Sediment storage and yield in an urbanized karst watershed

Abstract

In karst watersheds, sinkholes and other drainage features control the temporal and spatial pattern of sediment storage across the landscape. However, studies dealing with sedimentation in karst watersheds are scarce and the sediment storage function of sinkholes and caves has not been investigated using a sediment budget approach. In this study, we use estimates of channel erosion, sinkhole sedimentation, and suspended sediment yield to examine changes in sediment storage in the 9 km 2 Upper Pigeon Roost Creek fluviokarst watershed near Cookeville, TN. The study watershed has undergone urbanization over the last ∼ 50 years, and sinkholes and caves in the area show signs of recent sedimentation (buried tree roots, buried cultural artifacts, etc.). While sinkholes are generally considered to be sediment sinks, sinkholes examined in this study are shown to cycle between periods of net sediment storage and net sediment loss. Using copyright dates on trash items buried in sinkhole deposits, we estimated the residence time of sinkhole-stored sediment to range from 6 to 10 years. However, other evidence indicates that some sinkholes may store sediment for several centuries. We propose that sediment storage within sinkholes is controlled by several factors including sinkhole drainage area, sinkhole morphology, and basin sediment yield. In addition, changes in sediment storage in karst watersheds are contingent upon random events such as sinkhole collapses. Annual sediment yield was estimated to be 111 Mg km − 2 year − 1 for the entire study watershed and ranged from 11 to 128 Mg km − 2 year − 1 for 3 sub-watersheds. Sediment eroded from the watershed, perhaps during historic settlement of the area, is stored within a large cave system underlying the city. However, the results of a partial sediment budget indicate that the cave is presently a net sediment source. Overall, the findings indicate that the sediment storage function of caves and sinkholes varies spatially and temporally, and that these changes need to be incorporated into sediment budgets for karst watersheds.