reason.

They don't eat.

Once fish return to fresh water, their stomachs shrink

and they survive entirely off fat put on at sea.

No-one knows exactly why salmon

snatch at a combination of brightly

coloured feathers and thread.

It may be simple curiosity, it may be aggressive

defence of territory.

I'm hoping my Munro Killer is the right mix of flash and fancy

to help me catch my very first salmon.

Give a little tug.

Now, you just... Forward here like this?

Forward. Yeah.

Back...

..and go.

Lovely.

Shall I have a go?

This is the most terrifying thing I've ever done in my life.

This side? See, slowly you retrieve.

Slowly retrieve.

You retrieve a little line.

And then bi-i-ig, big...

Slow! Slowly.

Like that, but slowly and better!

No, it looked like you were missing your train. I know! OK.

So, slowly, slowly, slowly.

So, rod here, hand here. OK.

Slow, slow, slow.

OK. Yep.

And then back.

Wait, wait!

OK. I'm too fast! You did it!

I did it? OK.

Then slowly, slowly.

Watch the time.

Much better! Yeah, yeah, it's much easier...

Watch the time.

I've been a falconer all my life.

Like angling, it's a pursuit traditionally dominated by men.

So, it's personally so satisfying to find out

that, despite their scarcity on the river bank,

women often land the biggest fish.

This fact has been the source of much debate.

Entire books have been written on the topic.

One suggestion is that female pheromones might somehow entice

salmon onto the hook.

There's so many stories of women

attracting these absolute monsters.

This is very, very funny. This is like, erm...

It's not the women's skill that's making them catch it,

it's the fact that they're just passively attracting the fish.

This sounds to be like a very suspicious theory!

And so I believe that men had to come up

with this pheromones theory

simply because they were annoyed.

Because there are such worries about salmon numbers in the Tay,

anglers are heavily encouraged to return every fish to the river

so as many as possible have a chance to breed.

80 miles from its source, the Tay flows through

a flatter landscape.

Farmland covers the fertile soil of the gently sloping river valley.

Order and straight lines contrast

with the soft curve of the waterway.

But farmers aren't the only ones making their mark

on the Tay's landscape.

They've recently been joined by a creature whose industry

matches our own.

After a 500 year absence,

beavers are back.

It's late July and this adult pair have two new kids.

They're making their first, tentative splashes

outside the home lodge.

Beavers don't eat salmon, they're vegetarians.

Their diet consists of tree bark and fresh branches.

But, nevertheless, their presence here is highly divisive.

Beaver country looks like a modernist sculpture park

carved by rodent teeth.

Animals the size of a pet spaniel

have moved more than a tonne of mud

and wood to build this dam.

This used to be a tiny stream trickling down towards the Tay

but beavers have been here for the last decade

and they have done some serious river engineering in that time.

It's a very eerie landscape.

It's quite an unusual one.

It's not what I expect a river up here to look like.

There are fallen trees across the river.

Behind the dam, here, that they've built, there is a drowned tree.

Everywhere there are little stumps with chips of wood around them

where they've been hammering away with their incredibly sharp

incisors.

It's a weird, open landscape and it's astonishingly new

and yet very old.

And why do they create such extraordinary feats of engineering?

Because they feel intensely vulnerable out of water.

Their dams flood the land so they can safely reach evermore trees

and other plants for their food.

This can devastate riverside crops.

Beavers don't care for our neat, orderly division between

river and land.

But the truth is a landscape shaped by beavers is probably closer

to how the Tay would have looked before any human walked here.

This transformation has done extraordinary things

for the biodiversity in this place.

As soon as the water slows down, different creatures begin

to live in it.

Beneath the beaver pond surface dwell life forms

that would be washed away in a fast-moving stream.

And that means a completely different cast of animals occupy

the world above the water.

There are damselflies here, dragonflies here, newts here.

These are all species that would not otherwise be here.

Without the beavers, this would be a cold,

dark bit of forest with a trickle of water

and with them it's kind of a wonderland.

The beavers' return to the Tay hasn't been welcomed by everyone.

Farmers, in particular.

But my hope is that, over time,

there'll be room for us all.

As the days shorten, summer's heat is slipping away.

Because the water stays warm for a little while longer

than the sky above, on clear mornings clouds of steam fog

rise up from the river.

It's a bewitching sight for early risers on the river bank.

But it also heralds the beginning of a season that will transform

the river and all the life within it.

The clear waters of spring and summer are growing darker

and richer in hue.

It's trees that are driving this change.

The days are getting shorter, the temperature is getting much

colder and the trees really can't support the kind

of existence they had in the summer.

The chemical that they used to turn sunlight into food,

chlorophyll, breaks down, goes back into the tree

and what's left behind are the chemicals that are always

in the tree but we never see, which make them this colour.

One by one these leaves go, they're not needed,

and what's left behind...

..are the buds for the new leaves for next spring.

It's like winter and spring and autumn and summer all at once

on one tree.

Millions of leaves find their way into the Tay and its tributaries,

some falling onto the river's surface,

others washed in by rain.

As they float, they reveal new secrets about the river,

tracing currents, showing subtle eddies and whirls in the flow

that relate to the depth and shape of the riverbed.

Not every leaf that falls

into the river is washed out to sea.

In backwaters, where the water slows, leaves build up.

Each leaf attracts a miniature army of decomposers.

Insects, worms and fungi break the leaves into

ever-smaller fragments.

Even a single leaf is a vast source of food

for the river's microscopic decomposers.

But millions of leaves in the delicate balance of the river?

Too much of anything is often bad news.

The danger is that vast multiplying decomposers could use up

the water's vital oxygen.

If oxygen levels fall too low,

many creatures will suffocate.

Including salmon, biding their time in river pools before moving

upstream to spawn.

But the river has an unusual saviour.

A creature that can keep the river in balance.

And, to find it, it's wader time.

I'm searching for freshwater pearl mussels.

And when you look for them you can see what looks like a kind of

an oval dark stone covered in weed.

If you look on the top of it, it's got a crack on the top

and little, kind of, feelers, kind of,

filaments coming out and scooping

in the water to filter it.

It's such a treat.

Cousins of the more familiar saltwater species,

these mussels play an important role in keeping rivers clean.

But, because one in 1,000 contains a pearl, mussels have traditionally

suffered from overharvesting.

More recently, many rivers are suffering huge variations.

Either sudden floods or droughts due

to changing climate or being drained

for hydroelectric power.

In extreme cases, mussels can dry out or be washed away.

They're now so precious their remaining strongholds

are kept secret.

Scientists at this research station on Loch Lomond are mobilising

to save them.

Each individual mussel can filter up to about 50 litres of water a day.

So, that's equivalent to the amount of water you would use

in a shower every day.

So, it cleans the water, basically.

It siphons in the water, it filters out all the organic particles,

which it uses as food, and then it releases cleaner water

into the river system to the benefit of other organisms,

like fish, like invertebrates.

But sudden river level changes are a potential disaster

for these sensitive molluscs.

They will only thrive in the right conditions.

Fortunately, freshwater mussels have a secret weapon

that could be their saviour -

a flexible foot.

The team on Loch Lomond are monitoring how mussels respond

to water dropping five centimetres every hour.

The mussels have previously been thought as very

sedentary species, so that they don't move around much.

But what we're seeing in our experiments is that, as the flow

rates are changing and the flow levels are changing,

the mussels are moving around quite a bit and they are moving,

using their foot, to move around.

Now we know how fast these wonderful molluscs can move,

where possible, we can safely control water levels.

So, the mussels can continue to benefit the river.

Sadly, they have already disappeared from many UK waters.

But, on this secret Tay tributary,

thanks to careful conservation,

there are promising signs for the future.

So, under licence, I'm able to show you these little beauties.

This is a very rare sight.

Baby mussels.

This one's maybe three years old, this one's maybe eight

to ten years old.

And it's a real sign that this river's thriving.

I'll put them back.

As the air grows ever colder,

many species are finding a fresh

burst of energy.

Red squirrels are gathering food to see them through

the leaner months.

Warming water drove salmon smolts out of the river to sea

in the spring.

Now, autumn's cooling water is drawing adult salmon

upstream to spawn.

One of nature's most extraordinary sights.

The fish are guided back to this waterfall by the unique

scent of their birth stream imprinted on their memory

as they left it.

Now, pure strength and luck determine who makes it

over this final hurdle.

As fish head upstream, my journey continues down to catch up

with the Tay District Salmon Board

now carrying out a very different kind of fishing.

OK.

This net is rigged with a mild electric current.

It attracts and temporarily stuns the fish,

making them easier to catch.

Like tagging the smolts upriver,

this work is part of the determined

effort to stop salmon disappearing.

And it gives me a rare chance to get close to the fish whose presence

seems to define this river system.

Hello, Mike.

Really excited to see this.

This is a cock fish that you've caught. Yes, this is a cock salmon.

So, this is a male Atlantic salmon, getting quite close

to being ready to breed.

That's extraordinary.

It's just the most prehistoric, kind of, eerie...

You know, I've been reading a lot about salmon and I've seen them

from a distance but seeing this cock salmon in all its finery so close...

So, I remember those beautiful silver smolts

that we saw earlier in the year

and this was one very like that

only 14 months ago?

Yeah, just last May he would have gone to sea.

Travelled the big distance across the sea to his feeding grounds

and this is what he's become.

He's eaten a lot of food out there.

He's eaten a lot of food, he's quite a healthy-looking fish

as well, so, yeah, it's amazing how successful these fish

are in saltwater.

He's gone from, as a smolt, as you saw, about four inch long,

four inches long? A small silver fish and he's spent about a year

at sea and came back as a fish about five pounds in weight. Wow.

So, yeah, there's good feeding in the sea.

Come on, mister, there we are.

This beautiful animal has returned to the river 100 times heavier

than when it left for the sea.

If left in the wild, this male would be competing to mate with

egg-laden females in the next few weeks.

After spawning, both males and females would most likely expire

from the exhaustions of their journey.

But these fish will be reproducing in a slightly different way.

This is the Salmon Board's hatchery.

The team here are dedicated to boosting fish numbers

in the river.

When a female fish is ready to release eggs, hatchery manager,

Steve Kaye, delicately removes

around 6,000 of these orange spheres.

This is Steve's 48th year working

with salmon and he takes great care

not to harm the fish.

A male fish from the same river

is relieved of his sperm.

And the result?

Over 1 million fertilised eggs loaded in trays each autumn

and watched over by Steve's experienced eye.

Wow, they're beautiful.

There are a couple in here which are infertile. Oh, right.

You can see... Oh, yeah, they've got a white spot on them.

They've got a little white spot, so it's quite important

that I remove these, otherwise a secondary infected

fungus can come in on them.

In the unpredictable river environment,

not every egg will make it through the winter.

Whereas these million eggs will be safe until just before

hatching next spring

when they'll be released into the Tay tributaries most

in need of a population boost.

Not everyone agrees that stocking rivers by hand

is the best long-term solution.

But the hope is that it can give a much-needed boost to a fish

that's in trouble.

I'm reaching some of the last turns of the river

and, as autumn comes to an end,

a chance to reflect.

So, I have a confession to make.

People sometimes ask me what my favourite season is

and I know I should say summer, or spring in particular,

but it's not. It's none of those things.

It's always been autumn for me.

And there's something about the changing of the seasons,

these days, that really gets me here.

Everything's slowing down, shutting down.

Things are leaving.

Everything about it reminds me that winter's coming

and that our lives will eventually come to an end.

It's not very cheerful but it's sobering, you know?

To remind ourselves that life's very short.

What we see in the world... It's glorious but I think, also,

it's always slightly about us.

We always think of it in relation to our own lives.

We can't help but do it.

So, standing by the river right now,

I think I'm just loving it more.

I'm loving it more because, eventually, none of us will be able

to see anything like this anymore.

We'll all be gone.

As I follow this immense river downstream,

the seasons are turning once more and the Tay

is beginning its final chapter.

Cooler air and shorter days trigger

river bank life to slow down.

30 miles from its end, the Tay passes the first major

settlement on its long journey...

..Perth.

The city marks the point where the river's fresh water

meets the sea.

As the river pushes downstream, it becomes increasingly mixed

with the incoming saltwater in this wonderfully open landscape.

But, even as the sea starts to dominate,

the river still has enormous power to sustain life.

One of the few species that thrive in this brackish water

are common reeds.

Able to tolerate both salt and the changing tide,

they blur the boundary between land and river.

This half world feels wild,

but this is an industrial landscape.

Reeds were planted here in the 19th-century to stop

the river washing away farmed fields.

The plants now stretch for 15 miles along the Tay,

the largest continuous bed in Britain.

For decades, the reeds have also provided thatch for roofs.

Graham Craig started harvesting here in the 1970s.

He's come to check the crop he's lived beside all his life.

Reed-cutting always begins in winter, once the sap has fallen.

An ideal way to tell when the reed is ready to cut is it usually

has a nice rattle to it.

And you can tell when most of the moisture is out of it.

Also, obviously, we're looking for the leaf to come

off the reed, as well. There's still one or two bits.

If we do cut it with that on, it tends to make the bunch that bit

bulkier and it's not ideal for thatching with. So...

We harvest from December to April,

which is the worst months of the year!

But, I mean, you can be out here in January

with sunshine and it's beautiful out here.

And with the reed and the wildlife going about,

absolutely brilliant.

Despite their man-made origins,

these vast reed forests have become

a sanctuary for one of the UK's rarest birds...

..the bearded reedling.

Hundreds nest here,

30% of the UK's population.

They're bewitching birds,

with long legs and huge feet to clamber amongst stems

and the males have glorious, feathery moustaches.

With summer's flying insects long gone,

reed seeds provide most of the bird's winter diet.

Because they don't have teeth to grind away the tough seed cases,

the birds must swallow grit to do the same job in their stomach

and the nearest source is this crumbling old sewer pipe.

These rare birds have people like Graham to thank

for their stronghold.

Left to nature, accumulated dead reeds would provide a base

for less salt-tolerant plants and slowly turn to woodland.

Only regular cutting preserves the reed bed.

I love the idea that this wild haven is created and maintained by us.

Giving something back to the river.

It seems to me the wildest of all places, you know?

It's an impenetrable area for humans.

We can't really walk in it.

It's, sort of, neither land nor water, it's...

It's kind of exhilarating!

Beyond the sanctuary of the reedbeds,

our human presence dominates.

Downstream lies Dundee,

Scotland's fourth largest city.

Like many of Britain's great cities, it owes its fortune to the river

that flows through it.

150 years ago, Dundee's docks bustled with cargo boats

supplying textile mills that dominated the city's skyline.

But, as industries moved elsewhere, the city had less need

for the river and the 1960s Road Bridge blocked large ships

from travelling upstream.

There's rain and mist on the far side of the river here

and I can see the lights of all the cars coming into Dundee

over the bridge.

And the waters here are unchanging, they've looked like this for

thousands of years.

But the city has changed around them.

The city has undergone a rebirth in recent years.

Video games companies have clustered here

and this new temple to art and design has risen

from the very spot where ships once docked.

I'm standing underneath the extraordinarily stacked eaves

of the V&A building here.

What's going on is this wonderful regeneration,

the sense that the river, that was once an industrial highway,

has become an inspirational highway for the city.

And the river flows past, as it always has done,

completely unchanged

as culture ebbs and flows along its banks.

Beyond Dundee, river and sea become ever harder to separate.

And, with the coast just a few miles away,

the Tay's famous fish are already homing in

on their route back upstream.

The waters that are flowing past me into the ocean are carrying

with them the particular chemical signature of this one river.

And, somewhere out there, salmon are going to smell them,

they're going to make their way home, they're going to follow

those signatures until they get stronger and stronger

and they're going to end up in the river in which they were

hatched. And I guess it's that...

It's just a really moving thought!

That you can find your way home through the smell of water.

But one insistent question has followed me this whole journey...

..why are there fewer fish in the river?

Despite all that's done to boost their numbers?

We'll just go ahead and have a look at one of these.

Hints to an answer could be locked in the bodies

of the salmon themselves.

Doctor Nora Hanson studies salmon at Marine Scotland's lab

near the Tay.

She's interested in what's happening to fish beyond the bounds

of their birth rivers.

Growth has slowed down.

We know an awful lot about salmon

and we know especially

about their lives in freshwater

but their lives at sea

are still quite a bit of a mystery.

One thing we've been interested in doing is using what I like

to consider the natural data storage tag of salmon

which are their ear bones,

called otoliths.

These are little structures that sit in the inner ear of all bony fish.

Oh, it's exquisitely tiny!

They're quite small in this species and they can be quite

large in other species.

And, as it's growing and layering up, it's actually

incorporating information about the chemical properties

of the water in which it's bathed

and then we know from satellites and other measurements in the sea

quite a lot about sea surface temperature in the North Atlantic.

So, using those pieces of information,

we can recreate possible migratory paths taken by fish

just by looking at the pattern of temperature within the otolith.

This is an example of...

..an output.

This is a fish that emigrated to sea from the north coast of Scotland.

The red areas of the map indicate the most likely location

of the salmon as the months pass.

Pursuing its prey, it migrates to waters beyond Iceland

then moves east to the Norwegian Sea...

..then returns home.

One suggestion is that, as ocean waters warm

and currents shift, salmon are being pushed further

and further to find enough food.

So, there are problems that they're facing at sea.

Perhaps they're needing to travel further north in order to encounter

enough prey and enough resources in order to make it back.

Perhaps they need to travel to different parts

of the North Atlantic.

But only by being able to study where fish are migrating to at sea

will we really be able to start to unpick why it might be happening

to them there.

The implications are obvious.

Preserving salmon isn't limited to the length of the river.

The Tay's fate is intertwined with events in deep

and distant oceans.

But I've not quite reached the last gasp of our greatest river.

Three miles from the North Sea coast, the fast,

falling tide is exposing the bare estuary beneath.

And, in this desolate seeming place, flocks of winged visitors

are gathering to feast on life forms buried beneath the mud's surface.

Oh!

Those are redshanks!

Fast-beating, sharp wings.

There are hundreds and hundreds of birds assembling.

It's really thrilling!

Wading birds that have been breeding in the Arctic will stay

here for a few days, or a day or so,

and then they'll head south.

What I love about that is it makes me feel that the River Tay has,

sort of, tendrils, dots on the map, that extend way further than simply

the British Isles.

There are birds currently in Africa whose bodies are built of atoms

that have been drawn, molecules that have been drawn

from the mud of the River Tay.

As the river slows and hits the incoming salt,

much of its rich cargo of minerals and organic matter simply falls

to the floor,

fuelling huge populations of hidden invertebrate life.

To see these tiny creatures that are feeding a profusion

of birds, I team up with biologists Doctor Rachel Hale

and Doctor Andy Blight.

Oh, it's proper sand castles

but with mud rather than sand.

Oh, wow! It doesn't look like there's anything here

but you turn over a piece of sediment and you can see

all these U-shaped burrows

that loads of shrimps that live within the burrows have made.

They look a bit as if someone's pressed a hair pin into the mud.

So, what's made these burrows?

So, these have been made by a small amphipod called Corophium.

The thing I like about these guys is they look a bit

like a big sci-fi monster.

They've got huge antennae.

They've got these antennae!

So, there aren't very many species in this mud,

but there are lots of those species, lots of them, lots of individuals?

Basically because it's nutrient-rich and there's lots

and lots of organic matter.

So, there's loads of food!

But because it's quite a harsh environment to live in,

there are only a few species that adapted to live here.

But those that do live here are hugely abundant.

So, birds, basically, have a feast when they come here.

The river's final act is to feed vast numbers of buried shrimp,

worms and snails.

It can be hard to comprehend the sheer amount of life squeezed

into the top few inches of the estuary.

Rachel studies the living arrangements of these creatures.

In the lab at St Andrews University,

she gave me a glimpse into the incredible

complexity she's uncovering.

We've been using computer tomography scanning,

CT scanning, exactly the same as you would get in a hospital

but we use a scientifically dedicated machine.

And you can actually see these burrows within the sediment.

So, this is just of a single species

and, you can see, here's the sediment surface. Oh, wow!

And they create these huge networks of burrows underneath.

So, this is essentially making the sediment see through.

So, what organism is this?

So, this is the ragworm.

It's really complex! It is, yeah.

You can see it's using up pretty much all the space available

in that particular column of mud.

You know what?

I know this is going to sound really, really ridiculous,

but this is making me quite emotional!

I had no idea that this, sort of, complex beauty was hidden

beneath the featureless mud of an estuary or a salt marsh flat.

It's just...

It's just a thing of...

A thing to marvel at!

Those beautiful, branching patterns that Rachel has shown with ragworm

burrows remind me so much of other patterns

that I've seen on this journey down the Tay.

There's the patterns of winter branches.

The patterns in rock, of veins in rock.

The patterns, indeed, of the river and its tributaries itself.

This kind of branching pattern has been seen everywhere in nature

and people have recognised it ever since there have been people!

But it took a long time for us to answer the question...

..why this pattern is everywhere.

100 years ago, a man who lived and worked alongside the River Tay

for his entire life decided to explore why the same

shapes and patterns often repeat themselves in the living

and nonliving world.

Mr D'Arcy Thompson, a really fascinating figure

in the history of British science.

He was a polymath, he worked as a lecturer in biology

in both Dundee and St Andrews universities

and he was also quite eccentric.

I have a pet parrot at home, kind of renowned for it,

and he was well-known also for wandering around St Andrews

with his pet African grey on his shoulder.

There's a kind of kinship there.

D'Arcy Thompson's great quest was to find simple mathematical

and physical laws that guide the shapes and patterns

we see across nature.

He wanted to show that life isn't just sculpted by Darwin's

theory of evolution.

That other powerful forces also influence how organisms take shape.

D'Arcy Thompson's great work was this book,

On Growth And Form from 1917. Along with Darwin's On The Origin

Of Species, one of the most influential books ever written

on the natural world.

It's still an enormous inspiration for artists,

architects, engineers, anyone who's concerned

with natural form

and it's full of the most exquisite illustrations that connect

the mathematical with the biological

and nonbiological parts of the world.

His great revelation was that, even though the world

around us can appear incredibly complex,

nature's marvels often boil down to fundamentally simple laws.

It's why railway bridges bear

similarities to dinosaur skeletons.

And how the forces that shaped soap bubbles and water droplets

are the same as those sculpting tiny ocean plankton.

Architects and engineers have since been inspired

by his insights, creating structures that mimic natural forms.

And D'Arcy Thompson also explored

why objects branch in certain ways

as they grow.

Using simple mathematical formulae,

he showed it's possible

to replicate natural patterns.

They are simply the most efficient way to fill a space

with minimal material.

And this natural engineering that shapes trees,

lichens and bacteria also governs entire river systems.

A reminder that our greatest river isn't just one watercourse,

it's formed from thousands.

And here, at the end of its journey, the river's final moment

is just as complex and fuzzy to define as it was at its start.

This is such a strange, eerie place.

It seems halfway between the land and the sea.

It's not quite a river any more.

The sky's reflected in the water on the sand.

It's a kind of place that really seems like no place

and like every place

and it makes me think a lot about the journey that I've taken

and what I've learned in this year.

It's taught me about the complexity of the systems that surround us,

how a river is not just a discrete river in Scotland,

it also spans the migratory routes of birds and it spans the travels

of salmon right out to the Arctic seas and back.

A river is not just here,

it stretches all over.

D'Arcy Thompson showed us that inherent simplicity often

underlies the shapes and forms of the natural world.

But the key to preserving the future of our wild places is to understand

the complexity and interdependence that allows so many

creatures to flourish.

There's hope for the Tay, our most magnificent river.

Long may it flow and thrive.