[Legate of Amon Lanc]

Originally Posted by Tuor in Gondolin

There are several things here. One, who were these refugees and where
did they come from? Dunland? Perhaps a few. Rohan, maybe a handful of
Wormtongish- but doubtful. Perhaps people from some areas of South
Gondor going through Anfalas?
Two, Gandalf's comments suggest not a naturally barren land in Minhiriath
and Enedwaith but one almost unnaturally depopulated (shades of the
Dust Bowl?) Perhaps some New Deal CCC (Civilian Conservation Corp)
replantings and soil management was called for

You basically answer yourself with the second remark: I always believed that apart from a few Dunlendings, these men were mainly from Enedwaith or Minhiriath, exactly those bits of "depopulated" people. Mainly, remember that Barliman says that at first mostly they were poor guys running away from trouble. What trouble, one must ask? The Dunlendings had no reason to run away from trouble, unless they were persecuted in their homeland because they disliked the alliances with Saruman. But I could imagine that the Dunlendings on the rise, supported by Saruman, could become a threat to the neighbouring peoples - and that includes the primitive and simple folk of fishermen and hunters (as said in the UT) in Enedwaith/Minhiriath (I never remember which one is which).

And then there's that potential sea route, Forlindon to Pelargir...

Yes, but in late TA nobody used it, probably since the fall of Arnor. There was this big army of Gondor coming to defeat the Witch-King, and they sailed to Lindon, but there did not seem to be any more contact reported later.

Originally Posted by The Mouth of Sauron

I think I remember reading somewhere that Tharbad had a garrison of soldiers and engineers from Gondor until about a century before the War of the Ring. That would suggest that the maintenance of the bridge there was considered vital for North/South trade.

I don't think was that way by then. It's in the UT, but I don't think there was any powerful garrison there by that time. It was just some population remaining there, and after the great floods (2912 TA) the city was completely deserted.

Okay, I actually found the quotes. As for the population of the regions:

Originally Posted by Unfinished Tales, The History of Galadriel and Celeborn, Appendix D

Since the Great Plague of the year 1636 of the Third Age Minhiriath had been almost entirely deserted, though a few secretive hunter-folk lived in the woods. In Enedwaith the remnants of the Dunlendings lived in the east in the foothills of the Misty Mountains; and a fairly numerous but barbarous fisher-folk dwelt between the mouths of the Gwathló and the Angren (Isen).

And here is the one about Tharbad and its garrison:

Originally Posted by Unfinished Tales, The History of Galadriel and Celeborn, Appendix D

A considerable garrison of soldiers, mariners and engineers had been kept there until the seven*teenth 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.

Seventeenth century of the Third Age - that's actually quite a long time ago and Gondor was at its heights still back then. After the demise of the Northern Kingdom, it gradually lost its importance to keep a big garrison in Tharbad.