Fen, a type of wetland. The land is covered with shallow water containing slowly decaying plant material, some of which forms peat. The decay of the plant material is slowed by the shortage of dissolved oxygen in the water. Groundwater that drains into the fen typically contains calcium, making it alkaline, and other nutrients. These nutrients help support many kinds of plants, including mosses, rushes, sedges, and grasses. Fens are also home to a variety of animals, including many species of insects and water birds.

Fens are most common in the northern parts of the world, including Canada and northern Europe. The Fens is an area in eastern England that was once noted for its large number of fens; since the 17th century, however, most of the fens have been drained for farmland.

If the flow of groundwater draining into a fen is interrupted, the fen becomes less alkaline and loses nutrients. This process gradually causes the fen to develop into a bog. .)