History of Pleistocene Lake Bonneville
Ancient Lake Bonneville was located in north-central Utah and was fed by the Bear River, until 14,500 years ago, during the Pleistocene Epoch. Lake Bonneville was the largest stage of the ancient lake that is now the Great Salt Lake. After the Lake Bonneville Flood, the lake fell to the Provo shoreline level.
Figure 1 The extents of Lake Bonneville, Provo Lake, and the modern Salt Lake.
The fluctuations of the lake level are explained in the History of Lake Bonneville section, prepared by S. Khan and students at the University of Houston.
Click Here to see the History of Lake Bonneville section, which includes other lake level stages not discussed above. It also includes digital images of each stage, digital fly-through's of the Lake Bonneville flood path, and 3-D digital elevation models showing the stages of Lake Bonneville.
Extent and Timing of Lake Bonneville Flood
Climate fluctuations were extreme and rapid during Pleistocene time and caused fluctuations in glaciation and flooding of drainage systems during times of glacial melt. At 14.5000 ybp (radiocarbon years before present), Lake Bonneville overfilled and formed a body of water that broke through the rock at Red Rock Pass and flooded much of southern Idaho.
Figure 2. Map showing Pleistocene lakes in northwestern United States.
The Lake Bonneville flood filled the Pocatello Valley and flowed up through Marsh Valley and then out onto the Snake River Plain. The flood emptied the top 107 meters of water from Lake Bonneville , an estimated volume of 4,750 cubic km of water (Link and Phoenix, 1996). The peak discharge from the Red Rock Pass outlet is estimated to have lasted for about 8 weeks and to have been about 500 times that of the maximum discharge ever recorded from the Snake River at Idaho Falls.
Causes and Effects of the Lake Bonneville Flood
Several geological and environmental factors likely contributed to the Bonneville Flood. Rapid climate fluctuation during Pleistocene time produced large transgressions and regressions of the Bonneville shore level. Periodic overflow of Lake Bonneville water at Red Rock Pass controlled the maximum height of the shore line at 1550 m (Link et al., 1999). These events were apparently non catastrophic through most of the history of Lake Bonneville.
Subsidence of the Snake River Plain in the wake of the Yellowstone Hotspot produced changes in regional drainage patterns and volcanic eruptions produced natural dams that diverted steam drainages (Link et al., 1999). The Bear River previously flowed north into the Snake River. At ~50 ka, the Bear River cut through the Oneida Narrows , near the town of Thatcher (Figure 3) and diverted to the Bonneville basin. This increased the inflow of water to the basin by ~33% (Link et al., 1999).
Subsurface leakage of water at Red Rock Pass through alluvial fan gravels and karstic limestone initiated a catastrophic slope failure and emptied water from Lake Bonneville to the level of the Lake Provo shoreline (Link et al., 1999). The excess water from Lake Bonneville flooded the Marsh and Portneuf Valleys before emptying into the Snake River. Evidence of the flood is found as far away as Lewiston, Idaho (Link et al., 1999).
Figure 3. Modern paths of rivers and the ancient pathway of the Bear River , before it began to flow southward. Figure redrawn from Link and Phoenix , 1996 .
The Lake Bonneville Flood left behind many scabland featuressuch as dry waterfalls, alcoves, scoured bedrock surfaces, and boulder bar accumulations (Link and Phoenix, 1996). Evidence of this is seen in the Pocatello area; the removal of the Portneuf Valley basalt from the Portneuf Narrows area and the deposition of rounded basalt boulders (up to 3.5 meters in diameter) in the adjacent basin are prime examples. Other spectacular examples of Bonneville Flood features are the Snake River Canyon, alcoves, and waterfalls (e.g. Shoshone Falls) at Twin Falls and the "watermelon" boulders near Hagerman.
Currey, D.R., Atwood, G., and Mabey, D.R., 1984, Major levels of Great Salt Lake and Lake Bonneville: Utah Geological and Mineral Survey Map 73.
Digital Atlas of Idaho, http://imnh.isu.edu/digitalatlas, managed by Idaho State University Department of Geosciences and hosted by the Idaho Museum of Natural History, Pocatello, ID.
Link, P.K., Kaufman, D., and Thackray, G.D., 1999, Field Guide to Pleistocene Lakes Thatcher and Bonneville and the Bonneville Flood, Southeastern Idaho, in Hughes, S.S. and Thackray, G.D., eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History, Pocatello, p. 251-266.
Malde, H.E., 1968, The catastrophic late Pleistocene Bonneville Flood in the Snake River Plain, Idaho: U.S. Geological Survey Professional Paper 596, 52 p.
O'Connor, J.E., 1993, Hydrology, hydraulics, and geomorphology of the Bonneville flood: Geological Society of America Special Paper 274, 83 p.
Red Rock Pass and Bonneville Floods, Idaho by Steven Dutch, Natural and Applied Sciences, University of Wisconsin-Green Bay http://www.uwsp.edu/geo/projects/geoweb/participants/dutch/VTrips/RedRockPass.HTM
Continue to Module 15 - Snake River Plain Aquifer