

r.drain - 

r.drain traces a flow through a least-cost path in an elevation model. 
the elevation surface (a raster map layer) might by the cumulative cost
map generated by the \r.cost\ program.  The output result (also a raster
map layer) will show one or more least-cost paths between each
user-provided location(s) and the low spot (low \category values\) in
the input model.

Manual entry for \r.drain\
