A flat area watershed is a flat area, from which water could flow down to more than one drain target. It also includes the parts of the surface that drain to that flat area. A flat area is a connected set of triangles all of whose vertices have the same elevation. Flat areas abut parts of the surface that slopes downhill.
If for every edge on the boundary of a flat area, the opposing, non-flat triangle slopes up from the edge, then the flat area is the bottom of a depression watershed:
If some of the opposing, non-flat triangles slope down from the flat area boundary, but all flow to the same drain target, then the flat area is part of the watershed for that drain target “a”, as shown in the following illustration:
In the following illustration, the flat area, plus whatever part of the surface flows down to it, becomes a flat area watershed. This watershed is ambiguous because water flowing through it can flow to more than one drain target, “a” and “b”:
