Introduction: The Channeled Scablands in eastern Washington is an expansive landscape of flood-sculpted bedrock that has been of longstanding interest to geologists and the public at large. Early work in the 1920’s established that the landscape was carved by catastrophic floods issuing from an ice-dammed Lake Missoula and subsequent work demonstrated there were multiple floods at the end of the last ice age. Despite decades of work in the Channeled Scablands, there are many unanswered questions regarding the size of the floods that carved the vast channel networks, the processes by which the rock was eroded, and the timing of canyon incision.

Intellectual Merit: Mega-flood discharges are often estimated by assuming that the floods filled the canyons to high water marks with water; however, an alternative hypothesis is that canyon morphology adjusts during incision such that the forces exerted on the channel bed by flowing water exceed the threshold forces required for erosion by a small amount. We tested these hypotheses in Moses Coulee, a major canyon in the Channeled Scablands and found that accounting for erosion thresholds during canyon incision results in discharges that are five- to ten-fold smaller than estimates based on filling the canyons to their high water marks. These findings suggest that flood-carved landscapes in fractured rock might evolve to a threshold state for bedrock erosion, thus implying much lower flood discharges than previously thought.

Determining when different water-scoured bedrock surfaces in the Channeled Scablands were eroded by floods requires use of cosmogenic nuclide exposure dating – which takes advantage of the rate which high energy particles generate rare isotopes when they collide with bedrock exposed at earth’s surface. The bedrock in the Channeled Scablands generally lacks the minerals that are commonly used for exposure dating. To circumvent this problem, we calibrated the production rate of the cosmogenic isotope Helium-3 in the mineral ilmenite. The measured production of Helium-3 was 20–30% greater than expected from prior theoretical and empirical estimates for similar minerals. We found that the production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit Helium-3 and the long distances it travels before stopping within the rock. By accounting for the grain size of the ilmenite crystals, which are small – a fraction of a millimeter – we resolved the discrepancy between our calibrated production rate, theory, and rates from previous work. With the newly calibrated production rate, we are now measuring Helium-3 in ilmenite crystals in flood-eroded bedrock from throughout the Channeled Scablands to determine the timing of canyon carving.

Broader Impacts: For three consecutive summers, we have led a workshop for Eureka!, a partnership between Girls Inc. of the Valley and the University of Massachusetts-Amherst that has the goal of increasing college enrollment of women from groups that are historically under-represented in science, technology, engineering, and mathematics degree programs. In our workshops, which have been highly rated by the participants, we introduced over 60 8^th grade girls who are primarily women of color, to the earth sciences via investigation of the geology of the UMass campus, which record ice age glaciers and glacial lakes, and hence has similarities to the ice age geology of the Channeled Scablands.

Last Modified: 12/12/2019
Modified by: Michael P Lamb