Cook Inlet Wetlands
Left: An idealized cross-section of a Kettle wetland showing hydrologic componets and typical plants. Drawing by Conrad Field. Right: Range of wetlands mapped as Kettles.
Kettle wetlands are peatlands occupying depressions created when ice-blocks entrained in glacial till melted at the end of the last glacial advance (ice-block depressions). Kettles have a wetland or stream connection to Cook Inlet, unlike Depression and Spring Fen wetlands, which also formed in ice-block depressions. Depressions and Spring Fens are surrounded by uplands.
A Kettle south of Big Lake.
NWI and HGM
Kettles fit into the US Fish and Wildlife Service's National Wetland Inventory palustrine category. 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).
The LLWW Hydrogeomorphic classification (Tiner, 2003) would classify most Kettles as Terrene Basin groundwater-dominated throughflow wetlands. When the kettle is occupied by a central lake, greater than 20 acres and deeper than 6.6 feet, it would be named a Lentic Fringe throughflow wetland. Some may also form the headwaters to small streams, in which case the modifier "headwater" is appended.
Kettle Ecosystem wetlands (highlighted in blue) have deeply fluctuating water tables, and K2 and K3 can be flooded at the surface. Much late season water storage becomes available in these wetlands as the water table draws down during the summer dry period. Specific conductance values are relatively high compared to other wetland map components, indicating significant groundwater contributions. D = Depression, K = Kettle; S = Discharge Slope; LB = Lakebed; SF = Spring Fen; RT = VLD Trough; R= Riparian; H = Headwater Fen; DW = Drainageway.
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.
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 by Geomorphic Component. Kettle wetlands (highlighted in blue) have the third highest cation concentrations compared to other Geomorphic Components, indicating a strong groundwater influence on porewater chemistry. Kettles have the highest cation concentrations of any of the ice-block depression wetlands. The other ice-block depression wetlands (Spring Fens and Depressions) are isolated. 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).
Kettle Wetland Hydrologic Components:
K1: Kettle pond. If larger than 20 acres, the kettle is classified as a Lake.
K2: Water table at or very near the surface, often sedge-dominated.
K3: Deeper, more variable water table, often shrub dominated.
K4: Seasonally high water table, forested.
PO Box 15301
Fritz Creek, AK 99603
19 October, 2011