All language subtitles for BBC.The.River.A.Year.in.the.Life.of.the.Tay.720p.HDTV.x265.AAC.MVGroup.org.eng

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.