Matanuska-Susitna Wetland Mapping
SPRING FEN ECOSYSTEM Wetlands
Spring Fen Ecosystem wetlands are small peatlands surrounded by uplands. They are connected to other wetlands and to streams through shallow, unconfined groundwater movement through underlying permeable sediments. They occur in the region of moisture deficit, between Butte and Houston below 1000 feet elevation, where evapotranspiration generally exceeds precipitation. Wetlands are not expected in such an area, and must be driven by groundwater discharge. The thick glacial sediments underlying the area of moisture deficit are well-sorted and coarse-grained in places, allowing ample groundwater discharge where surface topography intersects the relatively shallow water table (Jokela and Munter, 1991). Spring Fen Ecosystem wetlands occur in these topographic positions. Because of the steady supply of shallow groundwater, water table elevations in Spring Fens vary the least of any wetland ecosystem.
An idealized representation of a Spring Fen Ecosystem wetland. Shallow groundwater discharges through realtively permeable sediments supporting peat development in areas of moisture deficit. Drawing by Conrad Field.
Spring Fen Ecosystem wetlands are similar to Depression Ecosystem wetlands in that they are surrounded by uplands. However, Spring Fens have correspondingly higher pH and specific conductance, which indicates a strong connection to shallow groundwater. This is due to the coarser, more permeable sediments underlying them.
Kettle Ecosystem wetlands occur on similar landforms as Spring Fen Ecosystem wetlands, but Kettle wetlands have a wetland or surface water connection to a navigable waterbody. Spring Fen, Kettle, and Depression Ecosystem wetlands all occur in kettle features. Although they all occur in a similar geomorphic setting, they are distinguished by slightly different underlying sediments and connection to other waterbodies. These small geomorphic differences determine important wetland jurisdiction and ecosystem service characteristics.
Numbers in paraentheses indicate number of samples.
Peat depth is a minimum, because some sites had thicker peat deposits than the length of the auger used (between 160 - 493 cm).
Water table depth is a one time measurement. At sites with seasonally variable water tables this measurement reflects both the conditions that year, and the time of year. If no water table was encountered, no value was recorded; use number of samples to aid interpretation. Deeper average water tables idicate higher variability.
pH and specific conductance measured in surface water or a shallow pit with a YSI 63 meter calibrated each sample.
Plant Prevalence Index calculated based on Alaska indicator status downloaded from the USDA PLANTS database, which may use different values than the 1988 list.
Spring Fen Ecosystem Wetland Map Components:
SF1: Spring Fen Ponds.
SF2: Spring Fens with water table near the surface. Often dominated by creeping sedge, marsh cinquefoil, and/or bluejoint reedgrass.
SF3: Spring Fens dominated by shrubs, especially thinleaf alder and dwarf birch.
SF4: Spring fens with deeper fluctuating water table, forested.