On the highest tides, a most unusual sight occurs in the Mersey Estuary: a tidal bore travelling upstream from Widnes and Runcorn to Warrington. Although on a smaller scale than the better-known Severn Bore, it is still a great example of this natural phenomenon and one of only about twenty that occur in the UK.
What to see
The tidal bore forms when water levels rise so quickly that a wave forms as the tide comes in, meeting the river flow. You can read more about how they form here, along with related phenomena such as mountain waves.
At its best, it reaches heights of 60cm or more, accompanied by the sound of rushing water and waterbirds startled into flight from the sands and mudflats of the estuary. This can provide great birdwatching opportunities as well.
The first signs appear close to the former lighthouse at Hale Head, to the west of Widnes, and the wave gains strength as it approaches Widnes and Runcorn. Good viewpoints include Pickerings Pasture Nature Reserve and Wigg Island in Runcorn.
The wave gains speed as the channel narrows towards Warrington, then ends suddenly on hitting Howley Weir, near the town centre. Sometimes a low reflected wave is seen further downstream a few minutes later.
Predicting the tidal bore
As with any natural phenomenon, there are many uncertainties in predicting when the tidal bore will occur. Some factors which can help include:
- A high tide of 10 metres or more at Liverpool (Gladstone Dock)
- Low river flows in the Mersey, such as after a dry spell
- Little or no wave action, such as when winds are light
The National Tidal and Sea Level Facility gives the following Tips for observers:
- It is better to arrive half an hour too early than a minute too late – rainfall, wind and other factors affect the time of arrival of the bore – its appearance cannot be predicted with certainty
- Bores can disappoint, because of various factors, even if the predicted tide is very high
- If you can go a number of times you will have a better chance of seeing something quite awe inspiring.
The best viewing times are typically in Spring and Autumn, from about three hours before high tide in Liverpool when the wave first forms to shortly before high tide by the time it reaches Warrington.
As with any activity near water, it is important to choose a safe viewing location, remembering that water levels rise quickly as the tidal front passes, by several metres in places, and never venturing out onto the sands or mudflats. The RNLI website has some excellent advice on water safety.
If you would like to learn more about the Mersey Tidal Bore, my book the Mersey Estuary: A Travel Guide gives more tips on when and where to see it, along with suggestions for places to visit, walks and cycle rides and background on the history, environment and wildlife of the estuary. It is available from most bookshops and online stores (www.troubador.co.uk). This YouTube video about the book includes some brief footage of the Mersey Tidal Bore.