Rapids occur because of a shallowing of the river or narrowing of the river. If it is a short span, then a bridge should be doable, especially if they have the aid of channelers. A river with rapids that is also wide would indicate a shallow river with a rock bottom. A bridge over this kind of river would be more difficult to build than a short span but still doable if they had both male and female channelers.

The Two Rivers may never have had a good reason to attempt a bridge but it would be in Ghealdan's best interest to ensure a trade route to an Allie.

If there was an existing trade route from Ghealdan to the Two Rivers than in would be in the best interest of Ghealdan and it's allies to start exploiting it as soon as possible. Whether through the Forest of Shadows or the mountain pass, Ghealdan would really benefit from a northern trade route. The more markets for Ghealdan's tobacco, wool, or precious minerals the better for Ghealdan.

Gateways could be used, but in a post TG world channelers would probably in great demand.