Wetland Classification and Mapping of Seward, Alaska

SEWARD WETLAND ECOSYSTEMS

Wetlands

Wetlands support a variety of functions from wildlife habitat to flood control.  Because these functions cross property lines and belong to many of us, they are a public good.  If an activity in a resource upstream affects downstream property owners, then that upstream resource belongs to more than just the upstream property owner.  Because so much activity has occurred in wetlands nationwide, and so many impacts to valuable resources are being felt on a widespread and costly nationwide scale, dredge and fill activities in wetlands are regulated under section 404 of the Clean Water Act.  Since these activities are regulated we need to have a definition of a wetland.

For regulatory purposes, wetlands are defined in a manual produced in 1987 by the Army Corps of Engineers (Environmental Laboratory, 1987).  The Corps, along with the Environmental Protection Agency, is responsible for enforcing section 404 of the Clean Water Act.  Using the 1987 Wetland Delineation Manual requires good technical knowledge in three primary areas: soils, hydrology and vegetation.  Techniques described in the 1987 Manual focus on using soils, hydrology and vegetation data so that, at any particular site, a line between wetlands and uplands can be drawn for jurisdictional purposes.  For our purposes, we apply the concepts outlined in that manual to define a wetland.  With some exceptions, anywhere that floods frequently, has over 16" (40 cm) of organic material on top, or a water table within about a foot (30 cm) of the surface for two continuous weeks of the growing season qualifies as a wetland on the Kenai.

Wetlands are interesting for reasons beyond the jurisdiction of section 404 of the Clean Water Act.  They affect a variety of activities from building septic systems to salmon fishing. Almost everyone wants to see salmon return year after year, and building on land that is high and dry is much easier and less expensive than building in a swamp. 

The US Fish and Wildlife Service National Wetlands Inventory (NWI) mapped 3363 acres of the 24,6600 acre Seward project area as wetland (14%).  NWI mapping is at a national scale and meant only to convey a general idea of which basic types of wetlands exist in a region, using a classification scheme applied across the country (Cowardin, et. al., 1979); and approximately what percentage of the ground is covered by wetlands

In this context we set out to classify and map Seward wetlands.  This project consists of three parts: an ecosystem level classification, a plant community classification and a map.  We used seven of the ten ecosystems named in the Kenai Lowlands Wetland Mapping Project. The ecosystems were developed using landforms and generalized hydrology.   Fifty-three plant communities were described using vascular plant presence and abundance and the communities already described by either the Kenai Lowlands Wetland Project or by Chugach National Forest.  We mapped 4522 acres on the 24,600 acre lowland project area.  We interpreted 3556 acres as wetland (14%; fig. 1), and another 966 acres as river terraces.  The river terraces do not meet wetland jurisdictional criteria.  However, given the dynamic nature of Seward area rivers and streams, these terraces may meet wetland criteria when flooding changes their course in the near future.  Although the terraces do not currently support a water table a foot from the surface, they are still relatively wet, and provide a large amount of floodplain storage capacity. 

Ecosystems

 

Seward area wetlands are grouped into seven Ecosystems.  For this project, Ecosystems are defined as landform units responding to similar history and environment to produce a unique signature on a 1:25,000 black and white aerial photograph.  As landforms (geomorphology) exert a dominant control on hydrology, Wetland Ecosystems should be useful units for predicting wetland functions, as part of a hydro-geomorphic classification tailored to local landforms.  Dominant Seward-area processes are described below, including a brief description of wetland ecosystems associated with each process.  Links are provided to more detailed descriptions of each Wetland Ecosystem.

Peatlands

Peatlands are relatively common around Seward.  They are locations where peat has built to a depth of 40 cm or more (about 18 inches).  Peatlands form where plant productivity is greater than decomposition.  Productivity is moderate, but decomposition is low,  due to interactions between a number of factors.  The low oxygen content and pH of saturated soils combines with the sequestration of minerals and amino-nitrogen by spahgnan, to preserve dead plant remains (sphagnan is a oxopolysaccharide with highly reactive carbonyl groups, present in the cell walls of spahgnum moss (Børshiem et. al., 2001)).  Plant remains accumulate as peat deposits, which may be many meters thick in places.

Peatlands can be classified into two categories: bogs and fens (Vitt, 1995).  Bogs are commonly defined as ombrotrophic systems, literally ‘fed by precipitation’.   Bogs are rare on the Kenai.  Bogs form when certain sphagnum mosses build up a layer of  nearly undecomposed peat that holds a lens of groundwater above the local groundwater table.  Because this sphagnum peat has very low hydraulic conductivity, nutrient poor precipitation stays nearly isolated from richer groundwater below.  Bogs can build rapidly where precipitation is high and temperatures moderate-- conditions which allow production to greatly exceed decomposition.  These conditions are more common further south in Alaska.

Kenai peatlands are typically fens and poor fens, as the growing season is probably too short, and precipitation too low for bog forming sphagnum mosses to thrive (not all sphagna are bog forming).  Fen groundwater has had some recent contact with a mineral substrate, so is more nutrient rich than bog water, and fen peat is composed of sedges, shrubs, and forbs as well as mosses, including both bog-forming and non-bog forming sphagna.  On the Kenai, tephra (volcanic ash) input is steady and this input along with marine aerosols, may create a more mineral rich precipitation, ameliorating bog conditions.

Worldwide distribution of peatland types has been mapped and zones have been delineated for many areas (Moore and Bellamy, 1974).  The Kenai Peninsula lays between the zone where bogs are common (in Southeast Alaska) and the zone where permafrost perches water.  Perched water aids peat accumulation by creating the anaerobic conditions that slow decomposition.  A host of unique landforms are generated where permafrost is responsible for perching the water table such as patterned ground and pingos.  Kenai Peninsula peatlands lie between these two extremes. 

Peatlands are estimated to hold about 30% of all carbon stored in soils.  The fate of that carbon is extremely important in global climate change models.  If peatlands are expanding, then they are acting as a carbon sink, ameliorating the effect of increased CO2 input into the atmosphere.  If peatlands are decomposing, then they are acting as a source, exacerbating the greenhouse effect (Makila, et. al, 2001).  Evidence suggests that peatland accumulation and decomposition is spatially and temporally variable. The same locale accumulates peat during some years and looses peat during others (Waddington and Roulet, 2000).

Although lower temperatures (lowering productivity) and precipitation probably both limit bog formation on the Kenai Lowlands, under past climatic regimes bogs were possibly more common, as they are on flatter landscapes in Southeast Alaska.  As climate warms, bog formation may (re-?)initiate.  Alternatively, warmer conditions could lead to drier soils, favoring decomposition over productivity, resulting in peatland decay, producing even more CO2.

Peatlands around Seward occur in four ecosystems.  The most common are in the Kettle and Depression Wetland ecosystems.  These are small peatlands found in low spots on ice-scoured bedrock knobs. 

The higher elevation sloping fens found on the ice-cut bedrock terrace above Fourth of July Creek are classified in the Headwater Fen Wetland Ecosystem.

A peaty forest with several sedge and shrub-dominated openings occupies the non-alluvial surface on the east side of the large rock drumlin between Sawmill and Salmon Creeks.  This peatland complex is classified in the Relict Glacial Drainageway Wetland Ecosystem.

Floodplains

Typical notions of floods and floodplain development do not apply in the valley-confined, aggradational environment of Seward area rivers and streams.   Retreating glaciers provide a vast supply of materials, especially gravels.  High rainfall events are common.

A high rainfall event (15" in 24 hours during early October, 1986) combined with shallow-rooted spruce and steep mountainsides multiplies flood flow potentials.  Debris avalanches dam streams. These debris dams are eventually breached by floodwaters, and flows much larger than the precipitation-driven flood already in progress are released.

These large flows carry readily available gravel, which drops out as the flow subsides.  The gravel raises the streambed, sometimes by a meter or more in elevation, often causing the stream to shift course to a lower position.

Streams are confined to relatively narrow valleys, emerging from them to flow together across converging alluvial fans.  A stream leaves its side-valley, and crosses the apex of its alluvial fan at a very narrow spot at the top of the broad fan.  If the stream leaves its channel at this spot, as it does during a flood, it can potentially flow anywhere down the fan.

The entire alluvial surface from Bear Lake to Resurrection Bay could potentially support a stream channel, including the alluvial fans at the mouth of Fourth of July, Lowell and Spruce Creeks.

Floodplain Wetlands

Approximately 620 acres of wetlands are classified as floodplain wetlands around Seward.  Floodplain wetlands help store flood flows, dispersing flood intensity over time and across the valley.  If these wetlands retained an average of about a foot of water for one day during a flood, that represents nearly  the same amount of water as normally flows in Salmon Creek at the Bruno Bridge, and about the same flow that washed out a portion of Nash Road during the 1986 flood (Jones and Zenone, 1987).

If the wetlands are filled, or water is diverted from them, that storage function is lost, and already severe flooding becomes more pronounced.  Position in the watershed affects how much downstream area will be protected by a wetland's flood storage capacity.  Wetlands higher in a watershed will affect more downstream area.  The wetlands adjacent to the Salmon River north of Nash Road will buffer flood intensity above the Seward Highway, Alaska Railroad, and Nash Road bridges, for example.

River terraces provide a similar flood storage value, although they do not meet criteria to be considered wetlands under the jurisdiction of the Clean Water Act.  Although these areas are not jurisdictional wetlands, they are included in the map because of their importance in ameliorating floods, and potential to become wetlands following the next flooding event.  In the Seward area an additional 620 acres of river terraces are included on the map; 198 acres of terraces around the Snow River are also included.

Floodplain wetlands, river terraces, and associated channels and side-channels are all classified in the Riparian Wetland Ecosystem.

 

Ecosystem Descriptions

The following descriptions of each ecosystem repeat a common format.  First they outline the dominant landscape process responsible for the existence of the ecosystem, then the dominant patterns within each ecosystem.  The ecosystem classification is then crosswalked to the two most widely used classification systems: NWI (the National Wetlands Inventory) and HGM (Hydrogeomorphic Model, as presented in a key by Tiner, 2003).  Next, the common geographic locations of the ecosystem are outlined, followed by an brief ecosystem characterization including the common soils found in each system. A description of dominant plant communities and relationships of individual plants within the ecosystem is outlined, including a summary table that links plant community names to their descriptions.  The descriptions end with a summary of the map components and units.


Table 1.  Summary of Seward area Wetland Ecosystem distribution.  Eighteen percent of the project area is mapped as wetland or river terrace (4522 acres of the 24,600 acre project area).

Ecosystem

Acres

% Wetland area

Number of features

Riparian (includes 466 acres of river terraces)

3180

70.3

211

Kettle

751

16.6

130

Tidal

266

5.9

26

Headwater Fen

110

2.4

41

Drainageway

166

3.7

3

Depression

45

1.0

40

Discharge Slope

18

0.4

3

Wetland / Upland Complex

7

0.2

1


A Key to the Wetland Ecosystems of the Seward Area:

1. Wetland periodically inundated by salt water.................................................................................... TIDAL
1. Wetland not periodically inundated by salt water.............................................................................. 2.
     2. A wetland on a bedrock knob or shelf, with no wetland connection to navigable waterway, although the wetland could be a navigable-in-fact lake............................................................................................. DEPRESSION
     2. Wetland connected by other wetlands to a navigable waterway................................................... 3.
3. Channelized flow present, with bed and bank morphology................................................................ RIPARIAN
3. If flow is present, not in a channel exhibiting bed-and-bank morphology..................... ..................... 4.
     4. Wetland in a broad valley bottom, but without a defined modern channel, with a water table near the surface, even when forested................. DRAINAGEWAY
     4.  If a wet forest is present, then wetland lies along a slope break, or is a peatland.......................................................................... 5.
5. Slope break influences groundwater discharge; usually at a  foot- or toe-slope landscape position on a terraced moraine, often over a mineral soil........................................................................................ DISCHARGE SLOPE
5. Wetland not along a slope break, is a peatland............................................... 6.
     6. A peatland above about 250m elevation in the headwater basin of a first order stream................. HEADWATER FEN
     6. Peatland below about 250m elevation, occupying a low spot on a bedrock knob..................................................... KETTLE

 

 


Do I Need a Permit?

 Seward Area Plant Communities

Introduction and Key to Seward Wetland Ecosystems

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11 December 2006 17:30