Fantastic thread. Collates a lot of information that will be very useful to a lot of people. One thing I need to ask is: what happens in wetland areas. What causes the high amount of groundwater in these areas and how does this impact on river flow and lake formation in them? Considering there are some very large wetlands in the world, and in a pre-industrial world such and the ones most people here seem to map, I'd think they'd be a major feature on the land around.