Relict Glacial Lakebed Wetlands

Relict Glacial Lakebed wetlands are extensive peatlands occuring on expansive flat surfaces that were formerly occupied by large proglacial lakes. Peatlands develop on these surfaces through a process known as "primary peat formation". In contrast to lake in-filling, the classical model of peatland formation, which is a process typical of ice-block depressions (Kettle, Depression and Spring Fen wetlands). Primary peat formation is the process responsible for most of the peatlands on Earth. Primary peat formation proceeds where a marshy area gradually fills with peat. The third peat-forming process is paludification, where sphagnum peat invades upland surfaces (Rydin & Jeglum 2006).

Relict Lakebed peatlands are mostly fens, often with patterning. The patterns consist of low-lying pools, which can dry up seasonally to form mud-bottoms (flarks), and intervening strangs (low shrubby ridges). Tree islands often form. Tree islands are often areas where bog peat is forming on top of the fen peat. This process is common on the areas that have been mapped in The Matanuska-Susitana Valley west of Houston. Bogs are almost completely absent from the area mapped on the Kenai Lowlands. This may be due to the nearly constant supply of tephra (volcanic ash) to the Kenai, unlike in the Matanuska-Susitna Valley. The tephra contains some calcium, which may prohibit germination of the sphagnum mosses that intiate bog formation.

Initial comparisons of 2004 satellite imagery with 1960 aerial photography in the Matanuska-Susitna Valley indicates that bogs and fens appear to be in stable equilibrium: neither appears to be expanding or contracting. This could be because relict lakebeds there support a drainage network sufficient to prevent bog mounds from forming. Bog mound formation is limited by distance to streams, and mounds typically develop on broad interfluves. It could also be due to relatively low rainfall and time since the intiation of peat growth there.

Relict Glacial Lakebed peatlands are larger than Kettles, which also form on abandoned lakebeds. The centers of large Lakebed peatlands are far removed from uplands, so surface water runoff originating from mineral sources appears distant. However, the mineral soil is never more than several meters away, beneath the peatland. Many fens, especially in the Matanuska-Susitna Valley, receive groundwater discharge from beneath, through sandy underlying sediments.

A Relict Lakebed peatland northt of Beaver Lakes. Patterning is evident with low-lying pools and sedge-dominated areas, higher shrubby "strangs" and tree islands.

The attentive reader will notice that both the Relict Lakebed and Drainageway 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 Lakebeds and Drainageways 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).

NWI and HGM

Lakebeds are palustrine wetlands in the US Fish and Wildlife Service's National Wetlands Inventory (NWI) classification, with 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 system of Tiner (2003) most Lakebeds fit loosely into the Terrene Flat/Slope groundwater-dominated throughflow category. They may have once been created by paludification, but that process is no longer active in most lowland peatlands. Many form the headwaters to small streams, particularly short-run streams near the coast.

Graph showing water table fluctuation. pH and specific conductance of wetland ecosystems with relict lakebed wetlands highlighted

Relict Glacial Lakebed Geomorphic Compnents (highlighted in blue) have variously fluctuating water tables, and LB2 can be flooded at the surface. LB3 components are bogs, where the water table fluctuates little. Forested components (LB6) have a greater range of water table fluctuation than any other Hydrologic component. Since peat has such high porosity, these wetlands can store large amounts of fall precipitation after a summer dry period. Specific conductance values and pH are intermediate, indicating both precipitation and groundwater sources. Groundwater discharge through underlying sediments is spatially variable. 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 and percent of major cations by wetland ecosystem

Cation chemistry by Wetland Geomorphic Component. Relict Glacial Lakebed wetlands (highlighted in blue) have low cation concentrations compared to other Geomorphic Components, indicating a strong precipitation influence on porewater chemistry. Cations that are present are contributed by groundwater discharge through underlying sediments. This discharge is spatially variable. 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).

Relict Glacial Lakebed 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.

LB2: Water table near the surface most of the growing season, often dominated by sedges.

LB3: Bogs, dominated by sphagnum moss and shrubs. Forested bogs are indicated by LB63 or LB36 map units, more extensive forests are mapped as LB63.

LB4: Dominated by shrubs, especially Labrador tea, leatherleaf and dwarf birch

LB5: Dominated by bluejoint reedgrass, often over relatively shallow peat in areas with large amounts of local groundwater discharge.

LB6: Forested, typically by black spruce. When a forested bog the map unit is qualified by the LB3 component: LB63 are forested bogs.

LBSF: A complex of patterned fen with small high ridges (strangs) alternating with low pools or hollows (flarks) and bog islands which may or may not be forested.

Cook Inlet Wetlands

RELICT GLACIAL LAKEBED WETLANDS

Left: An idealized cross-section showing the Hydrologic Components and common plants of a Relict Glacial Lakebed wetland. Drawing by Conrad Field. Right: Range of wetlands mapped as Lakebeds.

NWI and HGM

Wetland Indicators

LB1

LB2

LB3

LB4

LB5

LB6

Soils and Plant Communities

LB2

LB3

LB4

LB5

LB6

Cation Chemistry

Relict Glacial Lakebed Hydrologic Components: