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. An underfit modern stream is too small to have created the valley it now flows in; it flows across deposits laid down by larger glaciofluvial processes.

The attentive reader will notice that both the Relict Drainageway and Lakebed Geomporphic Components support six hydrologic components instead of the four that describe most of the other Geomorphologic Components (e.g. Kettles, Spring Fens, Depressions, VLD Troughs, and Headwater Fens). That attentive reader would also notice that the hydrologic components for Drainageways and Lakebeds describe the same types, but are identified with different numbers, so they might seem inconsistent. For example LB3 is the hydrologic component identifying a bog on Lakebeds, but DW5 is the hydrologic component identifying a bog on Drainageways; and LB5 is the hydrologic component identifying bluejoint reedgrass on a Lakebed, while DW4 supports the same plant on a Drainageway. This is because the realtionship of the components to the relative position and stability of the water table is different for Drainageways, whose porewaters receive ample groundwater inputs, and Lakebeds which receive a higher relative contribution from precipitation because they tend to have flatter gradients and underlying substrates that are less permeable. The hydrologic components attempt to describe relative wetness or dryness defined by the median position of the water table and its variability throughout the growing season. Therefore the same vegetation type (bog or Bluejoint) can grow in different relative positions on these different landforms. For example, a bog on a Lakebed (LB3) supports a higher water table with less fluctuation than the shrubby portion of a Lakebed (LB4), and a bog on a Drainageway (DW5) tends to support a deeper water table with more variation than the shrubby portion of the Drainageway (DW3) (see the figure: "Water Table Fluctuation and Chemistry" below and notice especially that although the mean values for water table depth are nearly the same for the shrubby components of the Lakebeds and Drainageways (LB4 and DW3) the variability of DW3 extends to above the surface, while the range of the variability of the water table in LB4 is positioned deeper).

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