You don't just die from falling in 4 feet of water. She was also wearing a massive padded coat, so it would have taken 10 seconds for the cold water to reach her skin, so long enough for her to get out and not have cold water shock. Also, in 4 feet of water, from the chest up, she'd be out of the water just standing up. And it isn't a fast flowing river, so she wouldn't have just gone in and then whoosh out to sea. She could have just walked out onto the bank.
The only plausible thing I can think of is that a sand bank could have collapsed and she's gone down into a sink hole with the sand and mud going in on top of her then the river has covered it up.
That would explain the dog and the phone still in the same spot.