Odd with the humans. A trader must have died that was stuck on the map, and so they got scared and stopped trading.

And the water thing is because dwarves are stupid. Occasionally a tile must be dropping to 2 or 1, but then filling up again once the dwarf tries to walk across there. I once lost half of a starting fort when they tried to walk across the river at the top of the map and got washed downstream. A sort of reverse waterfall effect.