Episode 2

KIRK: IT'S SPRING IN THE GREATER YELLOWSTONE ECOSYSTEM.

AS THE SNOW MELTS, NEW GROWTH MEANS NEW HOPE FOR MANY.

BUT OTHERS WILL BE FACING DOWN THEIR BIGGEST CHALLENGE.

A HUGE VOLUME OF WATER WILL SOON COME RUSHING THROUGH THESE VALLEYS.

AND THESE BEAVERS HAVE BUILT THEIR DAM RIGHT IN THE FIRING LINE.

THE QUESTION IS, CAN THEY SURVIVE THE FLOOD?

I'M KIRK JOHNSON... AND I'VE BEEN FOLLOWING THE WILDLIFE HERE AS IT BATTLES THROUGH ONE OF THE GREATEST SEASONAL CHANGES ON THE PLANET.

[WIND BLOWING] WHAT HAPPENS HERE IS REALLY UNIQUE...

THE TRANSITION FROM A FRIGID MINUS-40-DEGREE WINTER TO A 100-DEGREE SMOLDERING SUMMER.

IT'S ONE OF THE TOUGHEST SPRINGS ON EARTH.

[WOLVES GROWLING] IN WINTER, I SAW ANIMALS STAVE OFF COLD AND HUNGER.

BUT SPRING IS NO PICNIC EITHER.

ONLY THE STRONGEST ANIMALS WILL SURVIVE.

TO UNDERSTAND HOW THE ANIMALS COPE WITH THE CHANGE OF THE SEASONS, WE'RE GOING TO FOLLOW A NUMBER OF FAMILIES TO WITNESS THE THAW THROUGH THEIR EYES.

WE'VE BROUGHT TOGETHER A TEAM OF CAMERAMEN... [WHISPERS] Oh, my.

AND SCIENTISTS...

THIS ACTUALLY QUALIFIES AS WORK.

I'M TAKING MY COAT OFF.

YEAH.

WHO ARE USING THE LATEST TECHNOLOGY.

THESE WOLVES ARE JUST CREEPING IN RIGHT HERE.

TO RECORD AND ANALYZE THE CHALLENGES FACED BY OUR ANIMAL FAMILIES.

THEY MAKE A MISTAKE, THEY'RE KILLED.

KIRK: IT'S THE END OF APRIL.

TEMPERATURES REMAIN ABOVE FREEZING.

YELLOWSTONE IS ABOUT TO BURST INTO LIFE.

BEHIND ME ARE THE GRAND TETON MOUNTAINS.

THEY'RE COVERED IN SNOW, BUT LOOK AROUND.

LITTLE SPRIGS OF GREEN ARE POPPING UP.

SPRING IS COMING TO YELLOWSTONE.

HUMMINGBIRDS ARE BACK FROM SPENDING A WARM WINTER IN THE SOUTH.

AND MOOSE ARE OUT AND FEASTING ON THE FIRST GREEN SHOOTS OF SPRING.

[BIRD CHIRPING] BISON ARE MIGRATING BACK TO THEIR CALVING GROUNDS.

FOR THE ANIMALS LIVING HERE, THE CHALLENGES HAVE JUST BEGUN.

THEY'VE JUST SURVIVED A WINTER WHERE THE TEMPERATURES WERE DOWN AS LOW AS MINUS 30 DEGREES FAHRENHEIT, BUT NOW THEY'VE GOT TO RUN THE GAUNTLET OF THE SPRING.

7,000 FEET HIGH UP IN THE ROCKIES, THE GREATER YELLOWSTONE ECOSYSTEM INCLUDES TWO NATIONAL PARKS... AND IS BOUNDED ON 3 SIDES BY MOUNTAIN RANGES.

MY JOURNEY STARTS AT THE SOUTHERN END OF THE GRAND TETON NATIONAL PARK.

I'M ON THE BANKS OF THE SNAKE RIVER.

I'M LISTENING TO THE CHATTER COMING OUT OF THIS HOUSE OF STICKS.

IN HERE LIVES NORTH AMERICA'S LARGEST RODENT: THE BEAVER.

[BEAVERS SQUEAKING] AND I'VE GOT MORE THAN JUST A MICROPHONE IN THAT LODGE.

I'VE GOT A LITTLE CAMERA.

I CAN ACTUALLY SEE INSIDE THE LODGE.

INSIDE THERE'S A MOTHER, A FATHER, ONE 1-YEAR-OLD, AND TWO 2-YEAR-OLDS.

THEY'RE KIND OF GROOMING EACH OTHER, AND ONE HAS BEEN EATING A STICK, AND THERE'S BEEN A LOT OF YAWNING GOING ON.

I MEAN, HAVE YOU EVER SEEN A BEAVER IN ITS HOUSE BEFORE?

THIS IS AWESOME!

[BEAVERS SQUEAKING] 15 BEAVERS LIVE IN THIS IMMEDIATE AREA.

USUALLY IN APRIL, THEY'D STILL BE CURLED UP IN THEIR DENS UNDER A BLANKET OF SNOW.

BUT A RECENT WARM SPELL TRIGGERED THE THAW 3 WEEKS EARLY.

THAT MEANS FOOD HAS COME SOONER AS GREEN SHOOTS HAVE ALREADY SPRUNG UP, BUT HIGHER UP, THE PEAKS ARE STILL 10 FEET DEEP IN SNOW, AND THAT SPELLS DANGER FOR THE BEAVERS.

THEIR LIFE'S ABOUT TO CHANGE BECAUSE AS THE SNOW MELTS IN THE ROCKIES, WE'RE GONNA START TO SEE A HUGE AMOUNT OF WATER FLUSHING INTO THESE SYSTEMS, SO THEIR LITTLE PEACEFUL WORLD OF WINTER IS ABOUT TO BE TURNED OVER WITH THE GREAT RUSH OF SPRING MELTWATER.

ON THIS STRETCH OF RIVER, THE BEAVERS HAVE BUILT 5 DAMS.

THE RESULTING PONDS AREN'T JUST A NATURAL PANTRY, BUT A SAFE PLACE TO EAT AWAY FROM PREDATORS ON LAND, LIKE WOLVES, BEARS, AND COYOTES.

BUT THESE POOLS BENEFIT A MUCH WIDER ECOSYSTEM.

ANIMALS LIKE DEER EAT PLANTS THAT GROW AT THE WATER'S EDGE, AND THE PONDS ATTRACT FISH, WHICH, IN TURN, FEED A WHOLE RANGE OF OTHER WILDLIFE.

IN ADDITION, THE DAMS SLOW DOWN THE FLOODWATER.

IF THE MAIN DAM IS DESTROYED, THE OTHERS WILL GO, TOO, AND THIS IMPORTANT WATERY WORLD WILL DISAPPEAR.

WILDLIFE CAMERAMAN JEFF HOGAN HAS BEEN FOLLOWING THESE BEAVERS NEARLY ALL HIS LIFE.

THIS ONE UP HERE IS ACTUALLY THE BIGGEST, STRONGEST ONE, AND IT'S HOLDING BACK MORE WATER THAN...

SO THEY'VE BEEN IN THEIR LODGE ALL WINTER.

THEN--AND NOW WHAT'S GOING ON NOW?

WHAT'S ON THEIR MINDS AS THE SPRING IS COMING?

WHAT THIS GUY REALLY NEEDS TO DO RIGHT NOW IS, UM, FATTEN UP.

SO HE'S JUST COME OUT OF THE DEN, AND HE'S JUST SEEN ALL THIS NEW GROWTH, AND HE'S JUST GOING AT IT.

KIRK, VOICE-OVER: UNTIL NOW, THIS BEAVER HASN'T EATEN FRESH FOOD FOR OVER 5 MONTHS.

OVER WINTER, THE BEAVERS' FOOD SUPPLY IS LOCKED IN SNOW AND ICE.

THEY EAT TREE BARK, TWIGS, AND LEAVES, BUT WHEN THERE'S SO MUCH SNOW ON THE GROUND, THEY CAN'T GET TO IT.

YET BEAVERS DON'T HIBERNATE.

THEY MUST KEEP EATING TO SURVIVE.

THEIR SOLUTION IS INGENIOUS.

THEY STORE A MOUND OF LEAFY TWIGS LARGE ENOUGH TO LAST FOR 5 MONTHS NEAR THE UNDERWATER ENTRANCE TO THEIR HOME.

EVEN WHEN THE RIVER FREEZES OVER, THEY HAVE A PERSONALIZED ROOT CELLAR JUST A MOMENT'S SWIM AWAY.

BY THE END OF WINTER, THE ROTTEN TWIGS HAVE LOST MOST OF THEIR PROTEIN.

THIS BEAVER IS SURVIVING ON HIS OWN BODY FAT.

THIS YEAR, THE EARLY THAW HAS GIVEN HIM A LIFELINE.

THESE ASPEN LEAVES ARE JUST WHAT HIS HUNGRY FAMILY NEEDS.

THEIR LODGE IS A PREDATOR-PROOF FORTRESS OF STICKS AND MUD.

THIS ONE HAS BEEN BUILT INTO THE RIVERBANK.

[BEAVERS SQUEAKING] THESE GUYS HAVE LOST 1/3 OF THEIR BODY WEIGHT OVER THE WINTER.

THEY NEED TO EAT 2 POUNDS OF TWIGS AND LEAVES EVERY DAY TO GET THEIR WEIGHT BACK UP.

THE YOUNG BEAVERS SQUEAK TO DEMAND FOOD.

[BEAVERS SQUEAKING] BUT WHEN COMPETITION IS THIS FIERCE, IT'S EVERY BEAVER FOR ITSELF.

[BEAVERS SQUEAKING MORE FREQUENTLY] IN SPRING, THESE TWIGS ARE FULL OF SUGAR AND PROTEIN, JUST WHAT THEY NEED TO KICKSTART THEIR RECOVERY.

BUT FIRST THEY HAVE TO GO GET THEM.

[CRACKING] SO HOW LONG WOULD IT TAKE A BEAVER TO CUT THIS TREE DOWN?

YOU KNOW, I HAVE WATCHED THEM CHEW A TREE DOWN LIKE THIS IN ABOUT TWO HOURS.

WHY DO THEY CUT THE TREES DOWN?

UP IN THE TREE-- THAT'S--THAT'S WHERE THE VALUABLE NUTRIENTS ARE.

IT'S IN THE LEAVES.

IT'S IN THE BRANCHES, IN THE SMALLER TWIGS.

AND SO, WHAT THEY WANT, IF THEY'VE GOT A BIG, RICH TREE, THEY'VE GOT A LOT OF BRANCHES UP THERE, AND IT'S--ONE TREE THAT THEY CHEW DOWN CAN, YOU KNOW, SUPPLY THEM WITH A HUGE AMOUNT OF, UH, RESOURCE.

THAT IS SO COOL.

KIRK, VOICE-OVER: ONCE THEY'VE CUT THE TREE DOWN, THE BEAVERS EAT THE TWIGS AND LEAVES AND USE THE BRANCHES TO FORTIFY THEIR DAM.

WE'LL FOLLOW THEM OVER THE SPRING TO SEE HOW THEY'RE RESPONDING TO THE RISING WATER LEVELS.

AFTER A LONG WINTER WITH LITTLE FOOD, THE MAJORITY OF THE WILDLIFE IN GREATER YELLOWSTONE IS HANGING ON FOR THE NEW, GREEN SHOOTS OF SPRING.

I'VE COME TO SEE A CRITTER THAT IS SAID TO HERALD THE CHANGE OF SEASON.

SPRING IS SAID TO ARRIVE IN YELLOWSTONE WHEN ONE PARTICULAR ANIMAL SHOWS ITS FACE.

THIS LITTLE GUY IS A UINTA GROUND SQUIRREL, AND HE AND HIS BUDDIES COME OUT OF THEIR BURROWS IN APRIL.

KIRK, VOICE-OVER: THESE TINY MAMMALS ARE EMERGING FROM AN 8-MONTH-LONG HIBERNATION, AND THEY'VE ONLY GOT ONE THING ON THEIR MIND: FEASTING.

OVER THE WINTER, THEY LOSE 1/3 OF THEIR BODY WEIGHT...

SO THEY NEED TO PACK ON THE CALORIES.

WHEN THEY COME OUT, THEY'RE VORACIOUS.

THEY'RE ACTUALLY CHOMPING AWAY.

NOW, WATCH THIS LITTLE GUY MUNCHING AWAY.

HE IS EATING, IN 4 MONTHS, FOR AN ENTIRE YEAR.

KIRK, VOICE-OVER: COME SUMMER, THIS EARTH WILL BE PARCHED.

THE GRASS AND SEEDS THEY FEAST ON WILL BE GONE, AND THESE SQUIRRELS WILL HEAD BACK DOWN UNDERGROUND TO HIBERNATE THROUGH SUMMER AND AUTUMN AND WINTER, SO THE SOONER SPRING ARRIVES, THE LONGER THEY'LL HAVE TO FATTEN UP.

IT'S NOW MAY, AND DAYTIME TEMPERATURES ARE IN THE 50s.

[BIRDS CHIRPING] AS SOON AS THE SNOW MELTS AND FRESH VEGETATION APPEARS, WE HAVE WHAT SCIENTISTS HERE CALL "THE SPRING GREEN-UP."

IT DRIVES ALL LIFE IN YELLOWSTONE, FROM TINY TO HUGE... LIKE THE ROCKY MOUNTAIN ELK.

THERE ARE OVER 20,000 OF THEM IN YELLOWSTONE.

AS THE GREEN-UP MOVES FROM THE LOWLANDS TO YELLOWSTONE'S HIGH MOUNTAIN MEADOWS, THE ELK MOVE WITH IT.

AND AS THE SNOW LINE RETREATS FROM THE VALLEYS TO THE FOOTHILLS, THE FEMALE GRIZZLIES AND THEIR CUBS START EMERGING.

AS OMNIVORES, BEARS EAT A WIDE RANGE OF FOOD, BUT THIS SPRING, THEIR MEAT OPTION IS IN SHORT SUPPLY.

BY NOW THERE ARE USUALLY CARCASSES OF ELK AND BISON THAT DIED OVER THE WINTER, BUT THIS YEAR, THE MILDER TEMPERATURES MEAN MANY MORE HAVE SURVIVED.

THE BEARS WILL HAVE TO SCRATCH A LIVING, MOSTLY DIGGING UP ROOTS AND GRASSES.

GRIZZLY EXPERT CASEY ANDERSON IS ALREADY SEEING SIGNS THAT THERE ARE PLENTY OF BEARS AROUND.

THEY'VE BEEN USING TREES AS SCRATCHING POSTS.

CASEY: YEAH.

THIS THING HAS BEEN GETTING HAMMERED.

LOOK AT ALL THIS.

LEAVING THEIR SCENT BEHIND.

THERE'S ALL KINDS OF MUD AND HAIR STUCK ON HERE, SO YEAH.

IT'S GOING TO BE COOL TO SEE WHO IS BACK IN TOWN.

KIRK: TO FIND OUT EXACTLY WHO IS IN TOWN, I'M HEADING UP NORTH TO MEET CASEY IN THE FOOTHILLS OF THE GALLATIN RANGE.

CASEY HAS TRACKED A SINGLE FEMALE TO A WOODED AREA HALFWAY DOWN THE MOUNTAIN.

THE EARLY THAW THIS YEAR MEANS THERE ARE FEWER CARCASSES BUT MORE SPRING PLANTS.

KIRK: THE BEARS UP ON THE RIDGELINE-- ARE THEY GONNA COME DOWN LOWER?

CASEY: YEAH.

THE LAST OF THE BISCUITROOT IS REALLY KIND OF GETTING TAPPED OUT UP THERE, AND SO THEY'RE GONNA GRADUALLY KEEP COMING DOWN INTO THE VALLEY HERE.

AS MORE AND MORE BEARS SHOW UP, WE'RE GONNA START SEEING MORE CONGREGATION DOWN HERE IN THE FLATS.

THAT'S A SINGLE FEMALE.

WHAT'S SHE DOING UP THERE?

DIGGING BISCUITROOT.

YOU KNOW, JUST ONE PLANT, THE NEXT, FLICKING THE ROOT OUT, BUT DO YOU KNOW, SOMETIMES SHE GETS A NEW SPOT.

IF YOU LOOK, YOU CAN SEE SHE'S REALLY GOT TO DIG DOWN.

SO, YOU KNOW, A GRIZZLY IS DESIGNED TO EXCAVATE.

THAT BIG HUMP ON THE SHOULDER REALLY POWERS THOSE FOREARMS, AND THOSE BIG CLAWS ARE LIKE SHOVELS, AND THEY JUST DIG DOWN IN THE GROUND, AND THAT'S HOW THEY ACHIEVE A LOT OF FOOD THIS TIME OF YEAR, AND SHE'S--SHE'S DEFINITELY SHOWING OFF RIGHT NOW-- SHOWING OFF HER GRIZZLY POWER AND STRENGTH-- FLICK OF THE WRIST, USING HER CLAWS, POPPING IT OUT, EATING THE ROOT, AND ON TO THE NEXT.

THOSE ARE TINY THINGS.

THEY'RE, LIKE, HALF THE SIZE OF A PEANUT.

YEAH.

BY THE END OF THE NIGHT, SHE'LL EAT THOUSANDS OF THEM.

OH.

SO IT'S A LOT OF WORK UP ON THE HILL, BUT SHE'S GOING TO GET HER FILL.

KIRK, VOICE-OVER: BUT BISCUITROOT CONTAINS LESS THAN HALF THE CALORIES OF MEAT, SO SHE'LL HAVE TO EAT A HUGE AMOUNT TO REPLACE THE WEIGHT SHE LOST IN HIBERNATION.

THE FACT THAT BEARS ARE HERE AT THIS TIME IS NO COINCIDENCE.

KNOWING WHEN AND WHERE TO FIND FOOD IS ONE OF THE MOST IMPORTANT LESSONS THEY LEARN FROM THEIR MOTHERS.

CASEY: THE ENTIRE SEASON THAT THE GRIZZLIES ARE OUT HERE, IT IS JUST LITTLE-- THESE LITTLE POCKET WINDOWS OF FOOD SOURCE AVAILABILITY THAT JUST KEEPS CHANGING FROM DIFFERENT LOCATIONS ALL OVER THE ECOSYSTEM, AND THE BEAR HAS TO KEEP TRACK OF THAT, KEEP MOVING FROM ONE TO THE NEXT TO THE NEXT TO THE NEXT, AND IT'S REALLY A--A CRAZY MAP OF, YOU KNOW, FOOD SOURCES THAT THAT BEAR HAS TO KNOW, AND MOST OF THAT IS LEARNED WHEN IT WAS A CUB FROM ITS MOTHER.

SO WILL THESE BEARS, THEN, JUST DO A TRAP LINE?

THEY KIND OF WORK THEIR WAY AROUND THIS VALLEY AND THEN REPEAT THAT PROCESS?

I MEAN, DO YOU SEE THEM DOING THE SAME PATHWAYS TIME AND TIME AGAIN?

YEAH, DEFINITELY, AND ONCE A BEAR HAS LEARNED, YOU KNOW, THE FOOD SOURCES AND WHEN AND WHERE, EVERY YEAR, THEY'LL ALMOST FOLLOW THE EXACT SAME SPOT.

KIRK, VOICE-OVER: IT'S NOT JUST GRIZZLY BEARS THAT MUST EXPLOIT THE BEST POCKETS OF FOOD.

THE ROCKY MOUNTAIN ELK ARE STILL MOVING NORTH, FOLLOWING THE GREEN-UP.

BUT ANOTHER OF YELLOWSTONE'S RESIDENTS IS AHEAD OF THEM...

THE MIGHTY BISON.

THIS YEAR, THE BISON AND ELK ARE LUCKY.

NOT ONLY HAVE THEY HAD A MILDER WINTER, BUT THE EARLY THAW MEANS THERE'S PLENTY OF GRASS AROUND... AND NOW, IN SPRING, IT'S FULL OF PROTEIN.

THE PARK IS HOME TO ABOUT 5,000 BISON.

THE MAJORITY OF THEM LIVE IN THE NORTHERN RANGE.

ALMOST ALL HAVE MADE IT TO SPRING FIT AND STRONG...

WHICH IS VITAL FOR THE SURVIVAL OF THEIR NEWBORN CALVES.

THE MOTHERS CONVERT THE GRASS INTO MILK FOR THEIR YOUNG.

NORMALLY, A QUARTER OF THE BABY BISON IN YELLOWSTONE DIE IN THEIR FIRST TWO MONTHS, BUT THIS MILD WEATHER WILL GIVE THEM A MUCH GREATER CHANCE OF SURVIVAL.

WARM TEMPERATURES ARE GOOD FOR THE BISON...

BUT IT'S LEFT THEIR NUMBER-ONE PREDATOR, THE WOLVES, HUNGRY.

WOLVES RUN FASTER THAN THEIR PREY IN DEEP SNOW.

WITHOUT IT, THE ODDS OF A SUCCESSFUL HUNT ARE MUCH WORSE.

IN THE NORTHERN RANGE, 4 PACKS COMPETE FOR FOOD AND TERRITORY, AND SOME OF THEM ARE HAVING A REALLY TOUGH TIME.

DOUG SMITH HAS STUDIED THESE WOLVES FOR OVER 20 YEARS.

IN THE LAST DECADE, THEIR NUMBERS HAVE DECLINED BY ALMOST HALF.

ONE PACK, KNOWN BY PARK BIOLOGISTS AS LAMAR CANYON PACK, IS EXTREMELY WEAK.

IF THEY DON'T GAIN STRENGTH QUICKLY, THEY RISK LOSING THEIR TERRITORY TO ANOTHER PACK.

OUR CAMERA CREW IS DOWN IN LAMAR VALLEY, FOLLOWING THEM.

FROM A MILE OFF, THEY SPOT SOME UNUSUAL BEHAVIOR.

THE WOLVES ARE TAKING ON A WHOLE HERD OF BISON.

THEY'RE HOPING TO SNATCH A NEWBORN CALF.

DOUG IS HELPING US TO ANALYZE WHAT'S GOING ON.

DOUG: THESE BISON ARE DOING CLASSIC DEFENSE.

THEY--THEY GROUP TOGETHER.

THEY'RE BIG.

THEY'RE NOT MOVING, OTHER THAN TO ATTACK THE WOLVES.

KIRK, VOICE-OVER: THIS IS AN AGE-OLD DANCE BETWEEN POWER AND AGILITY.

WOLVES MAY BE QUICK, BUT BISON ARE STRONG.

GETTING THIS CLOSE RISKS A KICK TO THE SKULL.

DOUG: WOLVES WOULD LIKE TO SPREAD THIS GROUP APART, BREAK IT UP, AND THEN THAT WOULD FREE UP ONE OF THESE CALVES THAT THEY CAN GRAB.

KIRK, VOICE-OVER: A FEMALE BISON CHARGES, THREATENING TO CATCH THEM WITH HER HOOVES.

DOUG: YOU GET CAUGHT BY A FOOT, YOU GET CAUGHT BY A HORN-- INJURY, DEATH.

IT'S FASCINATING FOOTAGE TO WATCH.

UH, THEY'RE BEING AGGRESSIVE, YET VERY CAREFUL.

WOLVES DEFINITELY FEEL SAFER PURSUING A FLEEING PREY BECAUSE THEY CAN KIND OF COME IN FROM BEHIND, GRAB, AND PULL DOWN.

KIRK, VOICE-OVER: TO ATTACK A STANDING HERD LIKE THIS MEANS THESE WOLVES ARE DESPERATE.

DOUG: AND AS THEY GET HUNGRY AND IT GOES MORE DAYS WITHOUT FOOD, THEY'RE GONNA TAKE MORE RISKS.

KIRK, VOICE-OVER: THEY FINALLY CLOSE IN.

THIS IS THE BREAK IN THE HERD THEY'VE BEEN HOPING FOR.

DOUG: THE MOTHER CAN'T DO ALL THIS BY HERSELF.

THE WOLVES OUTNUMBER HER.

THEY MAY GET THIS CALF.

I CAN'T BELIEVE THIS.

YOU RARELY SEE THIS.

KIRK, VOICE-OVER: BUT THE CALF MAKES IT BACK TO THE SAFETY OF THE HERD... LEAVING 9 VERY HUNGRY WOLVES.

THE LAMAR CANYON PACK HAS FAILED.

DOUG IS CONCERNED THAT THESE WARMER WINTERS AND EARLY SPRINGS ARE PART OF A WORRYING TREND.

DOUG: WE CAN SEE CLIMATE CHANGE AFFECTING THE PARK NOW.

UM, OUR WOLF RESEARCH HAS ONLY BEEN GOING A LITTLE OVER 20 YEARS, BUT YOU TALK TO PEOPLE WHO WERE IN YELLOWSTONE 40 OR 50 YEARS AGO, AND WINTER SEVERITY IS--IS JUST NOT LIKE IT USED TO BE, AND THIS IS A BIG FACTOR IN HOW THIS SYSTEM WORKS.

THIS WINTER AND LAST WINTER WERE TWO OF OUR SHORTEST WINTERS WE'VE HAD, AND SO THIS, IN A BIG-PICTURE WAY, FAVORS THE BISON, SO WE LOST OUR SNOW AT LOW ELEVATION VERY EARLY THIS YEAR, AND SO ANYTIME THE SNOW GOES AWAY, IT GIVES THE BISON BETTER FOOTING.

UH, THEY CAN DEAL WITH THE WOLVES.

AND SECONDLY, IT EXPOSES THE VEGETATION TO SUNLIGHT.

THE CONDITION OF BISON WILL START TO IMPROVE EARLIER.

UH, AND SO THAT HURTS THE WOLVES A LITTLE BIT.

AND SO SITTING ASIDE AND WATCHING LIFE EVOLVE WITH THAT GOING ON, UH, HAS GOT ME SCARED STIFF.

KIRK, VOICE-OVER: LIKE DOUG, I'M CONCERNED ABOUT YELLOWSTONE'S FUTURE.

CLIMATES CHANGE, BUT IT'S THE SPEED IT'S HAPPENING THAT'S SO SHOCKING.

TEMPERATURES HERE HAVE BEEN RISING FAST, AND THE SEASONS ARE CHANGING.

THIS LAST DECADE HAS BEEN THE WARMEST SINCE RECORDS BEGAN.

WITH YELLOWSTONE'S WINTERS BECOMING SHORTER AND THE SPRING STARTING EARLIER, MUCH OF THE WILDLIFE IS STRUGGLING TO ADAPT.

AND IN MANY CASES, IT'S THE PREDATORS WHO FEEL IT THE MOST.

TAKE THE GREAT GRAY OWL.

THESE OWLS LIVE IN SOME OF THE COLDEST FORESTS ON EARTH FROM SIBERIA TO SCANDINAVIA.

YELLOWSTONE IS STILL COLD ENOUGH FOR THE OWLS TO SURVIVE...

BUT IF IT GETS MUCH WARMER, SCIENTISTS BELIEVE THEY WON'T BE ABLE TO HOLD ON.

BACK IN WINTER, CAMERAMAN JEFF HOGAN FOLLOWED A PAIR OF GREAT GRAY OWLS IN THE TETON MOUNTAINS.

GREAT GRAY OWLS RELY ON SMALL RODENTS LIKE VOLES AND GOPHERS THAT LIVE BENEATH THE SNOW.

BUT OWL BIOLOGISTS BELIEVE THAT THE CHANGING CLIMATE IS AFFECTING THEIR HUNTING.

JEFF: THESE OWLS HAVE TO BUST THROUGH THIS CRUST AND THEN MAYBE ANOTHER FOOT OF SNOW TO GET AT THEIR PREY.

KIRK, VOICE-OVER: ERRATIC WINTER WEATHER CAUSED THE CRUST TO MELT AND REFREEZE, MAKING IT IMPENETRABLE.

AND IF THEY CAN'T HUNT, THE GREAT GRAY OWLS WON'T NEST OR LAY EGGS.

NOW IT'S SPRING.

JEFF AND I ARE SEARCHING FOR OWL CHICKS IN THE BEARTOOTH MOUNTAINS.

THE FOREST HERE IS 300 YEARS OLD, AN IDEAL HABITAT FOR GREAT GRAY OWLS.

[KIRK WHISPERING] Oh, my God.

Look at those things.

[CHICKS CHEEPING] Right here I can see the stump of a tree.

It's got 3 chicks.

KIRK, VOICE-OVER: GIVEN THE DIFFICULTIES OF WINTER, FINDING A NEST WITH 3 CHICKS IS EXTREMELY UNUSUAL.

WHEN FOOD IS SCARCE, THE SMALLEST CHICK CAN BE EATEN BY ITS SIBLINGS, BUT THIS RUNT IS STILL ALIVE.

KIRK: THAT IS INCREDIBLE.

JEFF: AND HOW UNIQUE, HUH?

I HAD NO IDEA.

I EXPECTED A NEST, NOT THE TOP HALF OF A ROTTED-OFF TREE.

RIGHT.

IT LOOKS LIKE THE THIRD CHICK IS QUITE A BIT SMALLER.

IS THAT BECAUSE HE MISSED OUT ON SOME OF THE EARLY FEEDINGS OR WHAT?

JEFF: THE CHICKS HATCH TWO DAYS APART.

THE YOUNGER THEY ARE, THE SMALLER THEY ARE, AND ALSO, THE PRIORITY IS GIVEN TO THE FIRST ONE.

HOW OFTEN WITH 3 CHICKS DO ALL 3 SURVIVE?

YOU KNOW, WE'RE GETTING A THIRD-CHICK SURVIVAL RATE OF ABOUT 20%.

SO THIS GUY-- THIS GUY HAS GOT A 1 IN 5 CHANCE OF MAKING A GO OF IT.

RIGHT.

IT'S A BONUS IF YOU CAN GET THAT THIRD ONE TO MAKE IT.

RIGHT.

CHANCES ARE THAT THIRD ONE USUALLY DOES NOT MAKE IT.

KIRK, VOICE-OVER: THESE CHICKS ARE ABOUT TO FACE THEIR GREATEST CHALLENGE: LEAVING THE NEST.

THEY'RE KIND OF CROWDED IN THERE.

AT A CERTAIN POINT, THEY'RE GONNA HAVE A HARD TIME FITTING IN THAT NEST.

YEAH.

THAT'S WHY THEY'RE GONNA JUMP.

THEY'RE JUST GONNA LEAP OFF THAT THING?

BEFORE THEY CAN FLY.

THAT'S GOT TO BE 10, 20--AT LEAST 30 FEET UP.

OH, YEAH.

MAYBE--MAYBE MORE.

WHEN DO THEY DECIDE TO GO?

JUST ONE DAY, OR DO THE PARENTS ENCOURAGE THEM?

AT ABOUT 4 WEEKS.

THEY'RE GONNA-- THEY'RE GONNA BE BOUNCING AROUND, FLAPPING THEIR WINGS.

ONE, MAYBE TWO DAYS LATER, ANOTHER.

RIGHT.

OR IT MIGHT BE TOTALLY TERRIFIED AND SPEND ANOTHER WEEK IN THERE.

YEAH.

HUH.

YEAH.

SHE'S-- YEAH?

OH, THERE'S THE MOM.

YEAH.

THAT'S THE MOM.

JEFF: THE MOM, WHO USUALLY JUST STANDS BY AND KEEPS AN EYE ON THE NEST-- SHE'S BEEN COMING IN WITH FOOD THAT SHE'S CAUGHT.

KIRK: OH, BECAUSE THE PARENTS ARE DOUBLING DOWN, SO MOM AND DAD ARE BOTH HUNTING.

YEAH, AND THAT'S KIND OF UNIQUE.

WE DON'T SEE IT AS-- NOT AS OFTEN AS WE'VE BEEN SEEING IT WITH HER, AND SO MAYBE THAT COULD ACTUALLY BE MAKING THE DIFFERENCE BETWEEN WHETHER THIS THIRD CHICK SURVIVES OR NOT.

AH.

KIRK, VOICE-OVER: IN CONTRAST TO WINTER, WHEN ICE MADE HUNTING TOUGH, THE EARLY THAW HAS LED TO A BOOM IN RODENTS, MEANING BOTH PARENTS CAN BRING BACK FOOD FOR THE CHICKS.

BUT SPRING IN YELLOWSTONE IS UNPREDICTABLE.

AFTER I LEAVE, THE WEATHER TAKES A TURN FOR THE WORSE.

A STORM BLOWS IN, WHICH SPELLS BAD NEWS FOR THE CHICKS.

JEFF: THEY'RE JUST DRENCHED.

THIS RAIN MAKES--MAKES IT A LITTLE MORE DIFFICULT FOR ME TO WORK IN, BUT IT MAKES IT EVEN MORE DIFFICULT.

ACTUALLY MAY HUNKER DOWN A LITTLE BIT MORE DURING THE--A RAIN LIKE THIS.

KIRK, VOICE-OVER: THE RUNT IS AT THE BOTTOM OF THE PECKING ORDER.

AS SOON AS FOOD BECOMES SCARCE, SHE WILL BE THE FIRST TO LOSE OUT.

IF THE WEATHER DOESN'T CHANGE, THE RUNT MAY STARVE.

[CHEEP] [THUNDER] MAY IS ALWAYS THE WETTEST MONTH OF THE YEAR IN YELLOWSTONE, BUT THIS YEAR, IT'S FAR WETTER THAN AVERAGE.

THE REASON FOR THIS IS FAR TO THE WEST, IN THE PACIFIC OCEAN.

THE SOUTHWEST WINDS BLOW A SUCCESSION OF STORMS DIRECTLY TO YELLOWSTONE.

WHEN THEY HIT THE WALL OF MOUNTAINS... [THUNDER] ALL THEIR WATER IS RELEASED.

WE EXPERIENCE THE BIGGEST DELUGE THAT'S BEEN SEEN HERE FOR MANY YEARS.

RAIN FALLING OVER 3 MOUNTAIN RANGES DRAINS STRAIGHT INTO THE YELLOWSTONE BASIN.

OVERNIGHT, WATER LEVELS RISE BY A FOOT.

[THUNDER] [CAW] FOR TWO WEEKS, RAIN POURS DOWN AND THUNDERSTORMS REVERBERATE AROUND THE MOUNTAINS.

FINALLY, IN EARLY JUNE, THE WEATHER BREAKS, BUT THERE'S WATER EVERYWHERE.

ON TOP OF ALL THAT STORMWATER, THE MELTING SNOWS ARE NOW FLOODING INTO THE RIVER SYSTEMS.

HERE AT YELLOWSTONE FALLS, YOU CAN SEE THE IMPACT.

THESE ARE THE LOWER FALLS OF THE YELLOWSTONE RIVER, AND ALL THE SNOW THAT FALLS IN THE CENTRAL PART OF YELLOWSTONE NATIONAL PARK FLOWS INTO THIS SYSTEM.

THOUSANDS OF ANIMALS RELY ON THIS RIVER TO SURVIVE.

GREATER YELLOWSTONE SITS ON THE CONTINENTAL DIVIDE.

ITS WATERS FLOW WEST TO THE PACIFIC OCEAN AND EAST TO THE GULF OF MEXICO.

THIS HUGE RESERVOIR OF WATER IS A LIFELINE FOR AGRICULTURE AND INDUSTRY AS WELL AS WILDLIFE IN THE AMERICAN WEST.

SO IT'S REALLY IMPORTANT TO UNDERSTAND HOW MUCH WATER IS DRAINING INTO THESE RIVER SYSTEMS.

I'D REALLY LOVE TO KNOW HOW MUCH WATER IS COMING DOWN HERE AT THE PEAK FLOW IN THE SPRING.

BUT HOW DO WE MEASURE IT?

KIRK, VOICE-OVER: TEAMS OF SCIENTISTS HAVE MONITORED THE STREAM FLOW IN YELLOWSTONE'S RIVERS FOR THE LAST 86 YEARS.

THEY'VE NOTICED THAT RECENTLY THE PATTERN OF THE SPRING RUNOFF HAS BEEN CHANGING.

I'VE COME TO THE HEADWATERS OF THE YELLOWSTONE RIVER TO MEASURE ITS STREAM FLOW, BUT TO DO THAT, I'VE CALLED IN REINFORCEMENTS.

[METAL RATTLING] WOMAN: HEY, KIRK!

KIRK: HEY, STACY!

THIS IS STACY KINSEY.

SHE'S A HYDROLOGIST AT THE U.S. GEOLOGICAL SURVEY, AND SHE AND I ARE GONNA DO SOME PRETTY COOL SCIENCE USING THIS LITTLE BOAT TO ACTUALLY MEASURE THE VOLUME OF THE WATER GOING THROUGH THIS RIVER VALLEY.

STACY: YOU READY FOR THE ROPE, KIRK?

YEAH.

COMING AT YOU.

OH.

GOT IT.

OK. WHAT NEXT?

OK, SO IF YOU COULD GUIDE IT AROUND THAT ROCK, THAT WOULD BE GREAT.

KIRK: OK. OUT INTO THE FLOW THERE?

YEAH.

THAT WOULD BE GREAT.

OK.

I'M GONNA HEAD ACROSS.

KIRK: OK.

I'LL GET THE COMPUTER.

KIRK, VOICE-OVER: STACY WILL MONITOR THE RISING WATER IN THE YELLOWSTONE RIVER THROUGHOUT JUNE.

THE DATA SHE GATHERS WILL TELL US HOW BIG THE SPRING RUNOFF WILL BE.

THIS IS REALLY COOL.

THIS COMPUTER IS COMMUNICATING WITH THAT LITTLE BOAT.

THE BOAT IS SENDING A SIGNAL DOWN TO THE BOTTOM OF THE STREAM AND MEASURING BOTH THE DEPTH OF THE STREAM AND THE RATE AT WHICH THE WATER IS FLOWING.

WE USED TO HAVE TO DO THIS BY TAKING STICKS AND MEASURING THE DEPTH OF THE WATER AS YOU WADED ACROSS, BUT THIS LITTLE BOAT TALKS TO THE COMPUTER.

THIS IS VERY PRECISE DEPTH AND SPEED MEASUREMENTS.

IT'S GREAT SCIENCE.

HOW YOU DOING, STACY?

IT'S LOOKING GOOD!

IT'S A LITTLE ELEVATED FOR THIS TIME OF YEAR, BUT IT'S STILL ON THE RISE.

KIRK, VOICE-OVER: THE RAIN HAS ALREADY RAISED THE WATER LEVELS, BUT IT'S ALL THE SNOW THAT REMAINS IN THE MOUNTAINS THAT WILL MAKE THE REAL DIFFERENCE.

SO WHEN IS THE WATER GONNA GET DEEPER?

IT'S GONNA CONTINUE TO GET A LITTLE BIT DEEPER BECAUSE THERE'S STILL SNOWPACK UP IN THE HIGHER ELEVATION OF THE MOUNTAINS, AND SO AS THAT SNOWPACK CONTINUES TO MELT, THE STAGE OF THIS RIVER IS GONNA KEEP ON INCREASING.

GOTCHA.

AND WHEN DOES THAT NORMALLY HAPPEN?

WHEN DO YOU START TO SEE THE PEAK?

WE USUALLY SEE IT IN THE NEXT COUPLE WEEKS.

OK. KIRK, VOICE-OVER: IT'S NOW THE 11th OF JUNE, AND STACY'S DATA SHOW THAT THIS YEAR, THE SPRING RUNOFF IS EARLIER THAN USUAL.

ALL THIS WATER IS IN THE SYSTEM, AND THAT'S BEFORE THE FINAL SNOWPACK AT THE TOP OF THE MOUNTAINS HAS MELTED.

BUT ALREADY, OUR BEAVERS ARE BEING AFFECTED.

THE SNOWPACK IS NOW PRIMED AND READY.

ALL IT NEEDS IS A FEW DAYS OF CONSISTENTLY HOT WEATHER TO COME RUSHING DOWN INTO THE RIVER BASINS.

WITH THE WATER LEVELS RISING, I WANT TO FIND OUT HOW THE BEAVERS' DAMS ARE DOING.

WELL, LOOK AT THIS.

THE WATER HAS COME UP A LOT SINCE I WAS HERE LAST.

THE SNAKE RIVER HAS OVERFLOWED, AND WE'VE GOT A BUNCH OF MURKY WATER, AND THE DAM IS JUST BARELY HOLDING THE WATER BACK RIGHT NOW.

KIRK, VOICE-OVER: THE DAM HAS ACTUALLY BREACHED IN SOME PLACES, AND THE BEAVERS ARE RACING TO PLUG ALL THE LEAKS.

ON THIS STRETCH, THE BEAVERS HAVE 5 DAMS TO LOOK AFTER.

THIS IS AMAZING.

THE WATER IS ACTUALLY HIGHER THAN THE TOP OF THE DAM, AND IT'S JUST DEBRIS AND ALGAE THAT ARE KEEPING THE DAM FROM COMPLETELY OVERFLOWING.

IT LOOKS LIKE IT'S ABOUT TO START RAINING.

THE SPRING RAINS ARE GONNA ADD MORE WATER, AND THE MELTWATER COMING OFF THE MOUNTAINS IS BUILDING UP IN THE SNAKE RIVER.

PRETTY SOON, THIS DAM IS GONNA NEED SOME REAL REMEDIATION.

THINGS ARE STARTING TO LOOK PRETTY PRECARIOUS HERE, BUT BEAVERS ARE ONLY SECOND TO HUMANS WHEN IT COMES TO ENGINEERING SKILLS, AND THEY'RE UP TO THE TASK.

AS THE WATER COMES UP, THEY'RE GONNA GO TO WORK.

IF THE MAIN DAM IS KNOCKED OUT, THE WATER WILL EASILY BREAK THROUGH THE OTHERS.

THIS WETLAND ECOSYSTEM WILL DRY OUT.

SO I'M HEADED UNDERWATER TO FIND OUT HOW SOLID IT IS.

THE BEAVERS ARE STILL SLEEPING IN THEIR LODGE.

THEY COME OUT IN THE EARLY EVENING, SO I'VE GOT SOME TIME TO INSPECT THEIR DAM.

IT'S ONLY WHEN YOU GET UNDERWATER THAT YOU SEE THE SKILL THAT'S GONE INTO THIS DAM.

THE BEAVERS CHOOSE LOGS UP TO 6 FEET LONG AND PLACE THEM ALL AT AN ANGLE OF 30 DEGREES TO HOLD BACK THE WATER.

THIS IS PRECISION ENGINEERING.

AH.

THIS IS MIRACULOUS.

THIS IS A BIG LOG, AND IT WAS BROUGHT HERE AND INSTALLED BY THE BEAVERS.

THE WAY THEY DO THIS IS THEY ACTUALLY DIG CHANNELS ACROSS THE LANDSCAPE.

THOSE CHANNELS FLOOD.

THEY CUT THE LOGS, PUSH THEM INTO THE CHANNELS, AND FLOAT THEM OUT TO THE DAM, THEN PUSH THEM INTO PLACE.

AND THAT WAY, THEY'RE ACTUALLY MOVING TIMBER THAT'S WAY BIGGER THAN THEY ARE, USING THE WATER TO FLOAT THE LOGS.

THAT'S INGENIOUS, AND IT MAKES A REALLY BIG, STRONG, DURABLE DAM.

KIRK, VOICE-OVER: AND IT'S NOT JUST THE WOODWORK THAT SUPPORTS THE DAM.

THIS IS AMAZING.

LOOK AT THIS ROCK!

THIS ROCK WAS PUT HERE BY A BEAVER!

I MEAN, THESE ANIMALS AREN'T THAT BIG, AND THEY'RE MOVING ROCKS LIKE 5 OR 10 POUNDS, AND THEY'RE PACKING THEM TOGETHER TO MAKE THE FOUNDATION OF THE DAM.

AND THEY PACK IN MUD AND TWIGS AND SILT, AND THEY'VE GOT A STRUCTURE THAT IN THIS CASE IS ABOUT 50 YARDS ACROSS.

BUT THERE ARE PLACES WHERE THESE BEAVER DAMS ARE MORE THAN HALF A MILE LONG.

THESE ARE PRETTY AMAZING RODENTS.

I'LL GIVE THEM THAT.

KIRK, VOICE-OVER: BUT IT LOOKS LIKE MY TIME'S UP.

THE BEAVER HAS JUST POPPED OUT OF HIS LODGE, AND HE'S GETTING READY TO GET AROUND AND START LOOKING FOR PLACES WHERE THE DAM IS BREACHING WHERE HE CAN DO SOME REPAIRS.

I'LL GET OUT OF THE POND NOW AND GIVE IT BACK TO HIM.

KIRK, VOICE-OVER: THIS BEAVER DAM IS A WORK OF ART, BUT STURDY AS IT IS, IT COULD BE OVERWHELMED IF THERE'S A RECORD SPRING RUNOFF THIS YEAR AS EXPECTED.

IF THE DAM BREAKS, THESE BEAVERS WILL LOSE THEIR HOME.

AND IN THE SECOND WEEK OF JUNE, TEMPERATURES GET INTO THE 80s, FAR ABOVE THE AVERAGE FOR THIS TIME OF YEAR.

IT'S ALMOST CERTAIN TO CAUSE MORE RUNOFF, WHICH IS BAD NEWS FOR THE BEAVERS.

THE SNOW RECEDES HALFWAY UP THE MOUNTAINS, REVEALING HIGHER FEEDING GROUNDS.

AND CLOSE BEHIND ARE ALL THE ANIMALS TAKING ADVANTAGE OF THE NEXT STAGE OF THIS YEAR'S SPRING GREEN-UP.

AT 8,000 FEET, THIS VAST WILDERNESS PROVIDES FOOD FOR THOUSANDS OF ANIMALS.

ELK THAT WINTERED DOWN IN THE VALLEYS FINALLY ARRIVE BACK IN THESE HIGH PASTURES, WHERE THEY'LL RAISE THEIR CALVES.

THESE MEADOWS ARE A FEASTING GROUND FULL TO THE BRIM WITH WILDLIFE.

AT THIS TIME OF YEAR, WHEN SO MANY YOUNG ANIMALS ARE AROUND, IT'S A BRIEF BONANZA FOR PREDATORS.

EVEN CHICKS IN THEIR NESTS ARE FAIR GAME FOR AN OPPORTUNISTIC BLACK BEAR.

BUT ALTHOUGH MEAT'S A NICE TREAT, FOR GRIZZLIES, PLANTS STILL MAKE UP THE BULK OF THEIR DIET.

CASEY HAS BEEN FOLLOWING ONE YOUNG BEAR FAMILY FOR THE LAST WEEK.

THESE NEWBORN CUBS ARE ONLY 4 MONTHS OLD.

WITH NOWHERE TO HIDE, EVEN BABY BEARS ARE AT RISK FROM PREDATORS THIS TIME OF YEAR.

BUT THE MOTHER HAS NO CHOICE.

THIS SOW IS FOLLOWING THE MELTING SNOW TO MAKE SURE THAT BOTH SHE AND HER CUBS GET ENOUGH PROTEIN FROM THE NEW GROWTH.

CASEY: THEY'RE STILL SMALL ENOUGH THAT THEY'RE-- WHEN THEY'RE ON ALL FOURS, THEY'RE DOWN IN THE SAGEBRUSH.

YOU SEE THEM STAND UP, POP THEIR LITTLE HEAD UP, THEIR LITTLE NOSE UP, AND CHECK OUT ALL THE-- THE ANIMALS, ALL THE CHARACTERS THAT THEY'RE MEETING OUT HERE IN THIS ECOSYSTEM.

KIRK, VOICE-OVER: AT THIS TIME OF YEAR, THE CUBS ARE AT THEIR MOST VULNERABLE.

AND MOM'S DEFINITELY STILL REALLY ON GUARD.

SAFETY IS A BIG PART OF THAT.

SHE'S SCANNING THE HORIZON ALL THE TIME.

THERE ARE STILL A LOT OF DANGERS.

THESE CUBS ARE QUITE SMALL.

PACKS OF WOLVES-- THEY COULD COME IN AND, YOU KNOW, KIND OF CREATE A PANIC AMONGST THESE-- THIS FAMILY GROUP AND MAYBE GET ONE OF THE CUBS OFF TO THE SIDE.

YOU KNOW, AN ADULT MALE GRIZZLY BEAR COULD STILL COME IN AND TAKE THESE CUBS OUT.

IN FACT, ABOUT 50% OF LITTLE BEAR CUBS IN THEIR FIRST YEAR WON'T MAKE IT PAST THEIR FIRST YEAR.

MOM--SHE'S JUST CONSTANTLY EITHER EATING AND DIGGING OR LOOKING AROUND FOR DANGER, AND THAT'S WHAT A MOTHER GRIZZLY BEAR DOES BEST IS JUST PROTECTS THOSE LITTLE GUYS.

SHE'S OBVIOUSLY WILLING TO DEFEND THEM.

THAT'S WHAT THAT BIG SCAR IS FROM ON HER FOREHEAD.

KIRK, VOICE-OVER: THE REASON MALES KILL CUBS IS THAT WHEN A FEMALE BEAR LOSES HER YOUNG, SHE'LL COME INTO SEASON, ALLOWING THE MALE TO MATE AND PASS ON HIS OWN GENES.

SO MALE GRIZZLIES ARE ALWAYS ON THE PROWL.

CASEY: OUT HERE, THERE'S SO MUCH FOOD.

THIS IS A GREAT SPOT.

THESE LITTLE CUBS HAVE A GREAT CHANCE OF MAKING IT PAST THEIR FIRST YEAR.

WHEN IT COMES TO GRIZZLY BEAR MOMS, THERE MAY NOT BE A BETTER MOM THAN THIS ONE RIGHT HERE.

KIRK, VOICE-OVER: IT'S BECAUSE OF EXPERIENCED MOTHERS LIKE THIS ONE THAT YELLOWSTONE'S GRIZZLY BEARS ARE DOING WELL.

WITH TEMPERATURES NEARLY IN THE 90s, THE SNOW LINE HAS FINALLY RETREATED TO MOUNTAIN PEAKS.

THE SNOWPACK IS COLLAPSING.

THE GREAT YELLOWSTONE THAW HAS REACHED ITS PEAK.

AND MELTWATER CASCADES THROUGH THE CANYONS... BULLDOZING EVERYTHING IN ITS PATH AND CRASHING ON INTO THE VALLEYS.

THIS MELTWATER IS CRITICAL TO THE SURVIVAL OF MANY ANIMALS...

BUT IT ALSO BRINGS DANGER.

DESPITE THE FLOODWATER, YELLOWSTONE'S RESIDENTS CONTINUE THEIR DAILY STRUGGLE TO FIND FOOD.

EVERY YEAR, ANIMALS ARE SWEPT AWAY BY THE FORCE OF THE WATER.

AND OUR CAMERAS ARE FOLLOWING A HERD OF BISON WHO ARE ATTEMPTING TO CROSS A RIVER.

THEY ARRIVE TO FIND A TWO-MONTH-OLD BISON CALF ALREADY STRUGGLING.

[CALF MOOING] IT'S BEEN SWEPT AWAY FROM ITS MOTHER.

WITHOUT HER BULK TO PROTECT IT, THE CALF IS POUNDED BY THE FULL FORCE OF THE CURRENT.

[BISON MOOING] THE OTHER FEMALES HAVE THEIR OWN YOUNG TO PROTECT.

IT'S LOSING ENERGY FAST.

[BISON GRUNTING] AFTER AN EXHAUSTING BATTLE, THE CALF FINALLY MAKES IT SAFELY TO THE NEW PASTURES ON THE OTHER SIDE.

THIS ONE IS LUCKY.

AND FURTHER SOUTH ON THE SNAKE RIVER, THERE'S SO MUCH FLOODWATER THAT ONE OF THE BEAVER DAMS HAS BEEN BREACHED.

WITH DAMS BEING WASHED OUT FURTHER DOWNSTREAM, WHAT'S HAPPENED TO THE BEAVERS WE'VE BEEN FOLLOWING?

WATER IS FLOWING RIGHT OVER THE TOP OF THEIR DAM.

IT'S STILL HOLDING FOR NOW, BUT AS SOON AS THE WATER LEVEL DROPS, THEY'LL NEED TO FIX IT.

HIGH WATER LEVELS DO PROVIDE ONE ADVANTAGE: IT GIVES THEM A CHANCE TO DO SOME LAUNDRY.

INSIDE THE LODGE, ALL THE OLD BEDDING IS PUSHED OUT INTO THE WATER.

THEY LEAVE IT TO SOAK FOR A WHILE, AND WHEN IT'S CLEAN, THEY DRAG IT BACK IN...

GIVING THEM FRESH, NEW BEDDING FOR THE SUMMER.

BUT SUMMER ISN'T GOOD NEWS FOR EVERYBODY.

WITH TEMPERATURES NOW REACHING THE HIGH 80s, THE OWL CHICKS ARE IN SERIOUS DANGER.

ONE HAS LEFT THE NEST, BUT THE OTHER TWO ARE IN THE FULL FORCE OF THE SUN.

THESE OWLS ARE PERFECTLY ADAPTED TO THE COLD BUT STRUGGLE IN THE HEAT.

THEIR ONLY OPTION IS TO LEAVE AND LEAVE QUICKLY.

JEFF: THE SECOND CHICK IS GETTING UP ON THE LIP ON THE HIGHEST POINT OF THE NEST AND JUST FLAPPING LIKE CRAZY.

HE'S LOOKING OVER THE EDGE.

AND THIS IS WHAT THEY DO BEFORE THEY JUMP.

KIRK, VOICE-OVER: BUT THE CHICKS ARE VULNERABLE.

THEIR WINGS AREN'T STRONG ENOUGH TO FLY.

WHEN THEY JUMP, THEY'LL PLUMMET 30 FEET TO THE FOREST FLOOR.

[WHISPERS] Oh, he's gonna go.

He's gonna go.

Jump.

Jump.

Go, go, go, go.

Oh.

He's leaning way over the other edge.

Could go any second.

KIRK, VOICE-OVER: IT'S THE RISKIEST MOMENT OF THIS OWL'S LIFE, BUT IT'S NOT UNTIL THEY JUMP THAT OWLS START LEARNING TO FLY.

IT'S NOT THE MOST GRACEFUL MAIDEN FLIGHT.

JEFF: He's hanging upside-down.

Got all messed up.

OH, HE IS STRUGGLING.

HE'S HANGING BY ONE TALON.

THERE HE GOES!

IT'S ONE THING TO LAUNCH.

IT'S ANOTHER THING TO LAND.

KIRK, VOICE-OVER: THE CHICK LANDS SAFELY...

BUT SHE MUST HEAD STRAIGHT BACK UP INTO THE CANOPY, AWAY FROM THE PREDATORS ON THE FOREST FLOOR.

AND IT'S MADE ALL THE MORE DIFFICULT BY THE UNUSUALLY HOT TEMPERATURES AT THIS POINT IN HER DEVELOPMENT.

IF YELLOWSTONE'S WEATHER CONTINUES TO GET WARMER EVERY YEAR, THESE OWLS MIGHT NOT SURVIVE HERE IN THE FUTURE.

THE RUNT IS IN DANGER.

SHE DOESN'T KNOW WHAT TO DO OR WHERE TO GO.

KIRK, VOICE-OVER: SHE'S BEING STRUCK DIRECTLY BY THE INTENSE HEAT OF THE SUN.

PANTING HELPS HER TO COOL DOWN...

BUT HER DOWNY FEATHERS, WHICH ARE SO EFFICIENT IN COLD WEATHER, MAY PROVE FATAL.

JEFF IS WORRIED.

[OWL CHIRPING] OUR LITTLE GIRL IS UP AGAIN, FINALLY, BUT SHE'S HOT, TOO.

SHE--SHE WANTS OUT OF THAT HEAT.

I CAN TELL.

KIRK, VOICE-OVER: THIS IS A LEAP OF FAITH SHE'S GOT TO MAKE.

SHE'S BACK ON THE LAUNCH PAD.

THIS IS WHERE I THINK SHE'S GONNA GO.

SHE'S-- SHE LAUNCHED!

SHE LAUNCHED!

[CHIRP] SO THIS FOREST IS NOW HOME TO 3 MORE GREAT GRAY OWLS.

I'M IMPRESSED.

I'M PROUD OF THIS LITTLE GIRL.

YOU KNOW, MAYBE ONE DAY, SHE'S GONNA JUST COME BACK HERE AND RAISE A GREAT GRAY OWL FAMILY OF HER OWN.

KIRK, VOICE-OVER: AS THE LONGEST DAY OF THE YEAR APPROACHES, SPRING BECOMES SUMMER.

THE ANIMALS WE'VE FOLLOWED HAVE MADE IT THROUGH THIS TOUGH SEASON.

THE THAW CAME 3 WEEKS EARLY, ACCELERATING THE GREEN-UP... AND INCREASING THE RUNOFF.

AND FINALLY, FROM TOP TO BOTTOM... YELLOWSTONE IS VIRTUALLY FREE OF SNOW.

ONE THING'S FOR SURE: THE WEATHER IN YELLOWSTONE IS UNPREDICTABLE.

I MEAN, LOOK AT THIS.

IT'S A FEW DAYS BEFORE THE SUMMER SOLSTICE, AND IT'S SPITTING A COLD RAIN.

[THUNDER] BUT ONE THING I'VE LEARNED IS THE WILDLIFE IN YELLOWSTONE CAN TAKE WHAT THE WEATHER THROWS AT IT.

BUT THE NEXT MONTH IS CRITICAL FOR THE WILDLIFE IN THE GREATER YELLOWSTONE ECOSYSTEM.

IF IT STAYS LIKE THIS, IT'LL BE GREEN FOR ANOTHER MONTH OR SO, AND THAT'LL BE GREAT FOR THE ANIMALS.

BUT IF IT GETS HOT, IT'LL GET DRY, AND THAT MEANS ONE THING: FOREST FIRE.

JOIN US NEXT TIME FOR THE FINAL CHAPTER OF THE "GREAT YELLOWSTONE THAW."

JOHNSON: JOIN US NEXT TIME FOR THE FINAL CHAPTER OF "THE GREAT YELLOWSTONE THAW," WHEN SOARING TEMPERATURES FORCE THE ANIMALS OF YELLOWSTONE TO CHANGE THEIR BEHAVIOR.

TWO ORPHAN GRIZZLY BEARS STRUGGLE TO FIND ENOUGH FOOD.

THE WOLVES DISPLAY UNUSUAL HUNTING STRATEGIES TO KEEP THEIR 11 PUPS ALIVE, AND THE BEAVERS FLEE FROM THEIR LODGE IN A DRAMATIC TWIST IN THIS EVER-CHANGING ENVIRONMENT.