Researchers dive into cause of Lake Bonneville flood
More than 2 million people live along the Wasatch Front. However, tens of thousands of years ago, scientists believe the area looked very different: In fact, it was submerged under more than 1,000 feet of water.
Lake Bonneville once covered much of what is today Utah, including Cache Valley, and at one point rivaled the largest of the Great Lakes in size. The lake’s northern natural dam near Red Rock Pass failed around 14,000 years ago, an event today known as the Bonneville flood. The lake never recovered from the flood, and all that remains of it are the Great Salt Lake, Utah Lake and a few others.
Two USU geology professors recently published their research from northern Cache Valley and may have discovered what caused the Bonneville flood.
“An earthquake on the Riverdale fault could’ve been the trigger for the flood,” said Susanne Janecke, a geology professor specializing in plate tectonics. “There are lots of different ideas geologists have put forward about the possible trigger for the flood. Nobody had really settled on a preferred interpretation, and earthquakes were never a part of that mix of possible triggers.”
Janecke, along with emeritus geology professor Robert Oaks Jr., discovered the Riverdale fault along the northeastern edge of Cache Valley in Idaho.
“Gullies that formed after the Bonneville highstand during the Provo stage are not continuous across the Riverdale fault, so it must have formed about the time of the Bonneville Flood, prior to formation of the gullies rather than later,” Oaks said. “Thus, an offset on the Riverdale fault may have triggered the Bonneville flood, or the rapid reduction of weight of the water may have triggered offset on the Riverdale fault.”
The Provo stage refers to the level Lake Bonneville receded to after the flood. According to the Utah Geological Survey, the flood lowered the lake by 375 feet in less than one year. The peak strength of the flood has been estimated to match the average flow of all the world’s modern rivers combined.
Janecke said their data set is not as robust as they’d like, so right now they can say the evidence is consistent with the earthquake theory, but it cannot be proved yet.
“It would be nice to do some follow-up work and really pin down the timing of that big earthquake, because if it was a couple months after the flood or 100 years before the flood, it wouldn’t have been the trigger for the flood,” she said. “That has to be sorted out still.”
In addition to the earthquake hypothesis, Janecke and Oaks’ research revealed other findings about Lake Bonneville.
“Susanne Janecke discovered ancient river meanders between Clifton, Idaho, and Oxford, Idaho, from an overlook on the high ridge to the west,” Oaks said. “The meanders show northward flow during the Provo stage of Lake Bonneville, after the Bonneville Flood had lowered the lake from its highstand.”
Oaks said this indicated a further drop during the Provo stage, so the northern margin of Lake Bonneville at the Provo stage moved south from near Swan Lake to just south of Clifton.
“We also found that the Riverdale fault continues north-northwest from Riverdale Hot Springs toward the Swan Lake area and may continue even farther into the area near the Zenda threshold for the Bonneville highstand, about a mile north of Red Rock Pass,” Oaks said.
The Zenda threshold was the northern extent of Lake Bonneville, according to Oaks and Janecke. Oaks said G.K. Gilbert, the geologist who published Monograph I of the U.S. Geological Survey in 1890, was the one who figured out Zenda was the furthest extent of the lake.
Janecke said the lake’s northern edge was never at Red Rock Pass if their hypothesis is correct.
“The outlet of Lake Bonneville when the main Provo shoreline was being formed was about 22 kilometers south of where everybody thinks it is,” she said. “So if you’ve ever driven out the north end of the valley and stopped at the sign at Red Rock Pass that says ‘This is where the outlet of Lake Bonneville was,’ that sign is wrong and should probably be revised because the outlet, the northern edge, the northern shoreline of the lake was never there. It was two or three kilometers further to the north at a little town called Zenda.”
Janecke said her and Oaks’ hypothesis still needs testing, but the research is looking promising.
“There’s good evidence that Lake Bonneville flowed out at a stable level for a long period of time, roughly 1,000 years or so,” Janecke said. “There wasn’t anything about all that water rushing out there at Zenda that allowed the water to cut down and incise into the subsurface, so it’s a little unusual that the lake was stable for a long time and then abruptly failed and you had a big flood. The earthquake is a good mechanism to explain both the long stability and then the abrupt, catastrophic flood because you have this major event, the earthquake, that could destabilize the dam and destabilize the outlet.”
“It is looking promising,” she said. “It’s consistent with the observations, but that doesn’t necessarily prove that it’s right, so as good scientists, we need to do more work to test the hypothesis.”
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