This is a section of the river leaving Downfall, the Bullywug settlement in Wild Beyond the Witchlight. The Bullywugs have set up a guard post with a net stretched over the river using a capstan so it can be raised and lowered to try to prevent boats leaving the settlement without authorisation. It could, of course, be used for any river guard post.
(For anywhere else you might want to make the palisades protect the guards from up AND downriver, but the bullywugs of Downfall have set this up in a hurry, and only want to stop people leaving.)