The Great Barrier Reef is similar to some other coral reefs in growing right up to the edge of the shelf in a region of nutrient-poor surface water but large tides. It is suggested that the resultant strong tidal currents suck in nutrient-rich water from the depth where the tidal flow speed drops to the fastest internal wave speed. The nutrients could encourage the reef to grow vigorously at the edge of the shelf. Some observations in Cook's Passage (14¿32'S, 145¿34'E) were made to test the concept and are encourging.