No spoilers for Strange Aeons!
Chapter 3 of the Strange Aeons adventure path is out, and the gazetteer section is on the Sellen River (including the East & West branches). The East Sellen, of course, runs right through the Hooktongue Slough, site of Blood for Blood.
Anyway, there's an item in there that in 4714 (so, 3-4 years after the usually assumed start time of the AP, if I recall correctly), Noleski Surtova had "The Chain" installed across the East Sellen, just inside Brevoy's borders. Its stated purpose is to keep River Kingdom traffic out of Brevoy... although if said traffic could make it through the swamp and the lake, I don't think a chain is going to stop it! :P
The item also refers to Noleski as "King", which is a departure from the status quo in Kingmaker.
Chain actually is meant as "chain or boom" here. This is... well, read on wiki: https://en.wikipedia.org/wiki/Boom_(navigational_barrier)
To say it short: it is a number of floating items chained together or an actual chain set up in a way to block small to medium ships from passing. in a river that doesn't allow large and heavy ships to get past, this is pretty much a toll barrier.