This is some fantastic information here.
The infinity pool idea is exactly the sort of thing I was thinking. A huge, mellow lake that spills over the side of an island's cliffs. However, it seems like no matter what we're talking a bout a river here, not a lake or slow-moving shallow body of water, especially one that's relatively free of sediment. That's fine. It just means I'll have to split the two ideas, so I'll have a very wide river going over a basalt lip or using the travertine process causing a looooong waterfall over the edge of a cliff, and then I'll have a very shallow lake elsewhere on the map. It's not exactly what I had in mind, but I'd rather it make some geological sense. Then again, maybe I'll still find a way to make it work, but right now I'm leaning towards "no".
Korash, I do like your idea, but it's not what's going on in my story.
Any more thoughts on this thread are still appreciated!
Thanks.