Chapter XIV, last but one page: Still elves are lightfooted, and though they were not in these days much used to the marches and the treacherous lands between the Forest and the Lake, their going was swift.

It's "marches" at least since The Annotated Hobbit 2nd ed. and exists in later editions.

In the 1993 print of The Hobbit (HC paperback, Illustrated by JRRT), it's "marshes". It's also "marshes" in the draft (History of Hobbit, 3rd phase, XVb. King Bard).

This change is not recorded in The Annotated Hobbit and A descriptive bibliography. Although "marches" can be accepted, I think "marshes" is the correct reading.