Most likely only a little bit would actually be piped. You’d just stop water from entering one watershed and send it into another and let gravity do its work. You would lose more to evaporation; but that happens to water in pipes too if they’re above ground.

Would love to see the numbers.

It’s fun looking at rivers from the Rocky Mountains in California that don’t make it to the ocean anymore.

I recently proposed a transatlantic natural gas pipeline and people thought I was insane, but it would only be ~5x longer than the longest existing underwater natgas pipeline, which doesn’t sound impossibly viable.