DEPOSITION PATTERN OF FLOOD DEPOSITS FOLLOWING THE CATASTRAPHIC FAILURE OF THE UPPER TAUM SAUK RESERVOIR

A portion of Upper Taum Sauk Reservoir failed on the morning of December 14th, 2005, releasing ~1.5 billion gallons of water down the western slope of Proffit Mountain (Reynolds County, MO). The resulting flood waters stripped trees, soil, colluvium, alluvium, and bedrock from a 1.6 mile-long side canyon. Our preliminary investigation suggests that variable flow regimes have influenced sedimentation processes. Water flow varied as a function of the orientation of the reservoir breach to valley slope, valley topography, valley curvature, erosion resistance of various segments of soil and bedrock, entrainment of trees and other debris in the frontal and lateral lobe margins of the flood waters, and variable water volume. The different sedimentation patterns allow the valley to be divided into three segments. The uppermost segment was largely devoid of sedimentary deposits due to the steepness of the valley slope (~12^o) and rapid removal of easily eroded materials. The middle segment begins just below a decrease in valley slope to ~2.1^o. Flood deposits in this segment are characterized by a sheet-like layer of poorly sorted and relatively fine grained material (pebble to silt size), immediately overlain in sharp contact by a well-sorted layer of cobbles and boulders. These two layers appear to have been deposited in quick succession. The poorly-sorted basal unit was lain down by a forward advancing hyperconcentrated stream flow that was loosing turbulent energy and hence its suspended sediment load. This was overlain by a slower advancing traction load of coarse boulders and cobbles. Late stage erosion, reworking of sediments, plus removal of fines occurred as progressively clearer flood waters washed over and dissected through earlier deposited sediments. A pronounced curve in the Proffit Mountain valley reduced the subsequent down stream flow energy of the lower third of the valley. Classic cut bank-point bar deposits are spatially linked within this valley curvature and locations further downstream. The maximum boulder size increases progressively down valley throughout the entire system. Larger boulders apparently projected to greater heights into the water column and thus were exposed to higher velocity flood water. These boulders were subsequently moved further down the valley system despite their larger mass.