Simple Tides / Guides / What Is a Tidal Bore?

What Is a Tidal Bore? The Ocean's Most Extreme Tidal Wave

A tidal bore is what happens when an incoming tide gets funneled into a narrowing, shallowing river mouth with enough force to push a genuine wave upstream, against the river's own current.

What it takes to form one

Three ingredients need to line up: a large tidal range, a funnel-shaped estuary that narrows and shallows quickly upstream, and a gently sloping riverbed. The incoming tide has nowhere to spread out, so instead of the water level just rising, it arrives as a moving wall or series of waves. Bores are strongest during spring tides, when the tidal range is largest.

The most famous tidal bores

Qiantang River, China — the largest tidal bore in the world, with waves reported over 30 feet in the most extreme sections and a wall of water visible for miles.
Bay of Fundy, Canada — home to the world's largest tidal range, which produces bores on rivers like the Salmon River and Petitcodiac River.
River Severn, England — one of the best-known bores in Europe, regularly surfed and predictable enough to have its own published bore-timing tables.
Amazon River, Brazil — known locally as the pororoca, a bore that can travel many miles upriver through the rainforest.
Turnagain Arm, Alaska — a bore tied to the same extreme tidal range that makes Cook Inlet one of the highest-tide locations in North America.

People actually surf these

In places like the Severn and the Qiantang, surfers and paddlers ride the bore wave for long stretches — sometimes miles — since it holds its shape as it travels upstream. It's also genuinely dangerous: bores can arrive faster than people expect and the surrounding mudflats and banks can be hazardous on their own.