The tide does the work. A bore forms when a big flood tide is squeezed into a shallow, funnel-shaped estuary faster than the river can absorb it. The leading edge steepens until it breaks and travels upstream as a wave. Britain has around twenty of them — roughly a fifth of the world's total — because it has some of the world's largest tidal ranges.
Severn dates and star ratings are the published timetable produced by Chris Witts at River Severn Tales — the same figures reproduced by Tides Today and the Gloucestershire tourism sites. One star is a small bore, five is a very large one. 2026 is a quiet year: nothing above three stars, and the three-star bores all run after dark.
Times on the other rivers are computed here. Every bore in the list rides the same astronomical tide, so the Severn timetable also fixes high water at Avonmouth (Minsterworth, minus 45 minutes). From there each river's times come from its own published offsets — the Severn, Dee and Mersey figures are the National Tidal and Sea Level Facility's, the Trent Aegir's are the Crowle Community Forum's — carried across on port-to-port high water differences measured from Admiralty predictions. Expect these to land within about a quarter of an hour.
Turn up early, and stand well back. Rain, wind and barometric pressure shift a bore's arrival by fifteen minutes or more, usually earlier. A low barometer and a strong south-westerly make it bigger; a river swollen with rain flattens it. Behind the wave the water rises fast for over an hour — at Stonebench it regularly pours over the road.