http://img809.imageshack.us/img809/4364/river.png
In the above image, I can't move at all to the right or up due to some mysterious obstruction. Attempting to move around it only leads to more mysterious obstructions. Even trying to lug my boat over land results in me banging my nose into more invisible walls. There's no big flashing claim indicator or anything, just the same kind of response you normally get when you try to walk through a solid object. What could be causing this?