A river system is something that appears to have a "purposeful arrangement". But we understand that it is merely the result of water flowing downhill.
Ooooo! Good one!
How about mud puddles? They seem to possess a "purposeful arrangement" -- they are arranged perfectly to fit into the pot holes they occupy; however could that happen without a designer?
And then there are legs.... did you realize everyone has legs that are EXACTLY the right length such that their feet precisely reach the ground - no more, no less! What are the chances of that happening by random chance? Clearly, a "purposeful arrangement" to say the least!