AGES OF ALLUVIAL TERRACE DEPOSITS AND DYNAMICS OF THE CHEYENNE AND BELLE FOURCHE RIVERS, SOUTH DAKOTA

The Water Resource Development Act of 1999 (amended in 2000) mandated a study of contaminated sediments within the Cheyenne River Basin, which is being conducted by the U.S. Geological Survey in cooperation with the U.S. Army Corps of Engineers. Initial work for this study includes the measurement of concentrations of metals in “uncontaminated” sediment. Sites were identified as uncontaminated if no evidence of negative effects by anthropogenic activity existed and their elevations are well above historical maximum flood levels. Four terraces sites on the Cheyenne River were selected for the study, at 108, 117, 183, and 199 river-kilometers upstream from the confluence with the Missouri River. Terrace elevations at these sites were 85, 85, 67, and 67 meters above the channel, respectively. The chronology of terraces on the lower Cheyenne River is largely unresolved, but previous work assigned the name Farmingdale to terraces along Rapid Creek with ages as young as Late Pleistocene to Early Holocene. On Rapid Creek, the Farmingdale terraces rise in the downstream direction over at distance of 22 kilometers from the level of the active floodplain to approximately 55 meters above the channel, at the confluence with the Cheyenne River. This confluence is 226 river-kilometers upstream from the Missouri River. Previous studies of terrace deposits on the lower Cheyenne River include optically stimulated luminescence (OSL) dates that range from 8.3 thousand years ago (ka) to 14.1 ka on a terrace approximately 75 meters above the channel at 16 river-kilometers upstream from the Missouri River confluence. Ages of terrace deposits at the current sample sites were obtained to further evaluate the Late Pleistocene and Holocene dynamics of the Cheyenne River system. Terrace deposits at 183 river-kilometers, at 67 meters above the channel, had an age of 6.1 ka (2 OSL samples). Terrace deposits at 199 river-kilometers, also at 67 meters, had ages of 10.7 to 16.5 ka (3 OSL samples) and 27.8 to 32.0 ka (2 radiocarbon samples). Dates at other sites are forthcoming. These data indicate incision rates on the order of 0.2 to 1.1 meters per century. Causes of these high incision rates remain unclear but may include climate change, regional uplift, stream capture events, removal of glacial dams, and glacial isostatic rebound.