Lake Bonneville and the Bonneville Flood

LAKE BONNEVILLE

Imagine an ice-age lake in Utah whose surface area roughly equaled that of Lake Michigan. Also imagine that same ice-age lake releasing a surge of floodwater whose volume was six times the flood flow of the Mississippi River. Since no humans witnessed this event we can only picture it in our minds. But the lake and its destructive outflow were not someone's imaginary fantasies. Geologists know that it was all very real.

After geologic processes millions of years ago created the unique basin-and-range topography of the western Rocky Mountains, an immense basin dominated the landscape of western Utah. Exactly when the body of water--later named Lake Bonneville--began to form is uncertain. The evidence indicates that it existed at least 600,000 years ago. As the climate became cooler and wetter, Lake Bonneville expanded in size, gradually spreading over a surface area of about 20,000 square miles and overlapping into Nevada and Idaho.

Because Lake Bonneville was in a basin, it lacked natural outlets. Once captured, water from rainfall, stream flows, and glacial melting remained in the lake, except for those amounts lost through evaporation. At its greatest extent, the lake level stood at 5,090 feet elevation--nearly 1,000 feet higher than the current level of Great Salt Lake. Distinct shorelines, including sand and gravel beaches, were formed along the 2,000 miles of Lake Bonneville's rim. Animals such as bears, camels, deer, mammoths, horses and musk oxen roamed its shores.

In addition to being confined by higher ground at the basin rim, a "natural dam" at Lake Bonneville's northern limit at Red Rock Pass prevented outflows. The pass [ south of Downey, Idaho, on U.S. 91 ] was the site of overlapping alluvial fans--huge deposits of rock, sand, and soil that had eroded during the weathering-down of nearby mountains.

The 101 Ranch along US HWY 30 appears to be looking for help removing basalt boulders from their pasture near King Hill, ID. The material was ripped from bedrock upstream, then tumbled by floodwaters before being dropped near this location.

Melon Gravels.....As the water ripped through narrow reaches of the Snake's canyon, it dislodged large basalt boulders and tumbled them downstream. Within a few miles these boulders had been rounded with diameters ranging from three feet to ten feet. The process created large bars of sand and boulders, some of them several miles long and nearly 300 feet high. These remnants were given the name melon gravels after geologists observed a billboard that advertised one patch of boulders as "petrified watermelons."

Impressive deposits of melon gravels can be found near Hagerman; in Malad Gorge; in Melon Valley south of Wendell; at Swan Falls south of Melba; and near the towns of Hammett, Glenn's Ferry and King Hill.

RESOURCES:

Visit HUGEfloods Jim O'Connor page to view more photos of Bonneville Flood features and watch CWU's Nick Zentner interview O'Connor (USGS hydrologist). Jim describes both Bonneville and Missoula floods in the 25 minute program - Bonneville discussion begins at 14 minute mark.

THE AFTERMATH

The most intense flooding occurred over a period of several days--at most, several weeks, but over spilling at Red Rock Pass may have continued for as long as a year before the lake stabilized at 4,700 feet--the so-called "Provo shoreline"--and remained at that level for at least 600 years.

As the Ice Age ended the climate became warmer and drier. With less rainfall and glacial melting to sustain Lake Bonneville, coupled with increased evaporation, the vast lake began to retreat. The current Great Salt Lake's drainage area is roughly that of ancient Lake Bonneville. Other prominent remnants include Utah Lake, Sevier Lake, and the Great Salt Lake Desert.

BENJAMIN BONNEVILLE

Lake Bonneville was named for Benjamin L. E. Bonneville, a French-born U.S. Army officer, who in the 1830s was involved in exploring and trading ventures in the Pacific Northwest. Geologist G.K. Gilbert was the first to study these prehistoric lake features, and he decided to honor Captain Bonneville in this way.

EXPLORING SOUTHERN IDAHO

People who want to visit the Snake River region to see for themselves the evidence of the Bonneville flood and other geologic wonders in the area, may find David Alt and Donald Hyndman's Roadside Geology of Idaho to be a helpful guide.