Depression Ecosystem wetlands are surrounded by upland, there is no wetland connection to a navigable waterbody.  However, large Depression Ecosystem wetlands could be navigable-in-fact lakes.  They are uncommon and diverse on the lowlands (Figure 1A, 1B, 1D).    Depressions occur on younger glacial surfaces such as the large ‘Moosehorn’ recessional moraine east of Sterling (Karlstrom, 1964, Reger, et.al, 1996), or the pitted till between Nikiski and the Moose River.  These  younger surfaces have yet to become subdued by erosion.  Although Depression Ecosystem wetlands are not connected by other wetlands to a navigable waterbody, and thus may not be jurisdictional under recent interpretations of section 404 of the Clean Water Act (see EPA /ACOE SWANCC press release), they do meet hydrological, plant and soils criteria as wetlands and perform typical wetland functions, such as duck nesting habitat and groundwater recharge.  Larger navigable-in-fact isolated lakes are jurisdictional.

Depression Ecosystem wetlands can be difficult to discern from Kettle Ecosystem wetlands.  Depressions are hydrologically isolated at or near the surface, but connections can be difficult to detect without careful scrutiny of aerial photography or a thorough ground search.  Some connections between kettles and other wetlands were too small to map at a scale of 1:25,000.  If a connection was discernable using stereo-paired aerial photographs, or was discovered on the ground, the wetland was mapped as a Kettle even if the connection was not mapped.  If no connection was found, it was mapped as a Depression.  Depressions are most common north of Clam Gulch.

Both Kettles and Depressions were formed by similar processes, namely deposition at the margins of ablating glaciers.  Depression Ecosystems are of two types, neither connected by other wetlands to a navigable waterway.  One type is a small steep–sided feature created at late-Wisconsin glaciers melted.  After the glaciers melted, they left behind terminal moraines with material in closely-spaced parallel ridges.  The material was either pushed into ridges by re-activated retreating ice or deposited in crevasses at the glacier margin (Embleton and King, 1975).  The depressions between the closely spaced, and steep-sided ridges almost always cover less than 5 acres, and are separated by plenty of intervening upland material. 

The second type of Depression Ecosystem wetland is a large depression, often a lake, on the re-worked till between Nikiski and Sterling.  Daniel's Lake, near Nikiski, at 692 acres, is the largest Depression Ecosystem wetland in the project area.  These depressions form in bands where glacial still-stands produced weak terminal moraines as they re-worked the older underlying till.  Bands of Kettle Ecosystem wetlands form behind bands of depressions, just as kettles often form in the stagnant ice just behind a terminal moraine.  Some kettles within a band of depressions are connected to navigable water only because the Cook Inlet shoreline has eroded back to them.

Some Depressions barely meet wetland criteria, having thin peat layers and relatively deep water tables, while others contain central pools with several concentrically arranged peatland plant communities grading to the surrounding upland.  Species variability between different Depression Ecosystem wetlands is high, and many are unique (high beta diversity).

Other non-wetland depressions, especially on the terraced moraines near Ninilchik, contain upland Andisols of the Island Series with diverse graminoid and herbaceous plant communities.  These uncommon locales are being invaded by forest and the soils are rapidly developing albic and spodic horizons.  A hummocky micro-topography is present, which becomes more subdued further into the surrounding forest. 

Aerial photographs reveal that since 1951 about a third of the Island series soils have been invaded by spruce (figure 1C.).  A speculation that the Island Andisols were not long ago peatlands that have since oxidized, seems plausible.  The mechanism could be an interaction of soil texture and the increased frost-free season since the end of the little ice age.  When the frost-free season was shorter, a fine soil texture may have held heavy autumn rains, and remained frozen for most of the year, perching a water table, supporting hummocky peat accumulation.  When the frost-free season became longer, causing later freezing following fall rains, earlier spring thaw, or both, the soil drained, or was not frozen as long, and the organic matter oxidized into a rich Andisol, which is now being transformed into a Spodosol (and supports good hayfields).

NWI and HGM

The US Fish and Wildlife Service's National Wetlands Inventory classifies this ecosystem into its 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 HydroGeomorphic Model (Tiner, 2003) classifies the depressions commonly found on the Kenai as Terrene Basin isolated wetlands.  Depressions dominated by lakes larger than 20 acres and deeper than 6.6 feet are Isolated Natural Lakes.

Plant relationships:

Plant communities in Depression Ecosystem wetlands follow similar patterns as those in Kettle and Relict Glacial Lakebed Ecosystems.  In depressions with a complete sequence from open water to forest, the center is occupied by a pool with floating pond lily (Nuphar polysepala) and pondweed (Potamogeton spp.); while the edges of the pool supports water horsetail (Equisetum fluviatle) and emergent sedges, often beaked (C. utriculata) and livid sedge (C. livida). 

Towards the upland, a progression of dominant plants follows.  Completely terrestrial sedges, especially fewflower (C. pauciflora), manyflower (C. pluriflora), water (C. aquatilis), tufted bulrush (Trichophorum caespitosum), and tall cottongrass (Eriophorum angustifolium) are encountered as one moves from the edges of a pool toward the upland.  The dwarf shrubs sweetgale (Myrica gale), and dwarf birch (Betula nana), are encountered among the sedges, followed by Labrador tea (Ledum palustre ssp. decumbens) and crowberry (Empetrum nigrum), which occur above most sedges. 

Above the shrubs, a forested peatland with black spruce (Picea mariana) forest (rarely white spruce, P. glauca) and an ericaceous shrub understory grades to upland.  South of Clam Gulch a Lutz spruce (P. X lutxii) forest with a Barclay's willow (Salix baclayi) understory is more common.   Sometimes a sphagnum-dominated community (typically with round sedge, C. rotundata) occurs between the shrub-dominated communities and the forest. 

Many depressions are occupied by bluejoint grass (Calamagrostis canadensis).  Bluejoint grass is often found at the fringe of wetland and upland conditions.  Many depressions occupied by bluejoint grass may not meet the criteria outlined in the 1987 Army Corps of Engineers wetland delineation manual, and many others are only seasonally wet.

Most Depression Ecosystem wetlands are occupied by only one or two plant communities, lacking the full range from open water to forest.  By far the most common are depressions occupied by either bluejoint grass or shrubby peatlands, with or without black spruce forest.  Many contain some area of sedge-dominated peatland.

Figure 1A.  This depression, near Sterling, has open water in the center.  A sphagnum moss- woolyfruit sedge community, then a bluejoint community ring the central pool and grade to a white spruce upland.

Figure 1B.  A bluejoint grass depression on the moosehorn moraine east of Sterling.

Figure 1C.  Forest invasion of depressions near Ninilchik.  Left photo taken June 1951, right photo September, 1996.

Figure 1D.  A large depression near Sterling dominated by the sedge alpine bulrush, a frequent indicator of disturbance.

Cover

 Introduction and Key to Plant Communities  

Introduction and Key to Ecosystems

    Kenai Hydric Soils    Map Unit Summary    Methods    Glossary

03 May 2007 18:02