A tidal bore is a place where the rising tide causes so much water to
enter the opening of a river that a wave forms. This requires a large
estuary that constricts to a much narrower river and/or very fast and
high rising tides on the ocean. This Wikipedia article describes
the process in detail. I have seen similar phenomena in the past
including the Reversing
Falls in St John New Brunswick and in British
Columbia, but these were more like waterfalls that formed and
dissolved based on the tides.
Hangchow Bay which is the estuary of the Qiantang River is a nearly
funnel shape. We viewed the bore from Yanguan which is in the
narrower river channel. As the water from high tide enters the bay it
reverses the flow in the river faster than the water can enter the
river. Unlike the two Canadian sites I visited the water came
roaring in as something that looked like a small tidal wave.
You can not fully appreciate what is going on without seeing it.
When I looked at my slides my reaction was "so what". It was only
when I combined the videos that I got something more like what I