Fens are palustrine wetlands characterized by the surficial exposure of alkaline groundwater. These wetlands average about 10 acres in area. Water in most fens ranges in specific conductance from 1,000 to 3,000 micromhos. Coarse emergents such as phragmites and common cattail, and several shrubby species of willow usually dominate the central zones of fen wetlands. Muskgrass, a large alga, can usually be found in the small open water seepage areas of fens.
Fens have boggy or quaking bottom soils.
Thus, they receive heavy use by several species of marsh birds whose nests may be easily damaged by large grazing animals. Fens also are important watering areas for sandhill cranes during migration and are much used by mink, wintering deer, and upland game. Lacustrine wetlands are generally large areas of open water with active, wave-formed shorelines and no persistent emergent vegetation in the central or deepest zones. Permanent fresh wetlands are numerous in a small area in northern North Dakota called the Turtle Mountains, but uncommon throughout the rest of the Prairie Pothole Region. Many of these wetlands contain water of sufficient depth to maintain fish populations.