LATE PLEISTOCENE TO EARLY HOLOCENE TERRACE DEPOSITS OF THE LOWER CHEYENNE RIVER, SOUTH DAKOTA

The Cheyenne River is a tributary to the Missouri River with its headwaters in the Black Hills of South Dakota and Wyoming. The Farmingdale terrace is the youngest of three regionally-recognized terraces along the Cheyenne River. The height of the Farmingdale terrace above the active channel increases in the downstream direction. We studied four Farmingdale terrace sites on the lower Cheyenne River at 199 to 108 river kilometers upstream from the confluence with the Missouri River, rising from approximately 77 to 85 meters above the channel. Alluvial-sediment samples were collected at these sites to determine the ages of terraces using optically stimulated luminescence (OSL) and radiocarbon dating techniques. Five OSL dates from samples collected at upstream terrace sites ranged from 6.1 thousand years ago (ka) to 16.5 ka. Two radiocarbon samples collected from sediment near the base of the terrace at 199 river kilometers yielded ages of 27.8 ka and 32.0 ka. Previous studies of terrace deposits on the lower Cheyenne River include OSL dates that range from 8.3 ka to 14.1 ka on a terrace approximately 75 meters above the channel at 16 river kilometers upstream from the Missouri River confluence. These data indicate Holocene incision rates on the order of 2.4 to 12.7 meters per 1000 years for the lower Cheyenne River. Examination of U.S. Geological Survey streamgage records since 1914 on the lower Cheyenne River at Wasta, SD indicates an incision rate of 3.2 meters per 1000 years. Causes of these high incision rates remain unclear but may include climate change, regional uplift, stream capture, removal of glacial dams on the Missouri River, and glacial isostatic rebound. Stream capture may be of particular importance. For example, the capture of the ancient headwaters of the Little Missouri River by the Belle Fourche River, a tributary of the lower Cheyenne River that drains the northern Black Hills, increased the Cheyenne River watershed area by nearly 20 percent. Timing of this event is critical in the understanding of the dynamics of the Cheyenne River system.