You have to be a real "pro" to weight a body down in a "proper" way, so that the body is not coming up anymore, so to say. I know a few cases where body's came up after a while, even though they were weighted down.
Is there a current in the river? Could her body been taken away with the current? As an example: A lot of body's going into rivers in Germany finally end up in the Netherlands...taken by the current, going down stream, to lower land.
About her phone pinged in the general vicinity of a dumpster transfer station (still wonder which phone, btw) It could have gotten there in many ways....not necessary meaning that Jenna's body is there somewhere to.