Do you mean to automatically trace along the troughs of valleys?
I've manually done this, with map_c overlaid with the map_t in Photoshop (and the latter scaled up the necessary 4X, of course). Even then, I have found that the river line so created is generally offset by a couple of pixels on map_c, due to the way the map_t pixel is referenced to one of the corners of that pixel. This can be fixed by simply selecting the river(s) and shifting as necessary.