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Those who have witnessed tidal bore roaring up a ravine are astonished that water does not seem to respect the relative sea level during those times. What makes the water climb? Water pressure drives water to climb above its level because at the point where the pressure build, it takes the easiest path. When the force of pressure is extreme, compressing the water at lower levels, the path of least resistance is taken. During a tidal wave, this path is away from the bulk of water. A tidal wave moves inland until one of two situations occurs:
Where tidal waves meet mountains, this can result in tidal bore up ravines. Where tidal waves flow inland, this results in a flood tide going hundreds of miles inland. Where the tidal wave finds foot hills or barriers, the force of the wave is broken such that it is slowed, allowing a reduction in pressure behind the wave to arrive before the wave moves far inland. But where the tidal wave finds virtually no barriers, due to the land being flat, it becomes water on the move, and this very momentum carries it far inland, and above a height that would otherwise be expected.
Note: below added during the Nov 30, 2002 Live ZetaTalk IRC Session.
There is more to the level that water rises than simply water finding its level. Tidal bore carried water up-river in a rush, rising above the sea level with force, for many miles. Tidal bore carries water up ravines, in some cases shooting almost straight up, appearing in a gusher at the plateaus overhead. One factor to consider is the force or pressure that water is under, and this is equivalent to the water higher, elsewhere. This is the factor that allows cities to have tap water, due to water stored at a high point in the vicinity, which then pushes out into pipes. Water on the move does more than just push forward, it also creates a void behind it. At first, this water is on the move because there is pressure behind it, like sloshing water in the Gulf. But then, the force of this moving water takes on a life of its own. It has momentum, and moving forward, creates a void behind it, thus drawing the water in the direction of motion, thus continuing the motion. Thus, rolling across the flat low lands such as the State of Texas, and moving hundreds of miles inland, on a roll, it does not simply stop when it starts to reach foothills. Why would it stop? Because the overall elevation is more than the sea level? Weigh this, with the force of water, all on the move, behind the lip of the flood tide. This water on the move is greater than the resistance in front of it, so it continues.