Thursday, October 21, 2010

Lake Bonneville and the Great Salt Lake


A Google Earth Map of Lake Bonneville is Available
Since the advent of computers I must admit that one of my former favorite hobbies (reading books) has gotten the short end of the stick. I mean, once Wozniak came of age I began finding myself more and more often startled by the 2 am hour while sitting at the keyboard than in the recliner with books like "Hawaii," "Space," "Alaska" and "Mexico."

Ahhh… Michner…   now there was an obsessive/compulsive after my own heart.  When he would write a book about an area he would begin with one asteroid hitting another and forming, what else… the earth.   Then, he would touch on nearly everything in-between like a part of that collision eventually making its way to the gaping maw of a super-giant black hole and succumbing to its gravity; but, not before giving up a tiny gamma ray particle, blasting back in the direction it had come at the moment it was being torn asunder.   Me… I just want to tell you about a female trout in labor far up an isolated arm of a world which seemed to consist mostly of water.

With a final shutter of her tail she gave birth to an egg cluster, circled and with a swish of her tail buried it under a cover of gravel; then, for only moments, gazed lovingly at her handiwork before resuming her long journey, never to see her offspring. Only perhaps one in ten would ever live to see adulthood without her.

Mere weeks later, gentle stirrings began within the cluster; then, almost simultaneously, hatchlings began to emerge. One, with a coloration not here-to-fore seen on this world stood out from the rest – a mutant (remember the gamma ray); but, everyone knew he was destined for greatness the moment they saw him… we'll call him Harold.

Like many of his clutch-mates, Harold was almost brass colored – or silver in some light. Like a few less, he developed round spots evenly distributed over his body; BUT, unlike any fish, anywhere, he had red slash marks just below his gill covers on his lower jaw and teeth at the base of his tongue. A difference which would be both the bane and blessing of his existence.

He found that could eat a much greater variety of food than his cluster-mates, so almost never went hungry; but then also, the "eaters," as they called the fish with the large mouths who would swallow an un-watchful minnow whole, could spot Harold from great distances, especially if he was moving. So, he grew quick and agile in order to remain alive.

It wasn't until Harold became older that he discovered just how having that red "cut" on his throat could be helpful… The Ladies Loved It! He could pretty much have his pick of mates and would quickly have many strong offspring who would call him "daddy" – all sharing his red bandolero.

Over the years, with their new advantages, Harold's progeny grew in number and all had his spots and color even though some didn't have his extra teeth and appetite. They grew to inhabit nearly every part of this water-world — known in a much later time as: Lake Bonneville.

Now, if I were as skilled as Michner (and had a couple thousand pages), I would be able to weave several other characters into my pages, perhaps dinosaurs and woolly mammoths and eventually humans. You could discover that Bonneville was a late Pleistocene-aged Lake covering Utah, eastern Nevada, and southern Idaho.

He would angle the story a tiny bit to perhaps include "Earl" (the eel) and "Merl" (the mammoth) in order to allow explanation that massive lifting movements of the earth had brought a continent above the water line and created 'berms' enough to alter water flow – inward. You could also learn that the brutal "glacialpluvial" climate (about 32,000 - to 12,000 years before present) had led to the filling of this basin for about 17,000 years; so, that it was now lapping at its shoreline, 5090 feet above mean sea level, as Harold peacefully cruised along rising to emerging insects!

Michner, had he been telling you the story, would have you breathless at the sheer enormity of this water-body once it had reached its outlet at Zenda Falls. It was almost 20,000 square MILES at its surface; and it was close to a full MILE deep (above mean seal level) even though now that it was full it was still putting nearly 2,000 cubic feet of water over the falls – every SECOND!

By now, we would be so involved in the character's lives that we could barely stand the tension (after all we secretly know that Lake Bonneville no longer exists) and then he would begin to tease us.  At 15,000 years before present (BP) the earth shook as breathtakingly-massive amounts of water broke through a crack in the Zenda spillway, and Harold's great-(x 1010)grandchildren's family swam for their very lives while the lake dropped 40 feet in mere hours!  [We would be so relieved by the safety of Harold's family that we probably wouldn't even notice when Michner slipped in that this was the "Early Zenda Incision."]

The water-world had changed and so had the lives of the Cutthroat. Gone were the days of blissful simplicity. And they could never let themselves become so complacent, so un-watchful, again. Constantly moving up-stream, fighting the currents no matter what the effort, would be their natural instinct.

For generations afterward the Cutthroats would pass the memory of the catastrophe throughout the clan; until, it was coded in their very genes. All through the "Keg Mountain Oscillation" (the 250 years where the climate dropped the lake below the spillway to under 4950 feet and the 200 years when it rose back up again) the red-throated "cuts" migrated upstream. They loved to be near their "homeland" of deep water, but especially took their families to the safety of the far-reaching tributaries as far as they could go.

Michner might then switch the story to that of the inhabitants around the area of Red Rock Pass in Idaho and describe the glaciers receding; or, of those along the Snake River or in the Badlands – all portends of the cataclysm to come.

In any case, the Cutthroat's world would now have an insidious stability for about 50 years while open-water conditions were attained again at Zenda, and lake level was maintained at the 5050 ft level. No-one but us, who have taken 7th grade Utah history, even notice the deceptively stealthy groundwater seepage through Zenda's alluvium, gradually leading to the discovery that piping was occurring.

It takes courage to know when you ought to be afraid.”
James A. Michener
Early in the morning of 12,490 BC (14,500 years BP) Connor, Harold's very, very distant great grandson, awoke with a start and an uneasy feeling deep in his buoyancy bladder which wouldn't go away, even with a large stonefly for breakfast.   And even though it wasn't even close to the season, he just "knew" that it was time to… migrate!   "Yes," Connor told himself, he had to be on the move.   "And it has to be now!"   The more he gathered his school the more urgent the feeling became until he was nearly slamming into them to urge their haste.

No sooner had they rounded the point of their arm of the lake, low grumblings reached them, more of a sensation at first, then… an explosion of noise and vibration and cataclysm: the real Lake Bonneville Flood.

What had been a tiny trickle of water at the base of Zenda, exploded into a raging torrent which literally missiled tons of the rock spillway out over the countryside, followed immediately by more water, under more pressure, than even Michner could have us imagine.

Eleven hundred and thirty cubic MILES of water went thundering over Red Rock Pass, over Marsh Creek, over the Portneuf River, Snake River and Columbia River and into the Pacific Ocean! The flood's speed ebbed and flowed according to the width of the passage it was carving. A MILLION cubic meters per SECOND through the Red Rock Pass; 935,000 m3/s in the Snake River Canyon and 600,000 m3/s at Hells Canyon. [A comparison for Niagara Falls is the "measly" 2,800 m3/s – Bonneville was 357 times greater than Niagara!]

The characters, which Michner had made us so fond of, barely saved their lives by climbing up above the 410 foot level of the canyon wall in the Portneuf Narrows. Connor and his school narrowly escaped the torrent of exiting water many times. It was only their fierce instinct for fighting against oncoming current which kept them from the fate of their less vigilant and genetically capable cousins now strewn across the land, their bodies baking in the sun.

There are no insoluble problems. Only time-consuming ones.”
James A. Michener
Life in the lake was once again more than decimated, and constant vigilance was demanded for nearly a year as the lake's contents hurtled out over the badlands carrying with it Volkswagen sized rocks and depositing them, shaped by concussive slams into each other, like so many "melons" in a farmers garden.

When, finally, the roaring stopped it was at a new dam, 340 feet lower on Paleozoic Limestone bedrock; at a new location, 2 miles back at Red Rock Pass; and at a new lake: Lake Provo!

Provo was similar in shape to its father, Bonneville, although smaller in size. To Connor, not much had changed with his water world; except, that he no longer liked the open water and much more preferred to swim up the narrow arms of water where the gravel was clean and water wasn't so… so… noisy!

To Merl, however, everything had changed!   For the next 300 years, until 14,200 BP, even the mere "memory" of the event seemed to send landfalls into the Red Rock spillway.   At least three times the water in the lake rose, temporarily, about 30 feet (4710-4740 feet above msl) until the interloping earth could be eroded away.

Who could tell whether it was the weather which had changed because of the lake, or the lake because of the weather.   Whichever it was probably didn't matter – it was getting a lot warmer and dryer.   And, the flow out of Lake Provo ceased entirely when its level dropped below the spillway.

It only took about 2,000 years (between 14 200 and 12 000 bp) for the Bonneville lake-cycle to end as shorelines regressed to about 4200 ft, nearly the same elevation as the modern-day Great Salt Lake.

For the last 12,000 years until now, the inhabitants of the area may be heard to grumble about the changing lake level, never establishing a shoreline for very long, but the truth is that it rises in isostatic adjustment to the loss of lake water.   The Great Salt Lake varies in size slightly in response to climate variations; but — these are nothing, compared with those of the Pleistocene.

Ahhh… now there was a lake!


Learn A Little More

[Be sure and look at the Google Earth map of Lake Bonneville which includes the flood, scablands and lake Provo.]

Lake Bonneville covered roughly ten times the area of Great Salt Lake, roughly an area of more than 19,691 square miles, and was over 1,000 feet (305 m) deep. It covered much of present-day Utah and small portions of Idaho and Nevada during the Pleistocene Epoch, more commonly known as the Great Ice Age, between 32,000 and 14,000 years ago.

The lake was probably not a singular entity either; geologic evidence suggests that it may have evaporated and reformed as many as 28 times in the last 3 million years. It existed until about 16,800 years ago, when a large portion of the lake was released through the Red Rock Pass in Idaho and continued westward across the Snake River Plain generally following the path of the present Snake River. The enormous flood was first named in 1965 by the geologist G. K. Gilbert after Benjamin Louis Eulalie de Bonneville (1796–1878), a French-born officer in the United States Army, who was also a fur trapper, and explorer in the American West. Bonneville was noted for his expeditions to the Oregon Country and the Great Basin. Harold Malde (1968) of the U.S. Geological Survey published the first detailed account of the effects of the flood on the Snake River Plain.

Release of Flood Near Red Rock Pass The flood left a ground record of its effects in the Snake River Plain by a variety of depositional and erosional features. At Portneuf Narrows, a canyon 45 miles northwest of Red Rock Pass, the flood is estimated to have reached a height of 400 feet.

Malde estimated that the probable peak discharge of the flood was approximately one-third cubic miles per hour (15 million cubic feet per second). The total flood volume is believed to be about 380 cubic miles.

Malde suggested that most of the important depositional and erosional features of the Bonneville Flood were developed in a few days; however, the Snake River sustained a high rate of flow for more than a year.

Large rounded boulders of basalt characterize many deposits left by the flood along the Snake River Plain. H. A. Powers, recognized that these boulders were of catastrophic origin, and Malde applied the name of Melon Gravel to the boulder deposits (Malde and Powers, 1962). They were inspired to use this term after observing a road sign in 1955 that called the boulders "petrified watermelons."

The Melon Gravels average three feet in diameter, but some well-rounded boulders range up to 10 feet in diameter. Only several miles of transportation by the flood was sufficient to round the boulders after which they were dumped in unsorted deposits up to 300 feet thick. Melon gravel bars are as much as one mile wide by 1.5 miles long.

With the change in climate, the remaining lake began drying up, leaving Great Salt Lake, Utah Lake, Sevier Lake, Rush Lake, and Little Salt Lake as remnants. The shorelines of Lake Bonneville can be seen on the higher slopes of the Wasatch Mountains, more than 984 feet above the present level of the Great Salt Lake as well as on Antelope Island.

The appearance of the shorelines is that of a shelf or bench protruding from the mountainside, well above the valley floor. Four main shorelines are associated with the fluctuating levels of the ancient lake.

The Stansbury, Bonneville, Provo and Gilbert shorelines each mark a time when lake level remained constant long enough to deposit massive accumulations of sand and gravel.

The Bonneville Bench, at approximately 1,555 m (~5,100 ft.) elevation, is part of the preserved ice age shoreline. This shoreline marks the highest level attained by the Pleistocene lake approximately 15,500 years ago. During this period, the lake covered over 52,000 km² (20,000 square miles ) and was over 300 m (1000 ft) deep in places.

The lake level fell some 105 m (~350 ft.) to what is now the next lower bench (the "Provo level") in a flood that geologists estimate to have lasted up to a year.

The Provo level is the most easily recognized shoreline feature throughout the Bonneville basin and is distinguished by thick accumulations of tufa that formed near the shorelines during the 500 years that the lake was at this level.

About 14,000 years ago, the lake started to drop again due to changing climate conditions, and by 12,000 years ago, the lake reached a level even lower than that of the modern day Great Salt Lake.

A slight transgression or rise in lake level occurred about 10,900 to 10,300 years ago and formed the Gilbert shoreline. The Gilbert shoreline is the least conspicuous of the major shorelines but evidence of it remains at Antelope Island and in large coastal features, such as the Fingerpoint Spit near the Hogup Mountains.

In addition to geological traces, the lake has left a legacy of related fish distributed in now-isolated bodies of water. The term "Lake Bonneville drainage" is often used to refer to the assembly of disconnected lakes and rivers; since the draining happened relatively recently, most of the fishes have not had time to evolve into distinct species.

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