There used to be a 'river' in TG, where you could have a line carved from startpoint to endpoint. Such a feature would be very handy, though it needs to be more sophisticated; a way of picking some logical starting points from which (perhaps using a plugin) a mask can be derived that follows a logical, slightly widening line downhill, following the contours of rocks, outcrops gullies, etc. The only way to do efficiently now is paint them on the terrain.