In this case round isn't describing an object, it's describing what the water is doing. The obstacles could be anything. I think most people would generally use "around" as opposed to "round" as it sounds more natural.

It's describing how the water moves. Also the word "round" in this context is actually a colloquial way of saying "around" and is not referring to the shape of the objects in the water. If you can imagine large, stationary objects in a fast, flowing river, the water always become more chaotic and the current moves down and around the object the water passes around. The word "flicked" describes how suddenly the water changes behavior as it hits an object, since the verb "flick" refers to a sudden and swift movement. I hope this explanation helps. If anything else seems unclear, please ask.