Tidal bore on the River Seine, 19th-century illustration. Caused by the twice-monthly peak tides (spring tides), this phenomenon is where a large tidal range occurs in a funnel-shaped bay, river, or estuary. In this case, it is occurring on the lower reaches of River Seine in France. This phenomenon is known in French as 'le mascaret'. Artwork published in 'Les arteres du globe' ('The Arteries of the Globe', 1888), a work on hydrology and geology by Paul Bory.