Glacial Lake Missoula

[Music] foreign [Music] [Music] this is Missoula and the campus of the University of Montana a terrific setting in the Rocky Mountains and Ground Zero from much of the water for the Ice Age floods of the Pacific Northwest let's tell the story in a nutshell and then explore old shorelines high energy gravel deposits and delicate silk beds that all tell the incredible story of glacial Lake Missoula during the Ice Age The Valleys of Western Montana were filled with one thousand feet of fresh water glacial Lake Missoula formed due to an ice dam in northern Idaho the Purcell trench lobe that blocked the Clark Fork River and its tributaries across the border in Montana the ice dam area which we know today as Lake Ponderay was two thousand feet high 30 miles long and sealed off a mountain valley creating a backup of Lake water 200 miles to the east like filling a bathtub with the drain plugged a massive lake with long fjord-like arms a southern arm that sat in the Bitterroot Valley to Hamilton below Trapper Peak an Eastern arm to Drummond a northern arm into the Mission Valley and the mission range as the Water deepened behind the dam the pressure built against the ice sheet eventually the ice was no match for the massive volume of water in the lake the dam failed quickly the lake drained quickly just a few days to drain and rush over the floors of the Clark Fork River and Flathead River Valley the water barreled over Eastern Washington leaving deep Cuts in the desert and moving tons of rock from the Rocky Mountains into Washington and Oregon and that was one Missoula flood but it happened again at least twice probably dozens of times possibly as many as 100 times the Purcell ice dam reformed another glacial Lake Missoula and a new Ice Age flood burst through Idaho when the lake reached a critical depth rinse and repeat the floods took different routes based on their size and local conditions in the channeled scablands of Eastern Washington thick deposits of lus wind-blown silt were Swept Away a surprising amount of Basalt Bedrock was removed by the Missoula floods leaving impressive box-shaped Canyons like the Grand Coulee with Dry Falls fields of giant current ripples huge potholes drilled into the Bedrock my God how big were these floods regardless of size each flood put on its brakes at wallula Gap as the water funneled through the narrow gateway to the Columbia River Gorge that was Lake Lewis in southern Washington a brief delay before the now dirty brown Water continued on through the Columbia River Gorge and onto the Pacific Ocean okay that's the story it's almost impossible to believe right what can we find in Western Montana to prove that glacial Lake Missoula really existed let's start with the obvious ancient shorelines strand lines benches on the hillsides created by wind blowing across the surface of the old Lake tells us the water was a thousand feet deep here but there's not just one old Shoreline there are dozens of them different lake levels for the Montana valleys that had Glacier Lake Missoula in them the old shorelines are best seen on Northwest facing slopes like the hillside above the University of Montana the big m above campus 620 feet above the town of Missoula this is only two-thirds of the Way to the Top the high water mark the highest strand line is more than 300 feet above us hiking up this slope you might expect real obvious notches benches Cuts dug into this slope but they're subtle these old shorelines are more obvious from a distance then hiking right on top of them TC Chamberlain was the first geologist to note these faint watermarks in 1886. he had read reports describing Scotland's parallel roads of glenroy and correctly interpreted the elevated shorelines here in Montana each glacial Lake Missoula strand line was created by Lake waves eroding into the hillsides but shorelines are also places of deposition Beach gravels have been found Little Beach berms so far no preserved organic carbon or other dateable materials have been found at the old shorelines so telling a decent story here is difficult Missoula the big m above campus The Strand lines on the hillside even though we don't have specific dates most geologists agree that the highest trend line is the oldest Lake that the Strand lines get younger as you go down the hill the thinking is if there was a young Lake up here and then you drain the lake wouldn't you wipe out all these older strand lines that's the thinking older highest younger lowest yes that's the thinking but without dateable material even the most basic questions remain is each level a different glacial Lake Missoula or is this one lake with periodic lowering or a combination of the two somehow without dates for each Shoreline it's still unclear the highest strand line is at 4 200 feet elevation on a steep hill slope exposed to more than ten thousand years of thunderstorms it's pretty amazing how little eroded these strand lines are at its maximum glacial Lake Missoula had a surface area of 3 000 square miles the northern shorelines of the lake sometimes had an ice margin ice calved off into the lake icebergs with big rocks in them that set sail for various destinations in the lake large Boulders show where the big rocks fell off their ice rafts drop stones back when the water of glacial Lake Missoula was relatively calm and quiet up north impressive White Lake beds were laid down close to the ice margin Rock flower silts created from the grinding power of the ice sheet to the north wildlife and Dusty white deposits everywhere the drain was plugged the lake was big and the white silts collected on the quiet Lake floor but there are also deposits that speak of tremendous high energy events mud is usually at the bottom of lakes right swim in your favorite Lake that means dark mud is oozing up between your toes but at the bottom of much of glacial Lake Missoula deposits of rocks not mud dominates on the valley floors below the Strand lines why I bet you know why right high energy water is recorded in these Valley of bottoms that's what the rocks are telling us but when Glacier Lake Missoula was here it wasn't high energy it was low energy the water's just sitting there and layers of mud and silt are being deposited at the bottom but when we break the ice dam that water starts moving quickly fast enough to erode all those soft beds at the bottom and in their place a big batch of river gravels were brought in from elsewhere and sit at the bottom deposited during the high velocity flooding rocks the size of my fist or my head are even bigger so when we look down the guts of the Clark Fork River Valley it's high energy River gravels in the bottom instead of the mud okay make another ice dam make another Lake lay down more silts and muds that's fine but when we break that ice down the water's on the move and we erase those and bring in more River gravels all told we have more than 300 feet of big flood deposited gravels at the bottom of the Clark Fork River Valley how many floods does this represent under the tranquil scene of trees and flowers the marbles from high energy floods the fastest water probably peaked in the first few hours during the ice dam collapse the high energy gravels are piled thick in places where the water slowed right after being shot through narrow Valley bottlenecks a giant flood bar at Tarkio hundreds of feet high and more than a mile long is composed of fist-sized rocks in other places water speeds were fast enough to pluck car-sized boulders from very hard bedrock not enough to convince you still not sure that glacial Lake Missoula drained in a hurry well how about these giant current ripples formed on the lake floor as the lake drained quickly at Camas Prairie individual ripples are 35 feet high and spaced 100 feet apart cobbles and Pebble gravel shaped into these impressive forms that developed under more than 200 feet of water moving up to 60 miles per hour Joseph Pardee was the first to study these more than 75 years ago four sets of ripples sit below four separate spillways above Camas Prairie Western Montana's Bedrock is different than Washington's flood scoured Basalt glacial Lake Missoula sat in sedimentary Bedrock created more than one billion years ago it's pretty easy to visualize where the lake rushed out of Montana today's Clark Fork River Flows in the direction that the Missoula floods flowed rugged vertical walled Canyons like Eddie Narrows were particularly energetic spots for the flood water pockets of flood gravels remain stranded high and dry inside Canyons Gulch fills help show the depth and the speed of the water as it ran the gauntlet Downstream to Idaho Washington and Beyond the deepest glacial Lake Missoula was just a few hundred feet shy of spilling over the bitter at mountains at Lookout Pass where Interstate 90 crosses the Idaho Montana state line instead the lake drained through the bitterroots using existing river valleys to the north and once into Idaho the flood swung to the Southwest over Spokane and the broad openness of Eastern Washington so much field evidence for glacial Lake Missoula is visible from I-90 between Saint Regis and Missoula at the exit for Nine Mile Road one more very important study site these are famous silty beds west of Missoula partly because we're still debating the significance of them these are rhythmites there's 40 of them here with that zebra striping what's the story why are these delicate silts still here if this is a place where high energy flood water was cruising through these repetitive layers of silt and mud contain details with important Clues but debate continues on what these layers are telling us about glacial Lake missoula's history even the terms are confusing rhythmites varvs are they the same thing not here at Nine Mile Interstate 90 from the freeway you can see the rhythmites dark light dark light from the freeway those are the zebra stripes but within one dark zebra stripe varvs at a tinier scale dark light dark light those are annual patterns dark light couplet that's one winter summer pattern varvs rhythmites many geologists see the more than 500 varve couplets here as annual layers like counting tree rings in the mud but not everybody agrees that these tiny layers are annual why not seasonal storms they say or occasional debris flows into the bottom of the lake but the tiny layers are so clean some people say not a root not a leaf not a twig not a gopher hole no tracks no cut and fill gullies very little organic carbon at least everybody can agree the dark zebra strikes the dark rhythmites were deposited at the bottom of glacial Lake Missoula the dark bands are mud the light bands are silt and the rhythmites get thinner and thinner toward the top but that's it for agreement what do the light-colored silty rhythmites really tell us do they record lake drainings or lake fillings The Nine Mile rhythmites sit on top of the high energy gravels that we talked about earlier it's looking like the coarse gravels not these delicate rhythmites are the record for the truly huge floods the big drainings of glacial Lake Missoula but how many big floods it's pretty tough to tease out individual huge floods from a big pile of marbles with so much water speed it seems unlikely that these soft beds would survive especially since they sit in the area's most deeply scoured Canyon stretches are these beds at Nine Mile from the last and smallest glacial Lake Missoula almost an afterthought in the Ice Age flood story a progressively smaller Lake Missoula toward the end of the ice age is consistent with these rhythmites that progressively thin up section and have decreasing numbers of varvs per zebra stripe as you head up the slope and that agrees with The Strand lines getting lower and lower with time above Missoula each successive thinning ice dam existed for Less years resulting in a lower ancient Shoreline flood magnitudes must have decreased through time but did each Dam collapse completely with each big flood or was there a slower release of water that somehow tunneled through the ice sheet did each glacial Lake Missoula drain completely our partial Lake drains even possible it's tempting to correlate the rhythmites of glacial Lake Missoula with Northern Washington's glacial Lake Columbia and Southern Washington's Lake Lewis are these the same beds is each rhythmite from a major Missoula flood from Montana was each flood from Montana are there other potential sources of water to the north bed for bed correlation is almost impossible due to differences in the character of the sediments the varved muds at the bottom of glacial Lake Columbia show many many years of Lake water due to the Columbia River being blocked by the Okanagan ice sheet a different plug a different bathtub and at the bottom of Lake Lewis in southern Washington novarves at all the lake down there lasted just a few days at a time that bathtub had an open drain wallula Gap emerging dates do seem to suggest that some of the huge floods struck earlier in the Ice Age we just don't have enough dates to tell a more complete story not yet anyway answers will come from the next generation of field geologists new dates are trickling in surface exposure dating techniques are being used now on Basalt bedrock and Coulee walls and on the surfaces of erratics sitting in Washington's channeled scablands as more dates emerge from across the Ice Age flood's Country some of the Mysteries that remain will be solved about glacial Lake Missoula ideally with new techniques used by Future geologists new dates will come from the floor of the old Lake and maybe even from the Strand lines up high above Missoula glacial Lake Missoula Where it All Began and where Unsolved Mysteries still remain [Music]

Channel: Nick Zentner

Views: 290,250

Keywords: Nick Zentner, Tom Foster, Glacial Lake Missoula, Missoula Floods, Camas Prairie, Channeled Scablands

Id: nBfi0Zle2HI

Length: 18min 52sec (1132 seconds)

Published: Wed Apr 05 2023