One of the reasons that far more traffic is carried by Sartar’s Roads than by river is the steep slope of the Creek-Stream River. The Creek-Stream drops some 350 meters in about 48 km between Upland Marsh and Dammed Marsh. Although there are no cascades, there are numerous stretches of whitewater.
The reed boats of the duck people can ply these waters, but it is largely impractical for humans – particularly going up these fast moving waters!
In short, maybe it is easier to think of treacherous rivers like the Snake, the Gunnison, or the Yellowstone instead of more tame rivers like the Rhine, Elbe, or Thames.
Source, Wikimedia
So if we think about why those roads are so useful, it is like the Incan Roads. The roads make it possible for caravans to easily traverse the more rugged eastern half of Dragon Pass, and make their way to Prax or to the Oslir Valley and then to Peloria. They tie Tarsh and the Grazelands to Sartar.
