RECENT ALLUVIAL SEDIMENTATION IN A KARST WATERSHED

Karst regions can be viewed as a series of separate surface watersheds, each defined by the contributing drainage area of a sinkhole. Surface water can move rapidly into the subsurface at sinkholes if swallet openings are present, thereby connecting adjacent watersheds hydrologically. Such connections are often determined using dye tracing, for example. However, with respect to surficial materials (i.e., alluvium), adjacent sinkhole watersheds may only be integrated to the degree that sediment can pass through conduits at swallets. For example, if sinkholes are mantled with a thick soil cover and direct conduits to the subsurface are not present, sediment entering the sinkhole becomes trapped and does not contribute to the sediment budget of the regional watershed. This study examines the extent and nature of alluvium within 152 sinkholes on the East Highland Rim, near Cookeville, TN. Soil survey data indicate that 52 percent of sinkholes in this study contain alluvium, 24 percent consist of residual soils, and 24 percent hold a combination of alluvium, residuum, or colluvium. Sinkholes containing alluvium have a significantly larger average drainage area (19 ha) than sinkholes containing residuum (0.7 ha). The residence time of alluvium within three sinkholes, as determined by ¹⁴C analysis and the presence of recent cultural artifacts, ranged from tens to hundreds of years. Results indicate that some sinkholes cycle between periods of net in-filling with alluvium, followed by periods of net sediment loss to the subsurface via conduits. Sinkhole sedimentation has several implications for geomorphology and watershed management: 1) sinkholes contain a sedimentary record of past land use or other environmental changes in a watershed; 2) sinkholes may be separated from adjacent watersheds with respect to sediment transport, thereby affecting regional sediment budget calculations; and 3) sinkholes may act as sinks for heavy metals or other contaminants that adsorb onto soil particles.