"The best sleep I have ever gotten while running away from my problems, as those were silly enough to hire mules." — Lorekeeper Aeron
First, rivers are necessary for human and animal survival, becase most of the creatures of Avalonia do need water on a daily basis. Good thing they are criss-crossing the landscape all over the island. And when humans had enough of that living, they turn to their second most favourite thing: commerce. Rivers have become important veins for mankind for both purposes.
It's not easy to sell your goods in the next village over, if you want to make it back home by supper. It's even harder to make it anywhere further than that, since you need the permission from your local lord, protection from bandits, somewhat okay legs to walk long distances or even more okay-ish legs of a good sturdy horse. And then you need help carrying all that you want to sell. But sooner than later, everyone around you has the potential of not really needing whatever you need to sell, which means you need to travel even longer, or, give up.
When it comes to money and trade, humans rarely give up. Mostly when they stop breathing.
And so rafting was born - first to transport the wood itself, big long treelogs for construction of different types of houses and halls or to be sold of further away. And then people start putting more stuff on those tied up floaty logs to be also sold further away. Soon a transport guild was established and rafting transport of goods is an integral part of Avlan economy.
Maybe that's why it has become such a popular place for raiding foreign nations, because it is not only prosperous but local people are seen as too dumb to construct bigger boats. It is a very popular insult, but obviously do not say it outloud if you do not have an escape plan from Avalonia already planned.
