ABSTRACT
Dynamic behavior of the Missoula Floods, which inundated the Scablands of Washington about 15,000 years ago, cannot be understood by field observations alone. It is necessary to link such observations to a model of the physics of fluid motion. Such models can direct investigations and formulation of hypotheses.
Controls upon flood dynamics that must be understood include: ice sheet configuration and dynamic behavior, topographic boundary conditions and the physics of ice dam breakup. In the study reported here, a digital elevation model (DEM) at a spacing of 2 arc minutes of latitude and longitude is used to represent the topographic controls. Volume-depth-area relations, ice sheet configuration and fluid flow are all constrained by this DEM. A simple kinematic model of ice sheet dynamics is used to represent the control of ice margin configuration upon lake volume, ice dam elevation and flood flow paths. Global controls upon the growth of the ice sheet are represented with an empirical global ice volume model coupled to insolation variations due to the changing orbit of the earth.
This model predicts a cycle of at least 35 floods during the Fraser glaciation; even if the discharge of the Clark Fork River was no different than today. A doubling of its discharge as suggested by Waitt (1980) would produce 67 floods, about the number that Atwater’s (1986) data suggest. Floods were first very small, then increased, reached a peak magnitude and fell off in size more quickly than the rise. The largest floods within the Pasco Basin lasted about 11 days and were never close to an equilibrium condition during hydraulic ponding.
