The Best Thing About Surfing China’s Rivers Is That After You Get Dipped Into The Water A Few Times, You’re Rustproof
A tidal bore, from Wikipedia:
Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than 6 metres (20 ft) between high and low water) and where incoming tides are funneled into a shallow, narrowing river or lake via a broad bay. The funnel-like shape not only increases the tidal range, but it can also decrease the duration of the flood tide, down to a point where the flood appears as a sudden increase in the water level. Note the tidal bore takes place during the flood tide and never during the ebb tide.
A tidal bore may take on various forms, ranging from a single breaking wavefront with a roller — somewhat like a hydraulic jump — to “undular bores”, comprising a smooth wavefront followed by a train of secondary waves (“whelps”). Large bores can be particularly unsafe for shipping but also present opportunities for river surfing.
(Thanks to MisterArthur for sending that one along)