Just... why. Why can't I change this?

So... explanation time.

I want to remove part of a river on my island. Everything is going well, except for this. I can't get rid of the river opening.

Why not?!

How can I make this not look stupid?!

I really hope they add in a feature that lets you remove the river openings if you want to. Because right now, it looks really funny.

I just- I only needed more space on my island, which I can fix. But I don't want it to look stupid. What do I do? Any tips?