Annual patterns in hydrology, P circulation and sediment dynamics were studied in a southern Illinois, USA floodplain swamp dominated by bald cypress (Taxodium distichum) and swamp tupelo (Nyssa aquatica). The swamp''s interactions with the adjacent river were emphasized. For the year, major inputs of water to the swamp were throughfall (74.3 cm) and runoff (69.4 cm) with minor contributions due to groundwater (21.6 cm). Outflows were by evapotranspiration (72.3 cm), surface outflow (56.5 cm) and groundwater (21.0 cm), with the latter 2 draining primarily to the river. A flood occurred during the study period, passing 1.6 .times. 107 m3 of river water over the swamp and depositing 0.06 cm of sediments. An annual P budget was developed for the swamp from field measurements. The greatest input of P to the swamp was 3.6 g P.cntdot.m-2.cntdot.yr-1, due to deposition of high-P sediments during the flood. This was 10 times greater than the outflow of P to the river, 0.34 g P.cntdot.m-2.cntdot.yr-1, and 26 times greater than the throughfall input of 0.14 g P.cntdot.m-2.cntdot.yr-1. Total tree uptake from sediments was 0.87 g P.cntdot.m-2.cntdot.yr-1 of which 0.77 g P.cntdot.m-2.cntdot.yr-1 returns as litterfall to the swamp sediments. Duckweed productivity was estimated to take 3.3 g P.cntdot.m-2.cntdot.yr-1 from the water column and deposit this in the sediments during die-off. For the period 1937-1967, cypress growth, based on tree ring analyses, was closely correlated with several measures of flooding frequency and magnitude, all obtained from past river data. Tree ring data prior to 1937 showed poor correlation with flooding, probably because of logging activity. Cypress growth has decreased dramatically in recent years, corresponding to the rise in water level caused by beaver activity.
