Rivers Influence Reef Pass Formation in the Society Islands

Abstract

Reef passes are deep, navigable channels dissecting coral reefs around volcanic islands. Many reef passes are located offshore of large island river basins, suggesting a potential causal relationship. To clarify the mechanisms that form and maintain reef passes, we quantify the relationships between reef pass location and drainage basin size in the Society Islands. River basins draining toward reef passes are larger than those draining toward unbroken reef flats, suggesting that rivers help create and sustain reef passes. The correlation between reef passes and large rivers weakens for older islands, suggesting that oceanographic processes increasingly maintain passes as islands age and subside. We propose two river-driven reef pass formation mechanisms: reef incision, in which rivers erode into reefs during sea-level lowstands, and reef encroachment, in which corals growing in lower-elevation submerged river valleys preferentially drown during periods of rapid sea-level rise, leaving gaps in the accreting reef.