One Tree Reef, in the southern Great Barrier Reef, is roughly triangular, with the southeast corner forming the windward margin, and the northwest side the leeward margin. In March/April 1979, water circulation and sediment flux were monitored over the reef margins. Generally, water moving from the ocean across the perimeters decreases in velocity when it reaches the lagoon. Over the windward margin coral and algal flats, water movement patterns are similar; i.e., in both environments flow reversal characterises the spring tide flood/ebb change, whereas on neaps, flow is continually towards the lagoon. On the southern prograding sand sheet, between the coral flat and the lagoon, flow is continually towards the lagoon. On the leeward margin, the flood/ ebb flow reversal is again a dominant feature. Sediment flux for the different reef zones reflects the water flows. On the windward algal and coral flats, twice as much sediment leaves the reef towards the ocean on spring tides as is transported to the sand sheets, where there is a net loss; on neaps however, suspended sediment movement is continually towards the sand sheets. On the sand sheets, all sediment flux is towards the lagoon, with twice as much arriving in the springs compared with neaps. Spring tide sediment losses over the leeward margin are greater than neap tide gains made by the lagoon over the same margin. The distribution of mud in the lagoon, its concentration at the leeward margin, and its absence on the sand sheets can be explained by the observed energy pattern. Sediment is lost from the system mainly by spring tide flushing of the lagoon, most sediment leaving over the leeward margin.
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