Floating down from Big Current is a lost Garbage barge.
The characters checked it out - and something seems to have happened to it's handlers and the gelatinous cube.
They fought off smell mephits, mutant catfish that could balloon up and float to attack (although not very well, as they rolled a dice in order to be able to hit), large rush monsters, the handlers who were now in the shape of gelatinous cubes, and some metal junk yard dogs.
After managing to take possession of the heap - they ran it aground on a beach, down river. The rats, flies and smells were generally abhorrent, and the city officials quickly managed to arrange to get it toed back to Big Current. The company who manages the garbage was pleased to have it back (and sorry that the rust monsters were killed) and gave a sizable reward.