@Martin: no it is not good enough for me too. I have now tried 'fake water' with a reflective shader more or less transparent, and some vague color on the surface (by using luminosity). Much faster, that's the plus side of it. I also used a painted shader which works much easier, but still used the favor depressions as well. BUT. Still not good enough, and another one is rendering....
The melting water wouldn't actually have to be real 'lakes' as it would run across the terrain, and merely wet the soil and stones up to a level. I think it would be feasible to displace some puddles downwards (painted shader, through mask+camera or crater shader) and put some small lakes in there. Lot of work though.
@njeneb: I have tried a lake with the same displacement as the main terrain, but didn't have the time to pursue that. It might work though, I've done it before. But it would probably take a long render time.
---Dune