Hi Andrew,
I noticed a couple times in the results messages the error: "Failed to open raster dataset". This makes me wonder if there is a problem with your rivers input file. If it is a raster, make sure it is integer type.
If this doesn't work, and you correct the file and path names as Dan Patterson suggests... my guess is that it is probably a problem with the coordinate system. Judging from the default cell size I assume you are using a lat/long coord-system, so try projecting toUTM or another Cartesian type and try again.
Hope this helps,
James