@pyroholtz
That is the line between St. Tammany Parish and Orleans Parish. Theoretically, it would follow the historical channel of the Tangipahoa River which defines the Rigolets - the primary outlet for Lake Pontchartrain into the Gulf of Mexico, but the most recent bathymetric data shows the area isn't well defined - or at least defined well enough to reflect that channel.
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The original boundary was drawn in the 1700s, presumably when there was still a river there.
I did find that Legislative Act 159 of Session 1912-1932 established the boundary of the City of New Orleans and it follows that line. But it's not clear if they were following a pre-established line or otherwise. Presumably, they were following the original Parish line.
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