BEDROCK COHESION IN COLUMNAR BASALT, AND ITS INFLUENCE ON EROSION GENERATED BY EXTREME FLOODS: UPPER GRAND COULEE, WA, USA

Extreme floods can produce extensive erosion in fractured bedrock over short timescales by plucking. However, the degree to which bedrock properties such as cohesion between adjacent blocks limits erosion is poorly constrained, and likely highly variable even within the same bedrock unit. To assess the effectiveness of cohesion in limiting erosion, we consider the formation of upper Grand Coulee, a large canyon in the Channeled Scablands of eastern Washington, USA which was carved by Pleistocene glacial lake outburst floods into columnar basalt. Observations from modern dam burst floods as well as physical models demonstrate that plucking is the most effective erosion mechanism in high-discharge systems, and numerical models suggest that toppling entire stacks of columns may be required to produce a straight-walled canyon such as upper Grand Coulee. 2D numerical flood models constrain canyon-forming discharges in upper Grand Coulee to ~2.6 million m3/s, which predicts sufficient shear stresses to topple columns at the retreating waterfall’s edge using a torque balance approach including a minimum estimate of cohesion based on measured column geometry. This balance is fairly sensitive to many parameters in the cohesion calculation which are challenging to constrain, such as the fraction of the rock face along which cohesion is present, and its position relative to the canyon floor. Although our approach produces a minimum cohesion estimate, examples from the nearby Snake River in which canyons were incised in a single event (the Bonneville Flood) suggest that floods are capable of toppling columns with such cohesion.