I've come across a weeping willow that looks exactly like the one in this picture before. The weeping branches are always just like what is show here: they stop just before the top of the water, all at exactly the same level! I've often wondered why that might be. The tree probably doesn't grow that way, that seems unlike any other tree growth I've seen. So my guess is that little things in the water - fishes, frogs, and things like that - must just nibble away at the tiny bits of branch and leaves that get close to the water. And then, over time the efffect is that the branches are all neatly trimmed, just above the water line. Natural pruning at work!