The black squares are due to a bug in FT that will cause certain operations to result in an infinity or not-a-number in an editing cell. ProFantasy's tech support has access to a version that will correct that problem.

Incise Flow finds where a fluid would flow across the surface and removes more surface where the flow is higher. It computes the flow by dropping a little blob on each point on the map and letting the blob move downhill as far it will go. At each point on the surface that it visits, it increases a counter for each cell. The counter results for all of the little blobs on the surface are the amount of flow on the surface. After the flow is computed, a modified version of the flow for each point gets subtracted from each editing point, incising the flow into the surface.

The idea of using multiple incise operations at each level is to get varying widths along the path of a river with the general idea that rivers closer to the ocean will have wider valleys than those higher up. Wilbur's Variable blur feature tried to implement this idea directly by using the flow amount as the width of the blur, but it didn't quite work out the way that I'd hoped.