3) Filling of artificial depressions
Grid: FILL elev_grid fill_grid SINK
Artificial and natural depression are both present in DEMs.
In order for river channels to flow to their mouth at a water
body, the course of the river must follow a monotonically
decreasing path, as is defined by the flow direction grid.
Consequently, sinks must be eliminated except at terminal
points for water accumulation, such as inland seas.
Certain sinks may also be selected to remain sinks, either
manually or by using automated procedures which take into
consideration such things as the depth and area of the sink.