I have been trying to figure out how far inland water would reach if a tidal wave, or tsunami wave, let's say 100 feet high, would go. Would it be no more than 20 miles, or would it be more like 200 miles. If anyone knows, I'd appreciate if you'd share. Thanks.

I'd say it would be closer to 200 miles ...

That's what I thought too.

And it would depend on the elevation of the land over which the water flows--think of a coast with very gradual rise, as opposed to one with tall cliffs high above the water, and increasingly tall hills behind those cliffs.

Just thinking out loud here, if you look at 100' as the y axis and and the distance it travels (which you don't yet know) as the x-axis, then you could look at topographical maps of the specific area in question to get the slope of the coast land and land further in. You could easily see it by drawing it on paper: draw the exact point where the water enters the land, and at that point draw a vertical line, with height of 100'.

Now at the top of that 100' line, which is the y-axis, draw a second line, this one horizontal, and this is the x-axis. Allow that horizontal x-axis to go for as long as you have paper, since we don't know how far the tsunami will go yet.

Once you know, or guesstimate the average slope of the land from the water's edge, you can draw that slope as the land over which the water will flow. Draw that sloped line, beginning at the bottom of y-axis, and sloping upward, and representing the land from coast inward, and continue to draw the same degree of slope until the "land slope" intersects with the x-axis. It is after this point that, AFAIK, the tsunami would lose steam more rapidly.

IIRC the velocity of a tsunami begins at about the 600-some mph, (but check this for sure), remembering that the velocity will gradually decrease as the tsunami "loses steam" by hitting obstacles, etc. If for example, the average slope of land from the water's edge is almost nil, and most land in the area is around sea level, you can see how very far the water could travel. OTOH, with a 30 degree slope of land inward, you could see how quickly the wave could reach a point where it would lose energy quickly.

Hope this helps a tiny bit.