Types of Fens

Sloping Fens

Sloping fens are the most common type of fen in California. They form at the base of hills where groundwater discharges to the surface. Groundwater discharges into the fen from the landscape above it and sheet-flows through the fen. Sloping fens can also occur on hillslopes where groundwater discharges from alluvial fans, glacial moraines, and bedrock aquifers. Slopes can be steep or gentle and large areas of open water are rarely if ever present. Typically, sloping fens have water slowly flowing over the fen surface during most or all of the summer. Because sloping fens have high drainage rates, they require a constant flow of ground water to maintain soil saturation.

Diagram of a sloping fen complex.
Diagram of a sloping fen complex (Wolf and Cooper 2015). The green area at toe of the slope represents the fen area.

Five people looking at plants on a strongly sloping fen.
A strongly sloping fen in the Stanislaus National Forest (Sikes et al., 2013).

Basin Fens

Basin fens originated as lakes or ponds and formed as the pond was filled with partially decomposed plant remains. Basin fens are typically flat and occur near or on the margin of open water. Basin fens are widespread in North America and may be quite large, such as surrounding Domingo Lake on the Lassen National Forest. Often basin fens have floating mats that quake, and rise up and down as the pond water levels change, maintaining contact between the peat surfaces and the water level.

Diagram of a basin fen that is supported by both surface and ground water inflow. Inset photo of Domingo Lake.
Basin fen that is supported by both surface and ground water inflow (Wolf and Cooper 2015). Water levels in the basins tend to be very stable, and floating peat mats are common. Inset photo is of Domingo Lake on the Lassen National Forest (Sikes et al., 2013).

Spring Mounds

Spring mound in Stanislaus National Forest.
Spring mound in Stanislaus National Forest (Sikes et al., 2013).

Spring mounds, are localized points of groundwater discharge, often support small fens. Many spring mound fens are only tens of meters in diameter, but they are morphologically and ecologically distinct. Spring mound fens may form from within a sloping fen complex and indicate the location of a strong upward groundwater discharge. A spring mound can be considered a topographic feature of a fen meadow. Such topographic features that include berms and pools, are commonly observed in fens, and may range in size from only centimeters of relief to many decimeters.

Diagram of spring mound fen.
Diagram of spring mound fen (Wolf and Cooper 2015).

Lava Discontinuity

The fourth geomorphic type is found where lava beds overlie each other, such as in Lassen National Forest in the southern Cascade Range. Typically the younger overlying lava flowed over older lava, melting the surface rocks, forming a nearly impermeable surface, and creating a geological discontinuity. The upper lava may have fractured as it cooled, or was covered with glacial till, and became a large aquifer for snowmelt recharge. Lava discontinuity fens may occur on very steep slopes, yet have think peat bodies and well-developed fen vegetation. These spring complexes may be very large and produce perennial flows of water.

Diagram of a fen formed where ground water discharges at a lava bed discontinuity. An inset photograph shows the Willow Creek fen site.
Diagram of a fen formed where ground water discharges at a lava bed discontinuity (Wolf and Cooper 2015). Fens of this type are common in the southern Cascade Range on the Lassen National Forest. Inset photograph show the Willow Creek fen site (Sikes et al., 2013).

Next: Rich Fens and Poor Fens…