USU Professors Find New Lake Bonneville Answers
What caused Lake Bonneville to lose more than 1150 cubic miles of water in approximately one year?
New research published by Utah State University geology professor Susanne Janecke and emeritus geology professor Bob Oaks Jr. adds more data, evidence and hypotheses to the Bonneville flood mystery.
“For roughly a thousand years, Lake Bonneville flowed steadily into a river at the outlet point without lowering the lake depth,” said Janecke. “Then something drastic must have happened to produce the catastrophic Bonneville flood.”
Janecke first realized something was amiss while studying inactive faults on Oxford Ridge, Idaho.
Janecke observed a unique, previously unidentified curvy river pattern known as a meander belt in the valley below. The shape of the land suggested the river was occupied at the end of the Pleistocene epoch; the same time interval Lake Bonneville existed.
“Previous documented outflow points of the lake had been recorded much farther north than our findings,” said Janecke
The new information led the researchers to investigate the potential impact and meaning of the southern outflow point of Lake Bonneville.
“We started making digital elevation models and detailed topographic profiles of the landscape in northern Cache Valley,” Janecke said.
These maps gave way to even bigger discoveries.
“We discovered a new fault and named it the Riverdale fault for its crossing near the hot springs in Idaho,” Janecke said. “We were able to use prior geological mapping and gravity data to better understand the fault’s location, history and activity level.”
The Riverdale fault stretches across southern Idaho and has been active in the last 20,000 years. Using cross-cutting relationships to understand the fault activity, Janecke and Oaks found the fault to have ruptured about the same time as the Bonneville flood, within the error of their methods.
Janecke hypothesized that an earthquake on the Riverdale fault may have triggered the Bonneville flood.
“An earthquake would have had a large enough impact to cause the Bonneville flood,” said Janecke. “There are three mechanisms by which a large earthquake could have destabilized the outlet of Lake Bonneville.”
First, the shaking produced from the earthquake could have caused a lake tsunami cutting the dam. Second, shaking from the earthquake could have been strong enough to collapse part of the outlet, creating a large-scale flood. The final possibility is the fault might have cut through the dam, causing a massive release of water.
Janecke also researched the relationships between the earthquake and rising levels of Lake Bonneville.
“As the water levels rose in Lake Bonneville, the extra load of water could have induced the earthquake by making it occur a little ahead of schedule,” said Janecke. “The second way filling lakes can induce earthquakes is by the increasing pore pressure. That can cause a fault to slip during an earthquake.”
“Because of our work, the Riverdale fault has been identified as an active fault that could produce major but infrequent earthquakes that could result in major shaking in southern Idaho and northern Utah,” said Janecke.
The fault has been inactive for 15,000 to 20,000 years, but has the potential for another earthquake.
“The Riverdale fault may need to be investigated further for its activity level before new dams across the Bear River are constructed nearby,” said Janecke.
Janecke and Oaks’ unfunded research spanned a period of ten years and was published in Geosphere, an electronic publication of The Geological Society of America.
Janecke and Oaks will continue to research the Riverdale fault and have involved USU undergraduate students in their research on Lake Bonneville.
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Contact: Susanne Janecke, 435-797-3877, susanne.janecke@usu.edu
Writer: Jaron Dunford, 920-246-2863, jaron.dunford@gmail.com
Susanne Janecke and Bob Oaks Janecke and Oaks in front of paleo-liquefaction features in the gravel pit at Green Canyon, where they work with two undergraduates, and find evidence for several large earthquakes in delta sand of Lake Bonneville.
USU faculty member and geologist Susanne Janecke.
Bob Oaks Jr., emeritus geology professor.
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