SEDIMENTATION RATES ON THE SODA BUTTE CREEK FLOODPLAIN, YELLOWSTONE N.P

An earthen impoundment dam near the headwaters of Soda Butte Creek above Cooke City, Montana failed in 1950, producing a major flood and spreading metal-contaminated mine tailings along a 28 km stretch of the floodplain, portions of which lie in Yellowstone National Park (Marcus et al., 2001). The presence of a stratigraphic marker of known age provides an opportunity to examine the effectiveness of Pb-210 and Cs-137 in calculating floodplain sedimentation rates and identifying discreet flood events from radionuclide activity profiles. We collected high resolution flood plain sediment cores from 5 sites on Soda Butte Creek and 1 site on the Yellowstone River along an altitude gradient ranging from 2300 to 1550 meters above mean sea level. The activities of Pb-210 and Cs-137 were determined by alpha and/or gamma spectroscopy.

There is a large decrease in Pb-210 activity in the cores with decreasing elevation because of (a) greater delivery rates to upland soils from greater precipitation at higher elevations, (b) increase in the age of flood plain particles farther downstream caused by increases in transport distances and therefore transit times, or (c) differences in soil mineralogy at each sampling site. Comparisons of Pb-210 activity profiles determined from independent alpha and gamma spectroscopy techniques reveal striking similarities in both primary and secondary trends, suggesting that minor deviations from smooth activity curves may in fact represent discrete sedimentation packages deposited during flood events rather than random measurement errors. Cores with subsurface Cs-137 peaks provide dates that agree reasonably well with dates obtained by Pb-210.

Two cores contained orange oxidized zones and we interpreted the lowest appearance of these layers to represent the 50 year–old McLaren flood event. In both cores, sediment ages derived from Pb-210 dating agree with this interpretation.