Seymour Narrows is a 5-kilometre (3.1 mi) section of the Discovery Passage in British Columbia known for strong tidal currents.

The section known as Seymour Narrows begins about 18 km (11 mi) from the south end of Discovery Passage where it enters the Georgia Strait near Campbell River.

The narrows were named for Rear Admiral Sir George Francis Seymour who commanded the Pacific Station from 1844 to 1848.

, i.e. one hundred million, which is possibly the largest Reynolds number regularly attained in natural water channels on Earth (the current speed is about 8 m/s, 26 ft/s, the nominal depth about 100 m, 330 ft).

On April 5, 1958, after twenty-seven months of tunneling and engineering work, Ripple Rock was blown up with 1,375 tons of Nitramex 2H explosive making it the largest commercial, non-nuclear blast in North America.