[Tagging] Watershed or Drainage Basin relation draft proposal

Kevin Kenny kevin.b.kenny at gmail.com
Thu Sep 13 19:33:20 UTC 2018

On Thu, Sep 13, 2018 at 10:02 AM Martin Koppenhoefer
<dieterdreist at gmail.com> wrote:

> you’ll have to put the ridges to map the watersheds anyway, the catchment basin is implicit with the waterways, coastlines and ridges.
> If there are names or other properties for the watersheds and catchment basins in play, it could make sense to have dedicated objects nonetheless, I agree.

> >
> > 2), while a ridge has to have a certain amount of slope to be called a ridge (perhaps at least 5 or 10% grade?), watershed boundaries are sometimes very shallow
> how can we observe those sheds in shallow areas? Can it be done on the ground or does it require additional elevation data? Maybe in the context of shallow land the sheds aren’t stable?

Definitely there are exorrheic wetlands and ponds in which the
watershed line is entirely indefinite. I know of a number of wet areas
that have distributaries into multiple major river basins; e.g., the
Grand Gorge area in the western Catskills drains into both the
Delaware and the Schoharie (and thence via the Mohawk to the Hudson);
the Preston Ponds area of the Adirondacks drains to both the Hudson
and to the Cold River (thence by the Raquette to the St Lawrence), and
so on.

The nearest the US has to an authoritative source is the National
Hydrography Database (NHD) which does have watershed boundaries, with
a hierarchical reference numbering system identifying them and tying
them to the 'reach codes' of the rivers that drain them. ('Reach code'
is a persistent identifier of up to 16 (?) digits, identifying
branches of a river from mouth to headwaters. (I've never investigated
what reach codes do with distributaries - I simply haven't needed to

There have been projects in the past to import NHD data in bulk.  I
was tempted to do it in my area once - I have the good fortune of
living in a place where the data quality of NHD is pretty good. While
some bulk imports from NHD have been highly successful, I stumbled on
conflation issues and abandoned the idea. I still occasionally
copy-n-paste a stream from NHD into JOSM, but that's always
onesie-twosies, and compared with data that are already in OSM and
with aerials. (Severe storms in the last decade or so have caused some
fairly major watercourses around here to shift, and NHD still hasn't
caught up.)

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