When Robb presents his terms to Cleos, he declares all the Stark Lands as well as “all the lands watered by the River Trident from the Golden Tooth to the Mountains of the Moon” as part of his new kingdom. Why the weird description for the Riverlands? Why not just “all Tully lands”? Does the specific wording include or exclude any fiefs, such as Harrenhal?

Good question!

If I had to guess, Robb’s terminology is based on a historic formulation of the boundaries of the Riverlands, which tended to be somewhat more fluid than those of other kingdoms.

Prior to the advent of modern surveying techniques, natural features tended to be used quite a bit as boundary markers – hence part of the reason for the Mexican-American War was that Mexico and the U.S disagreed whether the U.S/Mexico border was at the Rio Grande River or the Nueces River, or why France historically tried to expand its borders to the Alps, the Pyrenees, and the Rhine even though its linguistic borders are considerably to the west, and why the Germans tried to do the same for the territory between the Rhine and the Danube. 

Moreover, the phrase that Robb uses is somewhat more complex: it’s the lands “watered by the River Trident and its vassal streams (emphasis mine).” To figure out what he means by this requires looking at a map:

Note that the Trident and its vassal streams snake up to that disputed territory between Greywater Watch and the Twins in the north, and significantly into the Westerlands territory up by Ashmark and Hornvale  if you compare it to the political maps (hence why he’s also claiming the Golden Tooth), it might even include Wickenden in the Vale if you push if far enough.

The bigger issue is what about that territory between the Red Fork and the Narrow Sea south of the Trident? Well, arguably it’s “watered” by the eastern bank of the Red Fork from the Mummer’s Ford north to Riverrun along its western border and then again on the southern bank of the Red Fork from Riverrun to Darry, and then you have the Trident Proper which runs from Darry to Maidenpool all the way out to Cracklaw Point (which historically was something of a border territory). Harrenhal down to Stony Sept would be a bit tricky, since Harrenhal itself isn’t bordered by the Trident (although its lands probably are). Likewise, where does the waterline end: Antlers? Sow’s Horn? Duskendale? (It was once part of the Riverlands, after all.)