The Klong Ngao estuary in Thailand is a 7·5-km long tidal creek facing the Andaman Sea and drains 11·5 km2 of mangrove swamps. Physical processes in the estuary differ greatly from the wet season to the dry season. In the dry season, vertical homogeneity prevails and the swamp behaves like an evaporation pond. Salt and water are trapped upstream, longitudinal gradients result and, through tidal dispersion, nutrient outwelling may result for SiO2, possibly NO2 and NO3, but not PO4. The outflow is trapped in a coastal boundary layer. In the wet season, short-lived local floods generate a strong stratification in salinity and episodical flushing of the estuary and may make measurements of nutrient budgets inconclusive. The Klong Ngao mangrove swamp traps land-derived sediments in the wet season. © 1990.