HUMAN-INDUCED SEDIMENTATION ON THE NORTH SULPHUR RIVER FLOODPLAIN: GEOARCHAEOLOGICAL INVESTIGATIONS OF A POST-SETTLEMENT ALLUVIUM AT LAKE RALPH HALL, FANNIN COUNTY, TEXAS

Human-induced sedimentation shapes floodplains throughout the world and is particularly prevalent in basins where lands are or have been under intense cultivation. Recent geoarchaeological investigations at the currently under-construction Lake Ralph Hall Reservoir in Fannin County, Texas, have identified an alluvial unit of variable thickness mantling the floodplain of the North Sulphur River. This alluvium is characterized by minimally developed soil imprints (A-C and AC profiles) and sedimentary structures commonly preserved throughout the unit. The unit abruptly buries a late Holocene alluvium, which is capped by a paleosol representing a period of floodplain stability minimally dating 1,870 to 40 radiocarbon years ago. Additional chronological evidence in the form of various metal artifacts recovered from the alluvial mantle during archaeological excavations firmly establish this unit as a post-settlement alluvium. We interpret the formation of this post-settlement alluvium as the result of a massive influx of sediments primarily triggered by land use changes beginning with the Anglo-American settlement of the area. Initially settled in 1836, Fannin County grew steadily from the Civil War to the turn of the century with cotton and corn production representing the main source of income in the county well into the initial decades of the twentieth century. Much of Fannin County, including the upper basin of the North Sulphur River is located within the Blackland Prairie, an ecoregion of Texas that has witnessed extensive crop production since the latter half of the nineteenth century. Today, less than one percent of the original tall grass community of the Blackland Prairie remains intact. Decades of agricultural practices resulting in little plant cover during parts of the year left Blackland Prairie soils susceptible to erosion. In fact, the Blackland Prairie has one of the highest rates of soil erosion and loss in the state of Texas generating the potential for post-settlement alluvium to be widespread throughout the region. The recognition of this post-settlement alluvium is a vital part of any archaeological investigation and enables us to make better and more accurate assessments of the archaeological potential of floodplain settings.