Wetland Classification and Mapping of Seward, Alaska

SEWARD WETLAND ECOSYSTEMS

Depression Ecosystem Wetlands

A Depression Ecosystem peatland on a bedrock ledge above the Snow River.

The Depression Ecosystem wetlands (outlined in blue) on the bedrock knob between Bear Lake (lower center) and the South Fork of the Snow River (upper right).  Aerial photograph taken in 1997.

Depression Ecosystem wetlands are surrounded by upland, there is no wetland connection to a navigable waterbody.  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, groundwater recharge, and streamflow attenuation (flood control). 

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. 

Depression wetlands only cover a total of about 45 acres of the 4522 acres of wetlands mapped around Seward.  Most Seward-area Depression Ecosystem wetlands occur as small peatlands on the bedrock knobs and terraces north and east of Bear Lake.    These knobs were ice scoured during the last glaciation.  The bedrock core of the knobs is arranged in layers tilted perpendicular to the ground surface, and weaker layers erode more easily.  More resistant layers remain as small ridges between the wetlands, lying in the eroded depressions between the resistant layers.  In many cases the weaker layer eroded uniformly enough to support connecting Kettle Ecosystem wetlands.  Other layers eroded less uniformly leaving behind disconnected Depression Ecosystem wetlands.

The weaker layers are frequently good conductors of groundwater, and springs originate at their bases.  Springs like these add to the water contributed by hyporrehic and flood flows from the Resurrection River and Salmon Creek to form the extensive wetland complex along Nash Road.

Plant Relationships

Depth to the water table controls plant species distribution in Depression Ecosystem wetlands, and in peatlands in general.  In Seward area depressions a sequence from open, deep water to forest is evident. 

Open water is colonized by the floating aquatic, pond lily (Nuphar polysepala) in deeper water and burr reeds (Sparganium spp.) in shallower water.  The emergents (plants rooted under water but emergent above the surface) water horsetail (Equisetum fluviatle), mare's tail (Hippuris vulgaris), and mannagrass (Glyceria spp.) are common. 

Where the water table is just below the surface Sitka sedge (Carex sitchensis) and tall cottongrass (Eripophorum angustifolium), sometimes with dwarf birch (Betula nana) are all commonly encountered.  Peat is typically present.

Shrubs dominate peatland depressions that support a deeper water table.  Dwarf birch (Betula nana), crowberry (Empetrum nigrum), and Labrador tea (Ledum palustre ssp. decumbens) are all common.   Fewflower sedge (Carex pauciflora) is often an abundant co-dominant.  Stunted mountain hemlock (Tsuga mertensiana) are also encountered. 

Where peat has not accumulated, bluejoint reedgrass (Calamagroatis canadensis) is frequently dominant.  These depressions support seasonal pools which dry throughout the summer.  Bluejoint can dominate the margins of wetter depressions in the zone where peat transitions to a mineral soil.

Woodland and forest vegetation occurs in depressions with a deeper water table, and along the margin where wetland conditions transition to upland and the peat is shallow.  Mountain hemlock with Sitka spruce (Picea sitchensis) occurs over a shrubby understory of either rusty menziesia (Menziesia ferruginea) Sitka alder (Alnus viridis ssp. sinuata) and/or early blueberry (Vaccinium ovalifolium).

 

Common Depression Ecosystem Wetland Plant Communities:

 

Sitka Sedge (Carex sitchensis)

Tall cottongrass (Eriophorum angustifolium)

Fewflower sedge - dwarf birch (Carex pauciflora - Betula nana)

Crowberry- Labrador Tea (Empetrum nigrum - Ledum palustre ssp. decumbens)

Sphagnum moss - Ericaceous shrub

 


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 near Seward as Terrene Basin isolated wetlands.  Depressions dominated by lakes larger than 20 acres and deeper than 6.6 feet are Isolated Natural Lakes.


Summary of Depression Ecosystem Map Components:

D1- Open water.  Floating or emergent vegetation.

D2 - Water table at or near the surface.  Sedge and/or dwarf birch dominated.  

D3 - Water table does not reach the surface.  Shrubs or bluejoint grass (Calamagrostis canadensis) dominant.

D4 - Redoximorphic features or deep peat the wetland indicator.  Woodland or forest.

Depression map component combinations used around Seward: D12, D1-3, D21, D23, D31, D32, D34, D43

 


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Contact: Mike Gracz
Kenai Watershed Forum 
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11 December 2006 15:24