Comparison of tidal records from One Tree Island and Heron Island, Southern Great Barrier Reef, shows that ponding of water inside the reef crest at One Tree Reef holds the lagoon water level near Mean Water Level for 5-6 h of each tide. The lagoon is therefore isolated from the surrounding ocean for half of each tidal cycle. The level of the tide with respect to the reef crest determines the impact of energy inputs on lagoon circulation. Details of the lagoon tidal curve are described and the implications for lagoon circulation, flushing and sedimentation are discussed. © 1979 ASEG.
