Cook Inlet Wetlands
LEFT: An idealized cross-section of a Depression wetland showing hydrologic componets and typical plants. Drawing by Conrad Field. RIGHT: Range of wetlands mapped as Depressions.
Depression wetlands are surrounded by uplands. They are common as peatlands in ice-block depressions on large moraine complexes. They are also found on smaller moraines scattered thoughout the lowlands, and on glacial outwash deposits around Palmer. Geomorphologically, Depression, Kettle, and Spring Fen Ecosystem wetlands are all "ice block depressions". Large blocks of glacial ice, which were entrained in till deposited as glaciers receded at the end of the last glacial maximum, melted leaving deep depressions on the surface. Some iceblocks were more isolated than others. The different names: Depression, Kettle and Spring Fen, help distinguish differences in wetland jurisdiction and ecosystem services occurring in this geomorphic setting.
Depression peatlands typically support lower pH and specific conductance values than Kettles or Spring Fens, indicating more bog-like, rather than fen conditions, especially west of Houston in the Matanuska-Susitna Valley. They are probably controlled by a semi-confining layer and low insolation which together produce a micro-climate with lower evapotranspirational losses. Very steep-sided depressions, such as those found in the Cravasse Moraine area, south of Palmer, can support permafrost. At least 30 cm of hard ice was encountered 27 cm below the surface under a black spruce canopy in one Depression in the Crevasse Moraine area late in the season, on 21 August 2007.
A Depression peatland dominated by Dwarf birch (Betula nana), fewflower sedge (Carex pauciflora) and Sphagnum fuscum on the pitted outwash surface west of Goose Bay State Game Refuge.
Depression Geomorphic components are similar to Spring Fen components because they are both surrounded by uplands. However, Spring Fens are fed by groundwater discharge, thus have a less variable water table and correspondingly higher pH and specific conductance values indicating a direct connection to shallow groundwater. Spring Fens are found only in the lowlands between Palmer and Wasilla, where potential evapotranspiration exceeds precipitation. Depression wetlands are also similar to Kettle wetlands, but Kettles have a wetland or surface water connection to a navigable waterbody. Kettles lack the strong groundwater discharge of Spring Fens, and lie in areas where precipitation exceeds evapotranspiration.
NWI and HGM
Depression wetlands fall into the US Fish and Wildlife Service's National Wetlands Inventory Palustrine category. Depressions 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).
The LLWW Hydrogeomorphic classification of Tiner (2003) would place Depression wetlands into its Terrene Basin isolated wetland category.
Depression wetlands (highlighted in blue) have moderately fluctuating water tables, with D2 components fluctuating the least and D4 the most. Specific conductance and pH values are low compared to other wetland map components, indicating a predominance of precipitation over groundwater contributions. DW = Drainageway, K = Kettle; S = Discharge Slope; LB = Lakebed; SF = Spring Fen; RT = VLD Trough; R= Riparian; H = Headwater Fen; D = Depression.
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. Depression wetlands (highlighted in blue) have the lowest cation concentrations, indicating a strong precipitation source, and less groundwater influence than other Geomorphic 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).
Depression Wetland 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.
D1: Ponds in Depressions.
D2: Depressions with water table near the surface most of the growing season. Often dominated by sedges, dwarf birch and/or sweetgale.
D3: Depressions with fluctuating water table, often dominated by shrubs.
D4: Depressions with deeper, fluctuating water table, forested.
PO Box 15301
Fritz Creek, AK 99603
19 October, 2011