Soooooo… you have this beautiful river system with scores of branches flowing into one toward the sea. Just perfect. Then, it runs into this tiny little lake just off the coast. And it comes to a dead stop.
… 
I can’t call this a bug but it’s certainly not a feature of nature. The rise and fall of water levels due to any or all of a host of events tends to open channels between any neighboring bodies of water not separated by some significant obstacle.
I suppose this is mostly characteristic of erosion, and with the pre-Hurricane Ridge Flow Restructure we had ridge carving to work with. I’m going to go back and see if that will help with my current headache, but I had to cool off and commenting on it helped. I wanted to propose something to capture the periodic changes in the volumes of water intrinsic to erosion over time. You know, flooding to complement weathering. In that, you’d see the impact of the mass and momentum of flows in these (geologically) instantaneous periods.
Yeah. Kind of an insane request to throw out there. But it’s at least worth mentioning. 
Nature does so much, well, naturally but simulating it, you have to come up with an endless series of techniques to cover them case by case. @Stephen, you have my sympathies!