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Distinguished Speaker Isaac Larsen Looks at "Landscape Evolution" This Week

Dr Isaac Larsen of Unversity of Massachusetts, Amherst wraps up our Fall 2017 Distinguished Speaker Seires this week. His topic "Landscape Evolution of the Channeled Scablands, Eastern Washington".

The surfaces of Earth and Mars contain bedrock canyons carved by catastrophic outburst floods. Reconstructing the magnitude of the canyon-forming floods is essential for understanding how floods modify planetary surfaces, the hydrology of early Mars, and influence climate change. Flood discharges are typically estimated by assuming that floods filled the canyons brimful with water; however, an alternate hypothesis proposes that canyon morphology adjusts during incision such that bed shear stresses are approximately constant. These hypotheses imply drastically different peak flood discharges and have yet to be tested.

Here we reconstruct the incision history of Moses Coulee – cut by the catastrophic Missoula Floods in eastern Washington – and test the two hypotheses using 2-D hydraulic flood models over the reconstructed canyon topography. We find that the brimful hypothesis predicts a significant and monotonic increase in flood discharge during progressive canyon incision, which is at odds with the lake-outburst flood source. Alternatively, the constant stress hypothesis predicts near constant discharge of floods during canyon incision, and flood depths that are 5 to 40% of brimful estimates, consistent with flow-depth indicators from gravel bars within the These findings suggest that megaflood-carved landscapes in well fractured rock may evolve to a threshold state for bedrock erosion, which implies significantly lower megaflood discharges than previously thought.

Join us for our last fall series speaker this Friday, November 17, 2017 at 12:30 PM in Room 204, Natural Science building on the East Lansing campus.

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