The Marais des Cygnes River (/ˌmɛər də ˈziːn, - ˈsiːn, ˈmɛər də ziːn/ MAIR de ZEEN, -⁠ zeen, -⁠ SEEN,[3][4] French: [maʁɛ de siɲ]) is a principal tributary of the Osage River, about 217 miles (349 km) long,[5] in eastern Kansas and western Missouri in the United States.

The Marais des Cygnes is formed about 1 mile north of Reading, Kansas, a city in northern Lyon County, by the confluence of Elm Creek and One Hundred Forty-Two Mile Creek, and flows generally east-southeastwardly through Osage, Franklin, Miami and Linn counties in Kansas, and Bates County in Missouri, past the Kansas towns of Melvern, Quenemo, Ottawa, Osawatomie and La Cygne and through the Marais des Cygnes National Wildlife Refuge.

In Osage County, Kansas, a U.S. Army Corps of Engineers dam causes the river to form Melvern Lake, which is the site of Eisenhower State Park.

[6] The Great Flood of 1951 happened in June and July 1951, killing 28 people and causing over $935 million damage (in 1951 dollars).

Main Street (Old U.S. Highway 59) in Ottawa has to be detoured or is simply closed down when the gates are shut.