The Second Assassin (spoilers for Bag A Legend)

I just had my final confrontation with the second assassin, and when I threw her overboard I got this echo:

The Boatman protests. &quotNot here! Not here!&quot Your quarry rises, but she has no weapons now; no horse to trample you with. You seize the front of her habit. She spits in your face. You push. A splash.
The Boatman watches the river carry her away. In a brittle voice, he explains what will happen to her when the slow current meets one of the other, older rivers that ring the far country.
You take your seat. The other passengers edge away.

I’m curious. What are these older rivers, and what will happen to her? And do we know if someone who got thrown on the river at any other point would be able to simply get back on the boat and recover normally?

The only thing that comes to mind is a season tie in, where we convinced death to approach the far shore and witnessed people begging for devils to take them. Wherever she ends up, its likely not pretty.