Tidal bore

As the tide rises, seawater moves inland through an estuary. In certain river systems, the incoming tidal wave is compressed and steepened as it enters shallow or narrowing channels, producing a distinct front that travels upstream against the river's usual current. The resulting feature is a positive surge: water level rises abruptly as the bore passes. Formation and strength depend on multiple interacting factors, including tidal range, channel geometry and depth, and the river's outflow; a large tidal range alone does not guarantee a bore.

A bore may appear as a single breaking front or as a series of smaller waves, depending on local conditions. It can advance upstream while the water behind its front continues to rise with the tide. The U.S. Geological Survey's global catalog describes documented bores as moving, tidally generated waves and notes that they often occur in shallow-gradient, meandering estuaries. Its survey identified reported occurrences in dozens of localities, while also noting that documentation is incomplete in some regions.

Tidal bores are geographically selective rather than a feature of every tidal river. The Severn River system in the United Kingdom and the Qiantang River in China are well-known examples; the Amazon's bore is called the pororoca. The exact configuration of the river mouth, estuary and upstream channel matters, and the strength of a bore can vary with the tide and changing river conditions. Named examples should not be taken to imply that all bores share the same dimensions or behavior.

A bore can stir sediment, rapidly change water depth and current, and affect conditions for aquatic organisms and river users. Strong fronts and currents can be hazardous to boats, swimmers and people standing near low banks. At the same time, tidal bores are studied for their hydrodynamics and ecological role. In a 2025 study, researchers used data from the Surface Water and Ocean Topography (SWOT) satellite to examine water-surface elevations in the Severn system over three days in April 2023, demonstrating a way to observe tidal waves across coastal and inland waters.