This paper considers the hydrodynamics of wave-driven flow across a coral reef and the resultant flushing of its lagoon. Current depth coefficients are introduced that describe the fractional change of across-reef and lagoonal current with change in the relative local sea level, i.e., the depth of water over the reef flat. The coefficients are derived from an analytical hydrodynamic model and compared with data from two reefs. The first is Ningaloo Reef in northwest Australia, which is a typical barrier reef with a narrow coastal lagoon, and the second is Kaneohe Bay in Oahu, Hawaii, where the reef is both unusually deep and wide. The current depth coefficients are shown to be sensitive to the form of the frictional law on the reef flat and can be deduced from measurements of the astronomical tidal currents in the lagoon. Both quadratic and linear friction laws are considered for the reef flat and good agreement obtained with a linear law based on the high-frequency motion due to surface gravity waves. ¿ 1999 American Geophysical Union