Vector extract upstream network

Q: How to extract the entire network upstream a given point on the river network?

A: Use v.net.alloc for this task:

Add two columns to the vector network table (layer must be the layer of the lines), one for line length, one for blocking. Upload line length to the column for line length (v.to.db). Set the blocking column to -1.

Run v.net.alloc, use the length column as forward cost as the blocking column as backward cost. This should give you the network downstream of the given point. Using the length column as backward cost as the blocking column as forward cost should give you the network upstream of the given point.