CHASING THE ELUSIVE BIG LOST RIVER FLOOD TO FALL CREEK, PIONEER MOUNTAINS, IDAHO

The Pioneer Mountains of Central Idaho are the well-established source of the Big Lost River Flood, yet the source location, magnitude, and repeatability of flooding is not fully understood. Recently published cosmogenic ages from flood boulders on the Snake River Plain, 100 km downstream of potential flood sources, indicate an event ca. 22 ka. Glacial Lake East Fork (GLEF) was a glacially-dammed lake, long invoked as the flood source despite being of unknown age.

However, a recent OSL age of 23.5 +/- 4.9 underlying a glacial landform sequence upstream from GLEF’s dam location suggests that ice did not advance across the East Fork valley ca. 22 ka. Further, multiple workers note the lack of evidence for a large LGM flood along the Big Lost River corridor as well as the insufficient size of GLEF.

Recently discovered lacustrine sediment in Fall Creek may be evidence of an alternate flood source. “Fall Creek Lake” persisted near the confluence of the Left Fork Fall Creek and Fall Creek following glacial retreat and damming near a bedrock constriction. Cut banks along Fall Creek expose thick beds of landslide debris and glacial sediment bracketing the preserved lake sediment, possibly indicating multiple slope failure, damming, and drainage events. Downstream of a potential spillway, sediment has been stripped from the bedrock and deposited as a bouldery fan overlying outwash surfaces.

We interpret Fall Creek as the potential flood source because it may satisfy the established ca. 22 ka chronology and contains strong evidence for catastrophic drainage. Fall Creek Lake was of significantly smaller volume (<.0207 km³) than GLEF (<1.05 km³), and thus may have left a minimal trace within the Big Lost River corridor. A pending OSL age from the lacustrine sediment, combined with cosmogenic ages from flood boulders and nearby moraine boulders, will describe the most recent outburst from the valley system.

More broadly, these findings suggest that flood sources other than GLEF have contributed to the surficial record of repeated outburst events preserved along the margin of the Snake River Plain. Interpreting flood evidence as composite may resolve longstanding issues with the Big Lost River Flood.