Spring Fen Wetlands

Spring Fen 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. As such, they can be thought of as outcroppings of the local water table. 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 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 geomorphic wetland type in this classification.

Spring Fen wetlands are similar to Depression 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 Spring Fens.

Spring Fens occur on similar landforms as Kettles, but Kettles have a wetland or surface water connection to a navigable waterbody. Spring Fen, Kettle, and Depression Geomorphic Components all occur in ice-block depressions. Although they all occur in a similar geomorphic setting, they are distinguished by different underlying sediments and connection to other waterbodies. These geomorphic differences influence important characteristics of both wetland jurisdiction and ecosystem services.

NWI and HGM

Spring Fens fall into the Palustrine category of the US Fish and Wildlife Service's National Wetlands Inventory. Spring Fens support a variety of plant dominants from herbaceous emergents (PEM) to shrubs (PSS) and forest (PFO), with hydrologic regimes ranging from saturated through permanently, semi-permanently, and seasonally flooded (PSSB, PEMH, PEMF and PEMC, respectively). PEMF is probably the most common.

The LLWW Hydrogeomorphic classification of Tiner (2003) places Depression wetlands into its Terrene Basin isolated wetland category.

Graph showing water table fluctuation range, pH and specific conductance by wetland map component

Spring Fens (highlighted in blue) have the least fluctuating water tables of any Geomorphic Component. SF2 is the most common component, further indicating that Spring Fen water tables are frequently stable near the surface. These stable water tables are maintained by a strong groundwater discharge connection. Specific conductance values and pH are high compared to the other isolated Geomorphic Components: Depressions. D = Depression, K = Kettle; S = Discharge Slope; LB = Lakebed; SF = Spring Fen; RT = VLD Trough; R= Riparian; H = Headwater Fen; DW = Drainageway.

Wetland Indicators

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.

Redox features with deep depths typically indicate deeper peat deposits, which mask redox indicators so the depth corresponds to the peat thickness.

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.

Soils and Plant Communities

Cation Chemistry

Two stacked bar graphs showing total major cation concentrations and percent composition by wetland ecosystem

Cation chemistry by Geomorphic Component. Spring Fens (highlighted in blue) have higher cation concentrations than the other hydrologically isolated Geomorphic Component, Depressions. This indicates a strong groundwater discharge influence on porewater chemistry. Although calcium and silicon show the greatest concentrations, magnesium and iron concentrations in our area are high for natural waters. DW = Drainageway, K = Kettle; S = Discharge Slope; LB = Lakebed; SF = Spring Fen; RT = VLD Trough; R= Riparian; H = Headwater Fen; D = Depression.

Samples were collected from a surface pool where possible, otherwise from a separate shallow pit excavated to just below the water table. All samples were filtered through either a 0.2 micron filter using a disposable syringe, or pumped through a 0.45 micron filter using a peristaltic pump. Samples were acidified with ultra-pure nitric acid and kept cool until analysis on a direct current plasma spectrometer to about 5% accuracy (except K, 10-20% accuracy).

Spring Fen Hydrologic Components:

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.

Cook Inlet Wetlands