[OSM-newbies] Mapping streams

[xeen]

I'm trying to map a river, but I'm not exactly sure how to do so. The
problem is, that it's not really accessible, so walking on the edge is
not possible (it's a smaller river, so using a boat or kanu doesn't
work either). There's a cycleway nearby, but it's not going "a fixed
distance" from the river, so it's not possible to map the river's arcs
without blindly guessing (when cycling along the way).
Is there a better way than just using the accessible points and
interpolate the rest?

Thanks!
xeen