Even assuming you map a river catchment completely, how is the source defined anyway? Furthest water drop from the sea (as measured along the river)? Except in

Even assuming you map a river catchment completely, how is the source defined anyway? Furthest water drop from the sea (as measured along the river)? Except in odd cases where that's an underground resurgence, it's going to be pretty unspectacular, so what's the attraction? Mapping the watershed on the other hand, that's a nice thing to know.