RAVINE ALLUVIAL FANS AS RECORDS OF HOLOCENE AND WESTERN SETTLEMENT DISTURBANCES

The Le Sueur River Basin in southern Minnesota preserves the records of early Holocene and modern anthropogenic disturbances. Seventy meters of base-level drop at the end of the last glaciation initiated millennia of incision that continues on the Le Sueur River and its tributaries today. Onto this template of on-going incision, Euro-American land clearing and drainage of previously stable upland prairie in the mid-1800s may have further increased erosion rates in the basin. Ravines that link the low-gradient uplands with the deeply incised channel network have experienced changes in erosion rates over time from both of these impacts, with the erosional history preserved in alluvial fans at the mouths of ravines where they terminate on fluvial terraces. We are studying six ravine fans to better understand the long-term evolutionary history and how fans responded to Euro-American land clearing. These alluvial fans were chosen to encompass a range of size, depth of incision, and distance from the mouth of the Le Sueur River. Radiocarbon dating of organic-rich sediment, wood, and charcoal is being used to obtain depositional ages near the base of the alluvium. Fly ash from coal combustion is being used as a stratigraphic marker recording an event (Western settlement) in the fan’s history; it was sampled at 10, 20, 40, 100, and 200 centimeters on the fan surface as well as in any incised channels cut into the fan surface. The scale of the impact that settlement has on ravines is largely unknown, but initial results show that in one ravine, there is at least 135 centimeters of post-settlement alluvium marked by a metal pin found at that depth. If regional coal combustion began in the 1850s, that produces an initial post-settlement alluvium deposition rate of 0.81 cm/yr. Combining detailed stratigraphy with basal ages and post-settlement markers, we will be able to determine if background rates of deposition on fans changed with the onset of land clearing for agriculture.