RAPID RESPONSE FIELD MAPPING OF THE TAUM SAUK RESERVOIR FAILURE

In the early morning hours of December 14, 2005, a portion of the northwest wall of the Upper Taum Sauk Reservoir failed, releasing approximately 1.5 billion gallons of water. A 2.6 km (1.6 mile) long scour channel draining the western slope of Proffit Mountain (Reynolds County, Missouri) was eroded locally of soil, colluvium, alluvium, and bedrock following the breach.

A team of geoscientists from the U.S. Geological Survey and Missouri S&T traveled to the site the day following the breach to assess the newly exposed geologic terrain. Mapping was conducted by traversing the scour channel while making detailed field notes and sketches using topographic maps and aerial photographs. Waypoints were collected along facies contacts using a handheld Global Positioning System unit. Post-field analysis used geographic information system technology to integrate waypoint data with precision light detection and ranging (LiDAR) elevation data, and high spatial resolution (15 cm) digital aerial images to compile a draft geologic map.

Results of the investigation suggest that variable flow regimes affected different segments of the scour channel in different ways. New bedrock exposures include Precambrian felsic porphyries, granitic sequences, a highly weathered mafic unit, a conglomerate-boulder field, and Cambrian-aged basal dolomite and sandstone in unconformable contact with the Precambrian rocks. Lower portions of the valley were partially filled with sediments derived from the erosion of the previously mentioned units plus debris from the failed dam structure. Mid-valley braided channel flood deposits are inversely graded, with 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. The basal unit was deposited as hyperconcentrated floodwater lost turbulent energy and hence its suspended sediment load. A relatively slow advancing traction load of coarse boulders and cobbles eventually overtook the earlier deposits. Late floodwaters eventually washed over and dissected earlier deposits. Cut bank and point bar deposits are spatially linked with a valley curve further downstream. Maximum boulder size increases progressively down valley throughout the deposition system.