Chincoteague Bay receives an estimated 90,000 m 3 of sediment annually, of which roughly half is sand and half mud. Sand is derived principally from the barrier, Assateague Island, with storm overwash being twice as effective as eolian transport in supplying sand to the bay. The mainland of Delmarva Peninsula is the primary source of mud-size sediment for the bay. Annual contribution of mud from shore erosion is approximately eight times that introduced by streams. Sediment transported from the Atlantic Ocean through the two active tidal inlets is important only in the immediate vicinity of the inlets. Present average rate of sedimentation in Chincoteague Bay estimated from annual sediment supply is 0.3 mm/yr. This is far less than the rate of 1.5 mm/yr, obtained from sediment thickness in borings in the bay and believed to represent average conditions over the past 5,000 years. The present anomalously low sedimentation rate and accompanying decline in salt-marsh growth are attributed to a decrease in ocean-derived sediment caused by the closing of former tidal inlets through Assateague Island. If this interpretation is correct, it underscores the wisdom of a recent decision against dune stabilization for prevention of storm overwash on Assateague Island. Overwash is a major means of providing sediment for landward accretion of barrier islands in response to relative sea-level rise. Elimination of storm overwash in the absence of sediment influx from the sea via tidal inlets almost certainly would have resulted in long-term net erosion on the lagoonal shore of Assateague Island.