Ice-Induced Thalweg Oscillations along the Fort Peck Reach of the Missouri River

This paper reports preliminary observations of thalweg oscillations along the Fort Peck Reach of the Missouri River. The oscillations, tentatively attributable to river-ice formation, appear to occur in sinuous-braided sub-reaches, where the channel bifurcates into two or more sub-channels. The observations were obtained during a comprehensive survey conducted to determine the extents to which river ice influences channel stability along the flow-regulated Fort Peck reach of the Missouri River, Montana. The survey was conducted during the winter of 1998 – 1999. Survey findings suggest that river-ice formation may cause cyclic oscillations of thalweg location between sub-channels around bars and islands. Typical periods of the oscillations have yet to be determined; the observations suggest they might be between two to twenty years, depending on location, flow conditions, and winter severity. The survey observations and data are of direct significance for engineering activities in the Fort Peck reach of the Missouri River and in similar alluvial rivers subject to frigid winters. The observations indicate how ice-induced switching of thalweg may affect the functioning of channel- and bank-stabilization structures, water intakes, and bridge crossings, as well as potentially interfere with dredging work.