Are NHD tags useful? Such as nhd:reach_code, nhd:gnis_id,… I know nothing about this, but someone is deleting them.
In the wiki they are described as useless (if the intermittent is set properly). Anyway not sure nowadays >10 years old data is a good source to add it. 
Keeping the “nhd:gnis_id” tag allows easier mapping back to the other databases which helps people building tools to help with audits/adjustments etc. (ex here).
I usually change it to the more common “gnis:feature_id” but that’s purely preference.
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This guy is deleting all of them.
I see the wiki says most of them are not useful, but does say reach_code is useful. Nothing in the wiki about gnis_id
https://wiki.openstreetmap.org/wiki/Key:nhd:reach%20code?uselang=en
Of these keys, nhd:fcode is perhaps the most useless. It ostensibly conveys the same information as all the standard OSM tags on the feature. It corresponds to an old revision of a feature classification system that NHD has revised several times since. As I recall, each revision has resulted in quite a bit of reshuffling as a few classes get split or merged. (This is a cautionary tale to anyone who would design a numeric classification tagging scheme for OSM.)
But sometimes even this key comes in handy. In 2021, @Emilius123 noticed that the NHD import included lots of water=salt that seemed like it should be changed to the approved salt=yes. But on closer inspection by OSMUS Slack’s intrepid researchers, these features didn’t have anything to do with salt water. Two FCodes, 39001 and 39003, got mapped to water=salt. But these codes mean different things:
| FCode | Hydrographic Category | Water Characteristics |
|---|---|---|
| 39001 | intermittent | salt |
| 39003 | perennial | salt |
39003 was removed from a subsequent NHD revision, which we think signals that they didn’t think it was trustworthy information. Some of these occurrences were therefore deleted.
Had it not been for these eye-grating, import-specific tags, we wouldn’t have been able to find all the features that had misleading OSM tags. I don’t think this justifies every import-specific tag, but it does mean we need to be careful around imports like NHD that don’t meet modern import standards.
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nhd:gnis_id is one of several imported keys that indicates the GNIS Feature ID. Apart from imports, when mappers add Feature IDs manually, they usually go for gnis:feature_id, since GNIS is the original dataset that assigned these IDs.
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