Geomorphic response to historical agriculture at Monument Hill in the Blue Ridge Foothills of Central Virginia

Abstract

Historical forest clearance and agriculture have had profound effects on the landscape of the southeastern United States. This study examines a site called Monument Hill located in the eastern foothills of the Blue Ridge Mountains in central Virginia in order to better understand the long-term effects of historical agriculture on soil and streams. This site, deeply gullied and drained by an incised first-order stream, was first settled in the mid-eighteenth century and became part of a large nineteenth-century plantation. A thick sediment fan at the base of the hill buried and preserved a historical soil surface, archaeological artifacts, and woody debris datable by dendrochronology. The geomorphology of the site, along with limited historical data and tree ages from the mature second-growth forest cover, has enabled reconstruction of the sequence of land-use changes and measurement of geomorphic response over two and a half centuries. The original forest at the site was cleared between 1767 and circa 1800, probably to grow tobacco followed by wheat or corn. Soil erosion and depletion resulted in land abandonment and gullying. Forest regrowth began circa 1831, with tree establishment occurring rapidly in the 1860s and 1870s and peaking around 1888. A volume of 33,700 m 3 of sediment was removed from the gullies at an average rate of approximately 130 tons/ha/year during the disturbance and reforestation period. Only about 40% of this gully material is present in the sediment fan at the bottom of the hill, so the rest must have been transported downstream as bed and suspended load. The small stream in the valley was inundated with sediment and aggraded. After the sharp decrease in sediment supply that accompanied reforestation, the stream incised deeply into its alluvial deposits at rates up to 90 mm/year during the late nineteenth century. The study site is typical of many first-order drainages found throughout the region but experienced its peak erosive land use relatively early (by circa 1830) compared to areas further south and west. Minor stream adjustment continues to occur at this site. It is therefore likely that low-order streams in other parts of the Southern Piedmont and Appalachian regions that experienced a more recent peak in erosive agricultural land use will continue to adjust their channel morphology for decades.