It is easier to explain how the bore forms if we separate the cause of the bore from what is going on inside it. Bores are caused by the flooding of tidal waters up a river. Normally tidal rivers don't have bores, either because the tidal range isn't very big (broadly meaning that there is a smaller rise in waterlevel over the same time period, and it can therefore creep more gently upriver) or because the angle of the land is steep enough that each metre of tidal rise only creeps alittle way inland. However, on rivers with a large tidal range1 and low-lying land, the water overflows the river water and rushes back up the low-lying land*. Because the river water is in the way (and the river bed has some friction) the water at the front of the tide can't move upstream as fast as the tide is rising downstream, and a wall of water, pushed from behind, builds up at the junction between the tide and the river***.
What's going on inside a bore?
Diagram: The water of a deep tide need to flow further inland on a low-lying riverFigure: As a normal tide hitting steep land (left) rises from 'a' to 'b' the water has less distance to travel inland than a tide hitting low-lying land (right). In addition, a higher tide 'c' has even further to go. (diagram after Lynch*)