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The Lake Bonneville Flood and Its Effects on Idaho Landscape

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Dan has been a homeowner for some 40 years and has nearly always done his own repair and improvement tasks. He is a licensed electrician.

Shoshone Falls.  Can you imagine 300 times this much water over these falls?  Say goodbye to the generation plant on the left!

Shoshone Falls. Can you imagine 300 times this much water over these falls? Say goodbye to the generation plant on the left!

Lake Bonneville

Lake Bonneville was an ancient body of water left by the Ice Age where the current "Great" Salt Lake now resides. Although located in the same general area as Salt Lake, it was considerably larger.

The Great Salt Lake in northern Utah occupies about 1700 square miles, while Lake Bonneville was over 19,000 square miles in size. It was nearly as large as Lake Michigan and much deeper at nearly 1,000 feet maximum depth.

Lake Bonneville came into existence around 32,000 years ago, just one of many ice age pluvial lakes in North America. The climate at that time was somewhat cooler and wetter than it is now, and several rivers and streams fed the lake. With no outlet (Lake Bonneville was a terminal lake, just as the Great Salt Lake is today) any water loss was by evaporation and the continual addition of water made possible the large size of the lake.

Some 14,500 years ago geological activity caused the Bear River to change its course and flow into Lake Bonneville, raising the level ever higher. Eventually it began to overflow the alluvial fan at Red Rock Pass in Idaho and as that happened the earth was quickly eroded away and the lake began to drain in a catastrophic flood into what is now the state of Idaho.

Eventually the level of the lake stabilized at a much lower level and as the climate changed, becoming warmer and dryer, Lake Bonneville evaporated. The small remnant left today is known as the Great Salt Lake in Utah.

The Lake Bonneville Flood

When Lake Bonneville began to flow over the alluvial fan at Red Rock Pass that was holding the water back, it was the beginning of a truly catastrophic flood. Any engineer can tell us that when an earthen dam has water running over it that the end is near, and so it was with Lake Bonneville. It doesn't take very much water to wash out a portion of such a natural dam and once the process has begun it accelerates very quickly. Rapid water flow cuts away dirt and even rock very easily and Bonneville had a lot of water to contribute to the river now draining it.

As the water drained through Red Rock Pass it generally followed the Snake River Basin and canyons through the state of Idaho, eventually emptying into the Columbia River in Oregon and Washington and on to the Pacific Ocean. On its journey it changed the state of Idaho in ways that are readily discernible even today, 14,000 years later. The amount of water, even though only a portion of the entire lake, was far beyond what the Snake River had ever seen. At its peak, the flood was producing around 15 million cubic feet per second (cfs); more than any river in the world today carries. The chart below gives some comparison with current river flows and what the Snake River was carrying during the flood.

Modern RiverFlow, in CFS

Snake River

74,000

Mississippi

700,000

Columbia

265,000

Amazon

7,000,000

Bonneville Flood Waters

15,000,000

As can be seen from the chart, the flow rate during the flood was 15 million CFS into a river bed that (before all the dams were built) had a maximum flow rate in 1894 of only 74 thousand CFS. Overall, it is thought that the flood released nearly 400 cubic miles of water from that ancient lake. That's nearly 60 trillion cubic feet!

Even though the flood lasted only a few days at it's maximum rate the result was inevitable. Water spread far beyond the normal river bed in flat lands, scouring topsoil as it went. In narrow, deep canyons the canyon was cut deeper than ever right through solid rock. Huge boulders were tumbled for miles, creating the smoothed "Melon gravel" so common through the area. In areas where eddies happened (perhaps where a side channel was cut through the land and then returned to the main river bed) large deposits of sand, silt and topsoil were deposited.

The map below shows the general flow along the Snake River Basin through southern Idaho, north to Lewiston, Idaho where the river turns west towards the Tri Cities in Washington state. It is here that the Snake River joins the Columbia and the great flood began it's final mad dash to the Pacific.

Path of the Bonneville Flood

Red Rock Pass

Red Rock Pass is a low mountain pass in Eastern Idaho near Downey. It is notable as the point where the Bonneville Flood started.

At the time of the flood this pass was some 300 feet higher, above the level of Lake Bonneville, and was the natural dam holding back the lake waters. As the Bear River began filling the lake it eventually rose above the level of the dam and the resulting water flow cut through the paleozic shale, dolomite and limestone. The pass was eroded to it's present elevation, losing 300 feet in height.

The flood waters cut the pass through the resistant bedrock in a narrow canyon some two miles long where the water entered the Marsh Creek Valley, filling it wall to wall. From there it proceeded north towards Pocatello, Idaho, where it joined the Snake River and generally followed the river westward.

Red Rock Pass, where the flood waters left Lake Bonneville and began the long journey to the Pacific.

Red Rock Pass, where the flood waters left Lake Bonneville and began the long journey to the Pacific.

Shoshone Falls Area

Shoshone Falls, located near the city of Twin Falls, Idaho, is a relic of the Bonneville Flood. It is difficult to imagine or understand the forces of the flood at that time as it cut the basalt rock away, creating the beautiful waterfall sometimes called the Niagara of the West.

Although Shoshone has had the Snake River passing over it for over 14,000 years that medium sized river hasn't produced even a shadow of the work done by the flood in the few days that it took to half empty Lake Bonneville. The forces of nature at work in those few days long before man walked the continent were beyond comprehension.

A little west of Shoshone Falls is the Perrine Bridge. Crossing the Snake River Canyon some 485 feet above the river, we can still see the original canyon depth before the flood. The marks are about halfway up the canyon wall; the flood cut the rest of that depth in the few days that it ran so fast and furious.

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North, across the river from Twin Falls, lies a large area of scablands. This is where the flood escaped the canyon walls and spread over the surrounding valley floor, scouring it of all topsoil and dirt and to this day very little grows there. At the same time, just downstream from the bridge is an area inside the canyon that is extremely fertile, supporting two golf courses and a fish farm. It is where that water returned to the canyon and created a large eddy, dropping the accumulated silt and soil from the scabland area.

Bruneau Sand Dunes

Bruneau Sand Dunes, a few miles south of Mountain Home, Idaho, is located near the Snake River. It is a popular attraction with the largest sand dunes in North America, two campgrounds and a national observatory that is opened to the public on weekends.

The sand dunes there are about 12,000 years old and are believed to have begun forming from the action of the Bonneville flood as it created the small basin the dunes sit in and likely deposited the first of the sand to collect there. Since that time the dunes have grown in size as prevailing winds blow first from the southeast and then the northwest and the dunes do not move as sand dunes normally do. Rather, they stay put in one spot and grow slowly, year by year.

Bruneau Sand Dunes, believed to have begun with the flood action.

Bruneau Sand Dunes, believed to have begun with the flood action.

Celebration Park

Celebration Park is Idaho's only archaeological park and is located near Melba, Idaho, a few miles south of Boise. It is on the shore of the Snake River.

Celebration Park is home to large amounts of Melon Gravel. These are boulders ranging from a foot across to some six feet or more that have been rounded and smoothed by the abrasive action of water flow and being drug along the river bottom. The river could not possibly move these large boulders with the current flow rates; it took the Bonneville Flood to do that. Plus, of course, they are hundreds of feet from the river, and considerably higher than the river has been for thousands of years.

Many of the "melons" have petroglyphs carved into them thousands of years ago by the indigenous peoples of North America and it is this that makes it such an interesting place to visit. There is also a small campground available as well as lessons in using an atlatl - the spear throwing weapon from centuries past.

Farewell Bend, Oregon

Farewell Bend is a tiny settlement on the banks of the Snake River in Eastern Oregon. It primarily consists of a truck stop and a very nice state campground, coupled with a few homes.

The area around the town clearly shows some of the effects of the Bonneville Flood. The river opens from a fairly narrow canyon into a river bed at least twice as wide and the canyon walls retreat considerably, forming a small valley. The entire valley would have undoubtedly been under water during the flood; it is not very large.

It was large enough, however, to let the flood waters spread and slow considerably. In addition, just as the river enters the area it makes a rather sharp bend, and the water would tend to travel the outside of the riverbed curve. The water on the inside of the curve, relieved from the massive pressure as the rest of the flood moved to the outside of the curve, over the river bank and into the valley, slowed and dropped large amounts of sand far above the present day river.

The photos below are of the Farewell Bend area of the river and the sandbar. They were taken from rather far up the "canyon" wall and detail is necessarily rather small in order to give an overall view.

Hells Canyon

Hells Canyon is arguably the wildest and certainly the deepest gorge in North America. Although the canyon existed long before the Bonneville Flood, it was not at it's current depth then.

The flood deepened the canyon considerably at many points, cutting through solid rock to do so. Even hundreds of miles from it's origin, the water of that great flood still had enormous power and force.

Some locations in Hells Canyon show gravel or sand bars deposited far above current river levels; an indication of the depth of water flowing through the canyon during that brief period 14,000 years ago.

Today Hells Canyon is a wonderful place to visit and rafting trips are at a premium. A float or jet boat trip through the canyon could be the highlight of the year; fishing is wonderful and the view and solitude is beyond compare. Jet boat day trips are available, but float trips generally take several days as access to the middle of the canyon is extremely limited. One must float most of the length in one trip.

The southern end of the canyon is dammed, with a large lake perfect for boating, swimming and fishing. Travelling north, a second dam is found within the canyon walls and this can be visited via car. Beyond that, though, there is virtually no access to the river itself for many, many miles although the road does approach the top of the canyon rim at times.

Note the sand deposits far above the current river level.  This is the result of the flood.

Note the sand deposits far above the current river level. This is the result of the flood.

Some of the extreme depth of Hells Canyon is the result of the Bonneville Flood.

Some of the extreme depth of Hells Canyon is the result of the Bonneville Flood.

Do you remember?

For each question, choose the best answer. The answer key is below.

  1. How much water did Lake Bonneville release?
    • 100 cubic miles
    • 400 cubic miles
    • 800 cubic miles
  2. What is melon gravel?
    • petrified watermelons
    • small river gravel, forming gravel bars
    • large boulders, rounded and smoothed by water action
  3. How large was Lake Bonneville?
    • 190 square miles
    • 1900 square miles
    • 19,000 square miles
  4. How much did the level of Lake Bonneville change during the flood?
    • 300 feet
    • 1,000 feet
    • one mile
  5. Areas scraped clean of soil by the flood are called?
    • badlands
    • scablands
    • crablands

Answer Key

  1. 400 cubic miles
  2. large boulders, rounded and smoothed by water action
  3. 19,000 square miles
  4. 300 feet
  5. scablands

© 2012 Dan Harmon

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