You can think of the impressive front of the bore as a change of behaviour on the tide's behalf. Tidal waters are being forced into river water (and the water from the end of the last tide) travelling in the opposite direction towards the sea faster than either water can reverse direction. A mass of water therefore it builds up. Because it can't push upstream fast enough, the pressure behind it causes it to change form into one that can ride over the riverwater and rapidly make up the distance it has lost (release the pressure behind it). The form largely depends on how deep the tide is relative to the river water it is overrunning¹. The drawings below show three different forms, with tidal height increasing left to right...

Three types of bore a) Undular jump: river water moves undulates through the bore. b) Weak jump: river largely flows under the ramp, but in places at the front curls back in turbulence to give small waves c) Steady Jump: The river leaves the bed in jets which run up through the interior of the ramp, with much surface turbulence (after Lynch*).

Sites with further information and viewing details for the Severn bore.