Potential future conflicts and problems in the rain wild chronicles] [spoilers]

['thul]

All indications are that the silvery river is in another (unnamed) city, reached by pillar. Otherwise fitz/nighteyes would have found it ranging out from the quarry. There's enough symbols on the pillars to make finding the river hard, particularly if the route is two-step.
In verity, it seems the skill and Elderling blood bred true in a way it did not in Fitz. Also, he was far more fully trained than Fitz.

CoD class spoiler

The impervious ships might easily travel far upstream, but they might have serious trouble towards the end, where it is really shallow. Besides, Kalo/Spit might have something to "offer" them. Say, a good dose of venom. It would not be surprising if, should the ship get to kelsingra and hest with it, that when hest tries to place sedric back under his "control", Carson rather objects, and spit gets to "play".
With most dragons/keepers, the relationship is based around the keeper being fairly mild, and the dragon assertive. With Heeby/Rapskal, it is a true friendship, very few strings attached. With thymara/Sintara, it is both pulling for their wants in opposite directions. But with Carson/spit, they use each other, to a large degree humoring the other by it. Carson ignores Spit to get his will done, Spit uses Carson, say to flush out prey (gallator/Greft case, DH).

Dragons might carry baskets, but you forget one major thing...

CoD class spoiler

There's only three dragons strong enough to carry loads at all. Icefyre, Tintaglia, Heeby. Even then, it is one thing to carry a single person, quite another to carry a true load. Neither of the two former would be willing to carry people all that far. Icefyre also has that endurance problem from his frozen hibernation. He's the physically largest of the dragons, yet he is in many ways worn out.
In order to carry a load any distance, they have to physically be far stronger, especially if the load is bigger than one person.