I took a look at the basin filling algorithm and I agree that it would help but I also think I am doing quite a lot of that math in the process of just doing the water flow so I am thinking that I should fix up the single pixel issue by arbitrarily adding some height to the low pixel.

My fluid flow will automatically breech the rim of a full basin and it ought to start eroding it. I think my issues have been in the area that if the water exits out of the top at about the rate it evaporates or somehow rounds down or something like that then its just not causing it to punch a decent sized hole in the side to let it flood out and wash away the bank. What I need also is for static water to drop sediment and then it would rise up and overflow all by itself. But I have found a neat way to mimic that semi automatically which does seem to work ok. I'll investigate and add some more lines of code to the sim if I can work out a stable process.

I think if I had a climate model that varies the rain fall more dramatically then I think there would be times where it pours down momentarily so that it overflows a lot and then dries up a bit for a while. That variation might be needed to stop this steady state thing going on.