Matanuska-Susitna Wetland Mapping

Cook Inlet Wetlands

DEPRESSION  ECOSYSTEM Wetlands

A 2004 aerial photo map showing Depression Ecosystem wetlands (shaded red) in the Crevasse Moraine area.

Depression Ecosystem wetlands are surrounded by uplands. They are common as peatlands on the glacial outwash deposits around Palmer, and moraines south of Big Lake. Depression peatlands typically support lower pH and specific conductance values than Kettles or Spring Fens, indicating more bog-like, rather than fen conditions. They are probably controlled by a semi-confining layer and low insolation which together produce a micro-climate with lower evapotranspirational losses. 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 Ecosystem wetland dominated by creeping sedge (Carex chordorrhiza), south of Bruce Lake.

Depression Ecosystem wetlands are similar to Spring Fen Ecosystem wetlands 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. Depression Ecosystem wetlands are similar to Kettle Ecosystem wetlands, but have a wetland or surface water connection to a navigable waterbody.

Geomorphologically, Depression, Kettles, and Spring Fen Ecosystem Wetlands are all "ice block depressions". These features formed as large blocks of ice entrained in till freshly deposited at the end of the last glacial maximum melted, leaving a hole in the till. Some iceblocks were more isolated than others. The names help distinguish differences in wetland jurisdiction and ecosystem services of wetlands occurring in a similar eco-hydrogeologic setting.

Explanation:

All data from wetlands throughout the Cook Inlet Lowlands, not just from the Matanuska-Susitna Valley.

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.

Depression Ecosystem Wetland Map 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.