Hello Angieb,
I get "waterfall" BMs occasionally too (great description!). In my case, it is usually mild pouchitis or something that I would describe as a "stomach bug" or "upset stomach" (no way to know exactly what it was, but my bowel felt the need to give itself a good cleanout!).
I believe the yellowy colour is because it is basically digestive juices (bile), dead bloodcell waste products (bilirubin) and water coming through.
If it is pouchitis, there's a distinctive smell as well, which I can't explain in writing ...!
I found the best thing to do was take it easy and have a rehydration sachet (from the chemist e.g. Hydralyte). Also stick to bland food for the next meal – e.g. stewed apple, mashed potato, toast with peanut butter – whatever disagrees with you least.
If you can make a note of what you ate prior to the waterfall, you might be able to identify a culprit. Otherwise, if you don't have any pain or other alarming symptoms, and don't get seriously dehydrated, and it gets better by the next day, it's probably "just one of those things" that comes with the j-pouch.
Cheers,
Sarah