Exceptionally high rainfall levels accompanying instability of the Intertropical Convergence Zone in 1972 greatly expanded the freshwater realm on Fanning Island. Changes in the head of the groundwater body (a Ghyben-Herzberg aquifer), as measured at frequent intervals in 11 wells, revealed variations in sediment permeabilities but proved an unsuccessful technique for determining amounts of freshwater discharge into inlets along the lagoon shore. The aquifer was found to store freshwater and then maintain reduced salinities in the inlets long after an initial salinity depression during a period of precipitation. The spatial and temporal distribution of salinities in the inlets closely resembled those of an estuary. However, the factors contributing to salinity fluctuations in each inlet are sufficiently complex and show both regular and irregular patterns of temporal variation, so that the inlets constitute highly unpredictable environments. The biological implications of this unpredictability are deemed interesting because of the close proximity to the predictable and reasonably stable environments of the shallow-water lagoon reefs.