swamp v. marsh v. bog v. fen

Wetlands are lowland habitats whose soil is permanently or intermittently saturated with water or covered with a shallow layer of water. There are several distinct types of wetlands:

• Trees are the predominant vegetation in swamps, which form along rivers.
• Marshes are found within estuaries and coastal waterways, or in the shallows of ponds and lakes. They’re usually shallower than swamps, with less open water and worse drainage. Marshes support smaller plants without woody stems, like grasses, reeds, and cattails.
• A bog is a shallow, stagnant wetland covered with peat, a spongy material formed by the partial decomposition of mosses. Bogs have acidic water, and are fed primarily by precipitation.
• Fens also accumulate peat, but they experience greater water exchange with streams and groundwater, resulting in more nutrient-rich soil and water with a neutral to alkaline pH.

The Canadian Encyclopedia on swamp v. marsh v. bog

USDA Forest Service on bog v. fen