One of the rarest and most unique wetland types in the Sandhills is the fen. Fens are characterized by slightly acidic (~pH6) water and peaty (undecomposed plant parts) organic-rich soils that form in areas fed with a nearly constant supply of groundwater, which continues even in the driest years. Because they are constantly saturated with cold groundwater, dead plant material decomposes very slowly, over thousands of years. These organic soils can be many feet thick; the deepest recorded in the Sandhills is 23 feet and dated to over twelve thousand years. In places the organic soils and associated mat of vegetation are suspended over a pool of groundwater, causing a slight mound that shakes when you walk across it. Most fens are one or two acres in size, but some are over five hundred acres. Fens are usually found in the upper portions of valleys or lakes where groundwater flows in, and they are often bordered by wet meadows and marshes. Fens harbor several plant species rare in Nebraska, including cotton grass, buckbean, and marsh marigold. The primary range of these plants is mostly in colder regions north of Nebraska. The populations in the Sandhills are likely Ice Age relics that have survived in fens since the retreat of the last glaciers.
Fens are especially susceptible to alterations due to ditching or declining water tables. If the organic soils dry out, they can rapidly decompose, and the fen will disappear. They also are impacted by non-native plants that also affect other wetlands in the Sandhills. Many conservation efforts for Sandhill wetlands, such as plugging ditches, addressing stream bed degradation, and controlling invasive species also help to sustain the fens of the Sandhills.