A tower with spars to measure rising water would tend to catch hold and drag just about anything that came its way. A pyramid shape will allow an efficient water flow, but its peak may not be easily seen from a distance and we'd be talking about a lot of square foot and cubic area. Tall buildings are going to be top heavy in flood conditions and likely to be unstable.

I live in the mountains and it's rare, but we do have floods in especially heavy storm conditions. People are killed just simply by driving into water where they don't see the pavement and have driven off the side of the mountain. You wouldn't dare take a boat on a flooded lake because you can't tell where the docks are located, and sometime the tops of boathouses (boat garages) will be covered. If the weather report tells that the water is rising so many feet, no one has much of a clue just what water level that will be.

The most serious problem is the potential for the push inland and uphill of water like the instance of the Charleston earthquake. It's been a few years since I've heard ol' timers tell the story, but the Charleston earthquake at the coast of the Atlantic flooded the foothills of the mountains 300 miles upriver. People knew the water was coming but didn't know where to get to with their farm animals. They could not tell when the water would stop rising.

If I was asked to build a high-water marker because we knew high water was coming, I would build a ring so that people could see it from the mountains miles away. A ring design, with the "V" form edge we see on the Ring of Life, will allow most anything to flip around and pass through or pass by the side of the high-water marker.