To go over to the river bend you must first get past the fly infestation. There is a bug sprayer over under the bridge that leads to Bastion Falls. Go accross the rafters and pick up the sprayer. Then go back and spray those bugs.
That sprayer was Amelia's father's. It didn't mean anything to you. It was just a way to get to that big rock. You, Amelia and Frank spent countless hours out on that rock.