r.drain

[Edwin Welles]

We would like to use r.drain to find the flow paths FROM
a point on a stream up the slope to the watershed divide.

We have tried several unsuccesful methods, inverting the 
elevation, slope and accumulation maps (accumulation map from
watershed).

Does anyone know a way to do this?

Can someone direct me to some references that describe the
algorithms used in r.cost and r.drain?

thanks
Edwin Welles
edwin at hwr.arizona.edu