Abstract

The Watrous spillway was rapidly incised during a short-lived outburst from glacial Lake Elstow. In its outlet area, the bed of Lake Elstow was composed of stagnant ice. The 40 km long spillway is incised across a divide, and terminates in the glacial Last Mountain Lake basin, where a coarse-grained fan, herein named the Little Manitou fan, was deposited. Clast sizes fine systematically from the mouth of the spillway across the surface of the fan, from boulders with a maximum mean intermediate diameter of 1.26 m to cobbles, over a distance of 7 km on the south lobe of the fan. Large clasts are concentrated on the surface, and probably represent deposition at peak discharge. The proximal margin of the south lobe of the fan is hummocky, suggesting that deposition of sediment at its apex was on stagnant ice with subsequent collapse. Paleovelocity, paleodischarge, and other parameters were estimated for the outburst flood using a critical shear stress approach. These approaches yielded paleodischarge estimates lower than those deduced from previous analyses of floods in channels of similar size in the region, and lower than values given by empirical equations relating shear stress to particle size. Due to the lack of high-water marks along the channel, the progressive flood erosion of the spillway floor, and the possible presence of stagnant ice in at least the headwater reach of the spillway during the flood, slope is the most difficult parameter to estimate for the flood.

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