If you built a damn and threw one bucket of water in is it actully possible for canyon to fill up? i've seen interesting things happen with water.
For instance I accidentally placed a tnt block while tunneling under the ocean. The water came strait down but didn't flood my tunnel like it would in reality.
In another instance dropped a bucket of lava and it went from the top of my mine all the way to the bottom. I didn't realize this till after I had dumped a bucket of water on it and it also flowed to the bottom of my mine. This was from the surface all the way down a diagonal tunnel so quite a long ways. But If I place it on a flat surface it just builds up and goes so far.
It would be neat to be able to fill one of these canyons with water but not at the expense of running buckets of it back and forth. I've noticed that on gradual drops water seems to go further then on a flat surface. That makes sense but it also gets shallower towards the bottom instead of the top.