Bores can disappoint, because of various factors, even if the predicted tide is very high.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.It arrives at the Saltney Ferry footbridge about one and a half hours before HWL and then takes a further half-hour to arrive, somewhat reduced, at Chester. The Dee Bore may be seen at the old road bridge at Queensferry about two hours before High Water Liverpool (HWL). However lower tides can produce good bores if other factors are favourable such as a period of dry weather reducing fresh water flow in the rivers. They are at their best when very high tides are expected above 10m at Liverpool, which occurs on only a few days each year. Two north-west rivers that produce bores are the Dee and the Mersey. This is a consequence of high tidal ranges occurring in several locations around the British Isles – three rivers with notable bores are the Severn, Dee and Mersey. Of the one hundred or so rivers around the world known to produce bores, around a fifth of these are in the United Kingdom. In a few rivers, the behaviour is remarkably different and the onset of the flood tide is marked by a distinct, sometimes very vigorous wave – a bore. In most tidal rivers the change from ebb to flood is gradual: the ebb current downstream slows, there is a period of slack water, then slowly the flood tide starts flowing upstream.
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