Flying boulders

I was just testing out making a map and I made a river. Then I was randomly grabbing objects and dropping them all over the map. I added a character and then possessed them to see through their eyes, and that's when I found a large boulder floating in the air over a river. I think what's happening is that because there are river banks that drop into the river, the boulder that was placed on the water is getting placed at the height of the ground, instead of the lower height of the water.