It looks like what you're remembering is the idea that Tharbad remained
inhabited down to 2912. This ultimately goes back to the Tale of Years, which states:
Quote:
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Originally Posted by Tale of Years
2912
Great floods devastate Enedwaith and Minhiriath. Tharbad is ruined and deserted.
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This is the aftermath of the Fell Winter, so you can imagine great floods of snowmelt rushing down the Gwathlo (which name, 'Greyflood', evokes exactly that kind of thing; snowmelt rivers are pretty distinctive), breaking the bridge and washing out the last few houses.
As
Morthoron says, those inhabitants are nowhere said to be Gondorian. In fact,
Unfinished Tales gives us this lovely description:
Quote:
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Originally Posted by Unfinished Tales: Lond Daer
Before the decay of the North Kingdom and the disasters that befell Gondor, indeed until the coming of the Great Plague in Third Age 1636, both kingdoms shared an interest in this region, and together built and maintained the Bridge of Tharbad and the long causeways that carried the road to it on either side of the Gwathló and Mitheithel across the fens in the plains of Minhiriath and Enedwaith.
A considerable garrison of soldiers, mariners and engineers had been kept there until the seventeenth century of the Third Age. But from then onwards the region fell quickly into decay; and long before the time of The Lord of the Rings had gone back into wild fenlands. When Boromir made his great journey from Gondor to Rivendell - the courage and hardihood required is not fully recognized in the narrative-the North-South Road no longer existed except for the crumbling remains of the causeways, by which a hazardous approach to Tharbad might be achieved, only to find ruins on dwindling mounds, and a dangerous ford formed by the ruins of the bridge, impassable if the river had not been there slow and shallow - but wide.
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The garrison you refer to leaves in the 17th century - almost certainly during the Great Plague, which caused Gondor to pull everything back into the heartland. As Morthoron quotes, the bridge was maintained for another 400 years or so, but ultimately fell into disrepair (and, indeed, into the river). The
crossing remained for much longer - the Nazgul actually used it on their way to the Shire! - but it was dangerous.
UT also provides the details that the road approached Tharbad via a causeway, which tells us that the whole area was swampy. Those floods, coming down from the marshes at Swanfleet, must have been absolutely devastating.
hS