lakes at SubRegion edges

[markkampe]

If a lake crosses a sub-region boundary, it does not fill with water in the sub-region.

• identify lakes at the edge of the new sub-region
• pick an entry point (within the sub-region) and turn that into an incoming river (with what flow rate?)
• confirm that lakes on the edge of the sub-region still exist

Analysis of the problem:

• height and rain attributes are interpolated from surrounding neighbors, whereas incoming water and suspended sediment flows are copied from the parent.
• lakes are filled as part of the process of finding an escape point, but the entry and/or escape points for lake on the edge of a sub-region may not be within the sub-region.
• there is no water flow within a lake, it has been redirected from its entry point to the escape point. As a result, points in the middle of the (parent map) lake have no incoming water and so the depression will not be filled.

Reasonable solutions might be:

• if the entry point for a lake is not in the sub-region, create a new entry point (at the height of the real entry point) on the edge of the sub-region.
• if the exit point for a lake is not in the sub-region, create a wall at the edge of the sub-region at the height of the real exit point.