Marsh locations in the Rhine Tribuanl

I'm looking to create a vis source in a small marsh. At some point I want to tie in the adventure "What lies beneath" from Tales of Mythic Europe. Does anyone have any good suggestions on where to locate one?

The section on Deventer (page 46) describes it as "This modest-sized city, surrounded by marshland". Friesland (page 66) is described as "consists mostly of swampland" and the Bohemian Forest (page 103) talks of "its numerous peat bogs, swamps, and marshes". I think you could place this almost anywhere, to be honest. I don't have "Tales of Mythic Europe", however, so don't know if you need something quite specific.

you can also go to google earth or an other map and see the small village with a toponymy near of Fehn (swamp in german)

I´ll play "What lies beneath" near the village Ketsch.
Another german swamp is near Bruchsal.
Tobias Mahl could have planned originally to drain the swamp by Bruchsal, then he got the opportunity to work for the abbey Limburg (near Bad Dürkheim) and relocate his actions to Ketsch.
This is my plan.