Fine Sediment Erosion and Transport to the Near Coastal Zone from Watersheds of St. Thomas, USVI

Abstract:

The US Virgin Islands‘ landscape is characterized by steep slopes and short distances from ridge peaks to fringing reefs. Fine-grained sediments eroded from predominantly thin soils may be transported rapidly by streams (locally called guts) to the sea and cause stress to corals. We have studied erosion and transport processes on St Thomas by three methods: (1) continuous monitoring of suspended matter in one of the island’s few perennial streams, Dorothea Gut, (2) measurement of ¹³⁷Cs inventories in soil cores taken across the landscape, and (3) evaluation of sediment captured in most of the island’s coastal ponds, through which a significant portion of runoff must pass. We find that, for areas that have not been recently disturbed, watersheds retain fine sediments surprisingly well. On the other hand, small patches of land, like building lots that have been recently disturbed and poorly managed, can produce disproportionate amounts of fine sediment. These results differ somewhat from nearby St John, USVI, where unpaved roads are the major source of eroded sediments.