Bogs, Sloughs, Marshes, and Fens

What is the Difference Between Bogs, Sloughs, Marshes, and Fens?

The Forest Preserves of Cook County cover nearly 70,000 acres which comprise 11 percent of the county. They are alive with wildlife and filled with diverse and unique ecosystems such as dolomite prairies; oak woodlands; and wetlands, sloughs, marshes, and fens which each have their own defining characteristics and support a diverse set of plants and animals


A bog is an acidic wetland with very low oxygen. Usually found farther north, bogs are home to distinctive plants such as sphagnum moss. In the Palos region, Cranberry Slough Nature Preserve includes one of only a handful of peat bogs in Illinois. A unique ecological community developed in this hollow, formed by an ice block stranded during the retreat of the glacier some 14,000 years ago. Mostly untouched by human development and agriculture, Cranberry Slough contains remnant woodland, savanna, prairie, wetland and peat bog ecosystems over 570 acres. 


Smaller than lakes, sloughs are wetlands that are often a backwater to a larger body of water such as a lake or river. They are home to a wide array of wildlife species.

A wonderful place to explore a slough is Saganashkee Slough that supports a great variety of birds, particularly waterfowl. During spring and fall migration, the slough is a stopover for loons, herons, grebes, mergansers, white-winged scoters, double-crested cormorants, goldeneyes and many others. Bald eagles have been known to roost in the area. The slough supports a number of aquatic species such as crappie, catfish, largemouth bass, northern pike and bullhead.


Marshes are usually a permanent body of water, surrounded by and interspersed with vegetation. A hemi-marsh has an equal amount of open water and plants, creating sheltered nooks for waterfowl to nest. The Sand Ridge Nature Center in the Calumet region is an excellent place to learn more about marshes.


A fen is rare wetland type distinguished by its alkaline, mineral-laden water. The water in a fen percolates through limestone and emerges as a seep, or spring. A good places to see a fen is the Dan McMahon and Fen Nature Preserve which is an Illinois Nature Preserve and is designated as one of the highest quality natural areas in the state by the Illinois Nature Preserves Commission, and the Sagawau Environmental Learning Center.

Learn more about the ecosystem of the Forest Preserves of Cook County here.