> Looking at the US NHD estuary is broadly defined NOAA keeps track of the estuaries. And the states have fairly extensive data available: https://www.coastalatlas.net/?option=com_jumi&view=application&fileid=8&e=20&Itemid=107
Estuary is a generic term that covers five basic types, which have quite different in the characteristics that define them, beyond the general 'river going into the see' aspect. A Norwegian fjord and a mudflat can both be estuaries. Once you identify the type ( and since they are hugely important to global fisheries and other stakeholders, almost certainly someone has already identified the type https://oceanservice.noaa.gov/education/kits/estuaries/estuaries04_geology.html ), how the various feature boundaries are to be handled in OSM isn't too difficult. The ocean edge of the estuary if defined by if it is tide-dominated, wave-dominated, or river-dominated. That determines if you have have a Bay of Fundy https://en.wikipedia.org/wiki/Bay_of_Fundy estuary, Bay of Fundy, a Mississippi River type of delta, or a Columbia River situation where the river channel pretty much extends to the sea. That can be generally indicated in a tide table according to three basic buckets >4 meters, 2-4 meters, and less than 2 meters. >>>> but I think we'll have problems defining it? Only if you try to make the same scheme apply to all five ( coastal plain <https://oceanservice.noaa.gov/education/kits/estuaries/media/supp_estuar04_coastal.html>, bar-built <https://oceanservice.noaa.gov/education/kits/estuaries/media/supp_estuar04_barbuilt.html>, deltas <https://oceanservice.noaa.gov/education/kits/estuaries/media/supp_estuar04_delta.html>, tectonic <https://oceanservice.noaa.gov/education/kits/estuaries/media/supp_estuar04_techtonic.html> and fjords <https://oceanservice.noaa.gov/education/kits/estuaries/media/supp_estuar04_fjord.html> ). The 'cartographic' derived concept that there is just some sort of simple idealized 'coastline' is a fiction, and at the scale of human beings, not a very useful one. There isn't land and sea, there is land, sea, and a third 'coastal', which is land and sea changing several times a day and dramatically on a monthly and annual basis. Michael Patrick Data Ferret
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