"We huddled together on the bank of the river in the cool spring night, drinking tea and warming our hands by the fire. The full moon shone intermittently through the clouds, illuminating the river below. Suddenly from downstream came the cry "Flood-O!" We ran higher up the bank with a low, rumbling sound in our ears. As we reached the top of the bank a wall of water four feet high passed below us, moving faster than we could run. The wave was moving upstream, carrying with it pieces of wood and other debris. In the few minutes it took for the wave to move out of range of our hearing the water level rose another two feet. Gradually the turbulence subsided into gentle eddies, and after an hour the river began flowing towards the sea again." (Lynch*)
The Severn Bore is a large wall of water (up to 3m)1 that regularly rushes up the river, temporarily reversing the flow so the river appears to flow uphill. It is, quite literally, a tidal wave - caused by the tide in the Severn estuary. Usually the tide comes in on beaches imperceptively slowly, save on very flat areas, where it can be seen racing across the sands. Even in most rivers invaded by tides the change is slow. However, in some rivers, where the coastline funnels and shallows inland*2, the water builds up into a wall, or wave, of water known as a "bore"3,4 which races up the river as a single big wave5.
You can find details of when and where to view the bore here, or go on to look at how bores form.