Midnight over Sanctaphrax

Twig has the caterbird tethered to his ship, the edgedancer, and is being led into a storm never seen before, on his way to find his father. Twig soon learns that this storm is the mother storm, and that the mythical place, called riverisse is real. And is torn between a simple yet complicated choice. Cut the anchor chain of sanctaphrax, and all of the edges knowledge, to save riverisse, or have the mother storm destroy sanctaphrax, die off and riverisse run dry, thus the only clean water source in the edge, which will surely lead to peril. What will he do?