Here’s something else to think about – the Upland Marsh is located in what was a broad and fertile river valley, some 130 km long by 45 km wide. The waters that run into this valley are fast and turbulent – and past the Damned Marsh the New River drops a lot of elevation quickly. So you basically get this big plateau with a shallow valley running through it.
This valley was arguably the center of Heortling culture in the First and Second Ages. In the Second Age it was called Velensar and it was the core of the Kingdom of Dragon Pass and later the Empire of the Wyrms Friends. Cities like Orin Jistil, Olorost, Voss Varainu, and Salor were important centers of trade and knowledge – and I am pretty sure Orin Jistil was a richer and more populated city than Old Pavis.
There are no records of Beast People (including ducks) inhabiting the valley in the First and Second Ages, beyond the inhabitants of the Zoo near Voss Varainu.
At the end of the Third Age, part of that valley bogged up and became a wetland. It is a sunken land, now inhabited by the dead, with ruins popping up here and there, with the largest being Delecti’s Ruins.
I imagine parts of the Upland Marsh (Delecti’s Ruins, Olorost, etc., ) look a lot like this:
