Runoff Capture by Sea Level Rise Alters the Area, Geometry, and Quantity of Coastal Catchments

Abstract Coastal catchments, which drain directly to the sea, mediate material fluxes across the land‐sea interface and support species diversity and economic activity in coastal zones. Here, we explore the evolution of coastal catchment divides at the local to global scale from 2000 to 2120 under multiple SLR scenarios using state‐of‐the‐art digital terrain and hydrographic models. We show that coastal catchment area loss due to inundation is partly counteracted by inland migration of coastal catchment boundaries and new coastal catchment formation. Encroaching shorelines intercept runoff from interior watersheds. Runoff within these shoreline runoff capture windows bypasses stream networks and discharges directly to the ocean, resulting in net coastal catchment growth along 14% of the world coastline within a century, even under high sea level rise scenarios. The rate of coastal catchment expansion and formation by shoreline runoff capture is nearly one‐half the rate of coastal catchment area loss from inundation by sea level rise, indicating that shoreline runoff capture plays a key role in coastal catchment evolution. Contrary to the notion of static watershed divides, these results reveal a counterintuitive tendency of geologically rapid expansion and reshaping of coastal catchments in future decades. The findings have important implications for watershed and coastal water quality management, as the dynamic response of coastal drainage boundaries occurs on timescales relevant to coastal planning and decision making.