Why do some tides create powerful tidal bores in rivers and estuaries?

Tidal bores happen when the incoming tide has nowhere to spread out, so the water piles up and moves inland as a moving wall or surge. They are most common in estuaries with a large tidal range, a funnel shape, and relatively shallow water. Famous examples include the Severn Bore in England and bores on the Qiantang River in China. The phenomenon is caused by the same gravitational tides that raise and lower sea levels worldwide, but local geography makes the wave dramatic. Bores can be dangerous to boats, yet they also attract spectators and surfers.