I wouldn't say it's large-scale, but I wrote a marsh section in "World of Whitethorn 1B: The Village of Oester" recently with five special encounters and a description of the terrain, encounter tables, etc:
Chapter VII. The Blackened Woods
a. Special Encounter #10: Veptuna the Seductress
b. Special Encounter #11: Baban the Outcast
c. Special Encounter #12: The Marsh Dwellers
d. Special Encounter #13: A Swarm of Vicious Insects
e. Special Encounter #14: The Smuggling Ring