Hey, long time no forum/terragening, but I was bored today taking a break from school work and was looking at this (and playing with new update) and then expanded on this clip a little.
Basically taking this clip, then extending the idea by allowing it to follow terrain (like a river flowing down a mountain side, all easily painted).
I used a plane object instead of a lake (I believe this is the same method dandelO used to make a waterfall), and sent the computed terrain as input into the water area mask.
I created a very basic distribution shader that was blended by the painted shader, which then attaches to a displacement shader right before compute terrain, which has a negative multiplier allowing it to make a ditch.
That distribution shader also goes into the displacement of the "water surface - plug in here" layer, which then has a displacement multiplier of positive 4 in this image I believe.
Anyways, just thought I would share that quick modification possible to turn this clip into a very nice paintable river for down hills/rapids/whatever you wish.
Heres a quick scene I whipped up to demonstrate it, and a screenshot of the relevant node network.
