Matanuska-Susitna Wetland Mapping

Cook Inlet Wetlands

RIPPLE TROUGH  ECOSYSTEM Wetlands

Wetlands over a shaded relief aerial photography showing the giant ripple features in the Meadow-Beaver Lakes area.

Ripple Trough wetlands lie only in the valleys in the Meadow and Beaver Lakes Area. These uniquely arranged hills and valleys were apparently created by the waves formed when an ice dam collapsed and catastrophically drained Lake Atna, a 9500-square km lake that occupied the Cooper River Basin near the end of the last glacial maximum (Wiedmer, et. al., 2010). The features have also been called ribbed moraine, formed sub-glacially and perhaps later mdified by subglacial flooding. They are currently mapped as Rogen moraine, but are not now thought to be this type of feature.

Many ripple troughs support lakes, the largest of which is 230 acre Seymour Lake. Ripple trough lakes are of relatively uniform depth; all of them are about 20 feet deep. Lakes are defined as areas of standing water greater than 20 acres (and deeper than six feet). Lakes are mapped separately from wetlands.

Ripple Trough Ecosystem wetlands are peatlands. They often support a bisecting stream. Because the adjacent glacial sediments are coarse-grained cobbles and gravels which allow rapid groundwater transmission, Ripple Trough peatlands are often fens, at least toward the east. They should receive ample shallow groundwater discharge near the surface. Accordingly, they usually show somewhat higher pH and relatively small seasonal water table fluctuations (Table 1).

Eastward, where precipitation barely exceeds evapotranspiration, fens dominate. Westward, Ripple Trough peatlands support bogs atop fen peat. Bogs have lower pH (less than about 4.2) and specific conductance (less than about 50 µS at 25°C) and lack fen indicator plant taxa. In the Matanuska-Susitna Valley, bogs are often forested by black spruce (Picea mariana), with a dense shrubby leatherleaf (Chamaedaphne calyculata) and/or Labrador tea (Ledum palustre ssp. decumbens) understory. Many burned in the 1996 Miller's Reach Fire.

A bog typical of the type found in the western portion of the Ripple Trough features (pH= 3.8, SC= 33.0 µS).

Bog peat accumulates due to two primary factors: excess precipitation and landscape position. Precipitation excess (in a cool climate) allows certain sphagnum moss species, especially S. papillosum and S. fuscum to thrive. When thick enough, these mosses can hold a mound of precipitation-derived shallow porewater above groundwater discharging from beneath. The condition of a mound of meteoric porewater above shallow groundwater discharge results in a zone within the peat profile where head reversals occur. In this zone, the porewater from the sphagnum mound sometimes recharges to below, while at other times groundwater discharges to it. Wet conditions favor recharge from above and dry conditions favor discharge from below. Water chemistry in the zone varies accordingly. Bog mounds are limited in extent by local topography. Low-lying flat areas distant from streams produce the largest bogs.

Explanation:

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. If no water table was encountered, no value was recorded; use number of samples to aid interpretation. Deeper average water tables idicate higher variability.

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.

Ripple Trough 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.

RT1: Ponds in Troughs.

RT2: Trough peatlands with water table near the surface most of the growing season. Often dominated by sedges and/or buckbean.

RT3: Troughs with deeper water table, often dominated by shrubs especially sweetgale and dwarf birch.

RT4: Forested Troughs, deeper, seasonally fluctuating water table, some are bogs.