Relict Glacial Drainageway Wetlands

Relict Glacial Drainageway wetlands are peatlands formed in relict, sometimes abandoned, drainageway features. These are linear features which drained once more extensive glaciers. Some may have formed along glacier margins. Some support modern streams but these streams are underfit. These peatlands are fens, with a stable high water table supported by ample groundwater throughflow that has had recent contact with mineral substrates.

A Relict Glacial Drainageway draining into Threemile Lake in the Matanuska Valley.

Some Relict Glacial Drainageway wetlands lie along the margins of modern underfit streams. Underfit stream valleys formerly carried large volumes of meltwater from glaciers. These modern streams are too small to have carved the valley they now occupy; they flow across deposits laid down by larger, glaciofluvial processes.

Relict Glacial Drainageways are palustrine wetlands in the US Fish and Wildlife Service's National Wetlands Inventory classification. They contain 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).

In the LLWW Hydrogeomorphic classification of Tiner (2003) Relict Drainageway wetlands fall into two major types: those along streams are Lotic Stream Slope/Flat throughflow headwater wetlands and those without streams are mostly Terrene Slope/Flat groundwater-dominated throughflow headwater wetlands.

Relict Glacial Drainageway Ecosystem wetlands (highlighted in blue) have moderately fluctuating water tables, and DW2 and DW3 can be flooded at the surface. Specific conductance values are the highest compared to other wetland map components, indicating a predominance of groundwater over precipitation contributions. pH values are correspondingly high. DW5 indicates bog-like Drainageway margins; DW5A are forested Drainageways. 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

Cation chemistry by Geomorphic Component. Relict Glacial Drainageway wetlands (highlighted in blue) have the highest cation concentrations of any Geomorphic Component, indicating a strong groundwater source, and less precipitation influence than other components. 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).

DRAINAGEWAY Hydrologic Components:

Map unit names are made of combinations of map components. A suffix 'c' idicates a created wetland, and a 'd' indicates a highly disturbed wetland.

DW2: Drainageways with water table near the surface most of the growing season. Often dominated by sedges, dwarf birch and/or sweetgale.

DW4: Drainageways dominated by Bluejoint reedgrass (Calamagrostis canadensis).

DW5: Bogs along Draingeway margins dominated by thick Sphagnum moss, and often black spruce forests.

Cook Inlet Wetlands