TIDAL BORES: the geology of aquatic bores


tidal bore diagram


By Scott A. Rowan

In our series of stories on tidal bores, we examine the geology, location, and dangers of bores. In this article, we’ll specifically look at the geological conditions that cause bores. all rivers along a coastline will experience a bore. There must be several specific land formations under the water for a bore to happen:

• the river/stream must be fairly shallow
• the river’s outlet to the open water must be narrow
• the river’s estuary (where river meets ocean/sea) is wide and flat
• the fluctuations between high and low tide must be large with a minimum difference of approximately 20 feet (6 m)

Tides along coastlines are predictable, but tidal bores are not (which a few exceptions below). The rarity of bores – seeing a tidal wave so large it reverses the flow of a river for a short period of time – can easily lead to death. Though the river level may only rise a few feet, tempting locals to river surf or kayak and amateur photographers to get a great photo: be warned – the energy alone in a bore can drown you, not to mention the amount of submerged debris bores dislodge (trees, rocks, structures, etc.)

Learn the what/where/why/when/how about bores.