I have a note somewhere in my own caving diaries of standing at Hull Pot watching a 10 metre wide river emerging from its downstream lip and roaring off down the "dry" valley beyond. (From memory this was one of those scenarios when it'd snowed for a week then dumped a load of rain on top, effectively getting many days' precipitation coming through as a single flood event.) Hull Pot itself was an ominous swirling lake with various icebergs floating about.