The temperature and the surface area of a closed-basin lake primarily control the amount of water evaporated from the lake. When precipitation is high, more water is added to the lake by direct precipitation on the lake and from rivers and streams flowing into the lake than is evaporated from the lake; the result is that the lake rises and expands across a larger area of the basin.
The surface area of the lake continues to increase until the amount of water evaporated equals the total amount of water entering the lake.
During the last 10, years the level of Great Salt Lake has gone through many cycles but the lake has not risen more than about twenty feet higher than its average historic elevation of 4, feet above sea level. When the climate of the region becomes dramatically cooler and wetter, such as during ice ages, the lake in the Great Salt Lake basin rises to much higher levels. One such rise occurred about , years ago when the lake in the basin rose to an elevation about feet above the current level of Great Salt Lake, and again about 65, years ago when the lake rose about half that high.
The highest and most recent high lake cycle began about 25, years ago and produced Lake Bonneville, a huge lake over 1, feet deep that extended over most of northwestern Utah and into Nevada and Idaho. Explorers as early as Captain J. Fremont in recognized shoreline evidence that a succession of deep lakes had once existed in the Great Salt Lake basin. However, G. Gilbert, first with the Wheeler Survey in the s and later with the U. Geological Survey, was the first to study these prehistoric lake features and describe the major features of Lake Bonneville.
He named the lake after Captain Bonneville, an earlier explorer in the region to the north, but one who never visited Great Salt Lake. Approximately 14, years ago, a catastrophic flood took place at the natural dam structure known as Red Rock Pass.
Increasing water levels and seepage at the dam resulted in structural collapse, producing a foot wall of water spread throughout the Portneuf River Valley and into adjacent valleys along the path. Many geological features found in the flood path are the result of this catastrophic event, believed to be the second largest in known geologic history.
This flood is thought to be caused by capture of the Bear River which greatly increased the supply of water to the Bonneville Basin. These flood waters flowed over Red Rock Pass in southeastern Idaho and continued westward across the Snake River Plain generally following the path of the present Snake River. Although this enormous flood was first described in the literature by Gilbert in , Harold Malde of the U. Geological Survey published the first detailed account of the effects of the flood on the Snake River Plain.
The name "Bonneville Flood" first appeared in the literature in Richmond and others, Large rounded boulders of basalt characterize many deposits left by the flood along the Snake River Plain. Powers, who recognized that these boulders were of catastrophic origin, and Malde applied the name of Melon Gravel to the boulder deposits Malde and Powers, They were inspired to use this term after observing a road sign in that called the boulders "petrified watermelons.
At Portneuf Narrows, a canyon 45 miles northwest of Red Rock Pass, the flood is estimated to have reached a height of feet. The release of water from Lake Bonneville was apparently initiated by sudden erosion of unconsolidated material on the northern shoreline near Red Rock Pass. Although Malde originally proposed a flood date of approximately 30, years ago, he has subsequently revised this age to 15, years ago.
Size of the Flood Malde estimated that the probable peak discharge of the flood was approximately one-third cubic miles per hour 15 million cubic feet per second. This is to be compared with a maximum historic discharge in the upper Snake River of 72, cfs at Idaho Falls in June of The total flood volume is believed to be about cubic miles.
The catastrophic flood from glacial Lake Missoula, which swept across northern Idaho, Washington and Oregon, caused far more disturbance than did the Bonneville Flood. When the ice dam failed that contained impounded Lake Missoula, cubic miles of water were suddenly released. Now and then, the lake's water became fresh enough that it stopped laying down minerals.
McGee and Quade see it in the rocks that they've already dated: One layer is sometimes far younger than the layer just beneath it, with no sign of the 1, years in between. If you want a record of those thousand silent years, you have to look for other indicators, such as the shells of critters that lived during that time, or layers of mud that settled into caves.
Air Force. Our military-owned Chevy Suburban bumps down dirt tracks that wind through a desert plain lush with waist-high grass and sagebrush, past a s-era F-4 Phantom fighter-bomber, a row of Army Jeeps perched on concrete platforms, and a concrete building shattered on one end.
Some of this is old hardware of no further use to the military; other objects are high-quality replicas built by a special military team using plywood and other cheap materials to create realistic stage sets for soldiers as they practice firing machine guns or dropping bombs. Cathedral Cave, today's destination, sits several hundred feet above the plain, sunk into the base of a limestone cliff.
It takes 10 minutes of breathless scrambling up a steep slope to reach it. A band of fossils runs across the cliff and intersects the cave -- Mesozoic coral broken off in an ancient storm, strewn on the seafloor and frozen for eternity in three inches of petrified mud. Cathedral Cave sat as far as feet below the surface of Lake Bonneville. The story of this year's expedition begins deep in its bowels. Quade sidefoots his way down a dusty slope into the cave. He walks through a vaulted room whose walls are covered in knobby tufa reminiscent of organ pipes in a church, and picks his way to the dark, narrow rear of the cave.
The light of his headlamp falls on a crystalline crust of stone that covers the walls and ceiling. The beige crust has broken in places, revealing a six-inch cross-section. These beige crystals formed all over the cave, but only here in its darkest recesses are they free of the silt or fossil algae that would complicate Quade and McGee's chemical analyses. Quade accidentally stumbled upon these super-clean crystals when he first visited Cathedral Cave in The rocks that sat for 13 years in his sample chest came from this spot.
Alternating layers of calcite and aragonite record 9, years of history when the cave was flooded. These mineral layers are capped with evenly spaced calcite knobs the size of coat buttons. On top of them lies a fine dusting of white crystal aragonite, like an autumn morning frost.
The buttons formed over a period of years as the lake contracted, grew salty, and fell below the cave; the frosting reveals one final hurrah when the lake briefly crested again above the cave for another years before drying out for good. McGee and Quade are analyzing them for any clues that they might hold. But for the moment they are simply beautiful.
The geologists already have samples of these minerals; they've come today to tease apart layers of silt, animal remains, and debris on the cave floor. The group begins digging two pits. Dust that tastes of acrid rat urine billows into the air.
Taking turns, they dig through two feet of dirt; through rocks fallen from the ceiling; more dirt; several inches of hard tufa which are chipped away with rock hammers; and below that, a layer of mud. A debate ensues about whether the mud was laid down when the lake was only a few feet above the cave, or several hundred feet above it.
He hands over a pinch of mud that's creamy to the touch. But the tongue's exquisite sensation, a sort of oral Braille, magnifies its grit, revealing sharp-edged grains of silt too fine for calloused fingers to sense. These grains must have washed in during rainstorms from higher up on the mountain, when the cave was just below the shoreline - or so the geologist's diagnostic palate would indicate. Guleed Ali, a Ph. Pretty soon everyone is sitting, sorting through bits of silt on notebooks and clipboards for other clues that might reveal its age.
Madsen holds up a mud clod containing tiny white shells of crustaceans called ostracods, which give clues about the salinity of the lake. Cathedral and other caves have provided a record of how the region's ecosystems evolved as Bonneville climbed up and down the mountains starting around 30, years ago.
Archaeologists have found thick mats of limber pine and spruce needles laid down by water. They have also found seeds and stems of meadow- and marsh-dwelling currant bushes, cinquefoil and bulrush crammed into packrat burrows and cemented together with crystallized urine.
The caves also reveal when humans arrived, leaving behind charcoal, antelope bones, scraping stones, and in the case of Danger Cave in the Silver Island Range, a veritable biblioteca of dried turds. Archaeologists used to reconstitute them in water to study them: One team of researchers tried to distinguish their culinary contents based on aroma. Each one represents a day in the fecal diary of the people who arrived as Lake Bonneville waned around 14, years ago.
There are short, squat turds stuffed with bat hair that seem to cry out for Metamucil, and high-fiber, narrow-gauge turds made almost entirely of a marsh plant called pickleweed that grew near the water's edge.
The pronghorn antelope that the early human inhabitants ate still roam the area today. But many of the other species preserved in Danger and Cathedral Cave are nowhere to be seen.
The limber pine and spruce climbed several thousand feet up the slopes of the Silver Island Range as they chased the retreating rainfall. Eventually, those islands of cool, wet climate evaporated off the tops of the range's 7,foot peaks, and the trees became extinct around here. The only limber pine and spruce in the region reside in the Pilot Range, the Oquirrh Range, the Wasatch Range and other mountains where peaks above 9, feet still wring enough moisture from the air to sustain the trees through hot summers.
The Great Basin in the time of Bonneville wasn't just a wetter place. The greater amount of water flowing and seeping through its mountains and hills supported what biologists call a more productive ecosystem: It could sustain more tons of plants and animals per acre than today. That means it could probably have sustained more humans, too.
Bonneville's history reflects a global truth. Look at a map of the world and you see that many of the great deserts occupy two latitudinal bands 15 to 35 degrees north and south of the equator. These global desert belts arise from a massive atmospheric conveyor belt called the Hadley cell, which lifts warm, moist air from the equator, and wrings the moisture out as rain or snow over the tropics as the air rises to 40, feet.
The Hadley cell then dumps the dry air back down to the earth's surface further north and south -- creating deserts with cloudless skies. The dry beds of Bonneville sit at the northern edge of this arid zone today.
But they didn't always. The Laurentide Ice Sheet, a slab of ice several thousand feet thick that would cover modern-day Canada and the Great Lakes, may have done this during the last ice age. The Laurentide diverted a northern belt of wet air called the jet stream southward, away from what is now Oregon and Washington -- pointing it at Utah instead. As that fire hose of moist air hit the 10,foot crest of Utah's Wasatch Mountains it dumped snow, forming the glaciers and rivers that fed Lake Bonneville.
Bonneville's heyday may represent the wet extreme of what the Great Basin is capable of experiencing. It is easy to assume that the deserts and arid mountains of today represent the other end of that spectrum -- the dry extreme. But there is reason to think that today's condition is only the middle of that range -- that the West can, and will, become much drier. Satellite studies suggest that rising temperatures have caused the Hadley cells to widen by to miles over the last 30 years.
If this trend continues, the atmosphere's conveyor belts will dump their dry air farther and farther from the equator, shifting the most intense bands of dryness from northern Mexico toward Nevada, Utah and Colorado. Rising temperatures will also reduce precipitation directly by increasing the capacity of air to hold water, says Richard Seager, a climate dynamicist at Lamont-Doherty.
It may sound minor, but in the West, small change carries the day.
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