GRASS 6.4.1 Hydrology Program Predicts a River Channel on the Sea Beach Line

The GRASS 6.4.1 spatial analysis program has an hydrology program that uses local elevations.  Just bringing in the subway line layer as a "river" produced a map that shows a low level (in yellow) river channel at the location of the Sea Beach Line.  The "river channel" is more pronounced in the lower part of the map where the elevations are generally lower.  Notice the other rapid transit lines, indicated by X's as station stops do not have this effect.  To the left of the Sea Beach Line in the West End Elevated, to the east is the Culver El.  Further to the east is the Brighton Line on embankment and another color shows that it is not a channel.  The green line north of Sheepshead Bay is the Belt Parkway.  The hydrology "carve" program  thus shows potential river streams, in this case, the Sea Beach Line that became flooded during Hurricane Sandy and is presented in an earlier post.