Seismic profiles and drilling show that lagoon reefs have grown on topographic highs constructed by fluvial, deltaic, and/or marine processes. Levees of rivers produced topographic highs which became major sites for late Pleistocene coral colonization. Holocene reefs have grown on these carbonate mounds, atop deltaic sediments and on marine sand bars. The unique (rhomboidal and long-sinuous) reefs are derived from the underlying early Pleistocene river morphology (Victoria River) rather than from a fault-dominated karst morphology. Lagoonal reef formation and/or modification of reef foundation controlled by alternating sea level fluctuation and coastal plain fluvial processes.--Modified journal abstract.