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Airai Bay, Palau, is a small (3km²), semi-enclosed, mangrove-fringed, meso-tidal, coral lagoon on the southeast coast of Palau. It drains a small catchment area (26km²) of highly erodible soils in an area with high annual rainfall (3.7m). River floods are short-lived and the sediment load is very large, with suspended fine sediment concentration exceeding 1500mgl⁻¹. The resulting river plume is about 2m thick. The brackish water residence time is about 7 days; during this period the plume remains a distinct surface feature even after river runoff has ceased. About 98% of the riverine fine sediment settles in Airai Bay, of which about 15–30% is deposited in the mangroves during river floods. This mud remains trapped in Airai Bay because the bay is protected from ocean swells and the tidal currents and locally generated wind waves are too small to resuspend the mud in quantity. The mud is smothering coral reefs …