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You should integrate the rivers as an extraordinarily large cost value, several many orders of magnitude greater, that will force the algorithm to never include rivers in the calculations. Rasterize your rivers the same spatial resolution and registration (snap raster) as the cost surface - if you haven't already. Run a map algebra/raster calculator in a way ...


Micha, That's exactly how I've proceeded and I've still met the same walls that Knives met. So frustrating!


I ended up working through QGIS. Grass itself is quite overwhelming. Still having basic issues with projections, getting a hang of the data structure and executing r.walk. Thanks all.


For some basic tutorials and exercises for GRASS, you can check this link which I think is a great place for beginners. There are also a couple of other sources which describe the r.walk and r.cost functions and their associated parameters in greater detail (although I'm assuming you have already read these).

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