Mistfall

You may be looking for the audio story of the same name.

A swamp during mistfall. (TV: Full Circle)

Mistfall was an event that took place on the planet Alzarius approximately every fifty years, when another planet drew Alzarius away from its sun. The cooling generated the mist. The water bubbled, spiders hatched from river fruits and Marshmen rose from the swamps. The native Alzarians feared this event and laid out plans to avoid the gases, which they falsely believed were toxic. (TV: Full Circle)