The U.S. Fish and Wildlife Service says Cheyenne Bottoms is not fully functioning and the site could be threatened by oil drilling.
Heather Whitlaw with the Fish and Wildlife Service in Manhattan says her agency opposes a recent request by a Kansas company to drill for oil there.
The group also has concerns about nearby oil production outside the Bottoms; the site is on the state's list of impaired waters because of siltation and oxygen depletion.
Cheyenne Bottoms is a 41,000-acre land sink that's also the largest interior marsh in the U.S. It's where about 250,000 waterfowl stop during seasonal migrations.
The Army Corps of Engineers is expected to have a decision on drilling by the end of the year.