Peatlands

Peatlands are defined here as locations where peat has built to a depth of 40 cm or more (about 18 inches).  These wetlands are common and extensive on the Kenai Lowlands.  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, found locally within a few larger peatland systems.  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, and are typified by patterned fens (different than patterned ground) with relatively thin, moderately decomposed peat layers. 

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.

Some evidence suggests that peatlands on the Kenai Lowlands are undergoing substantial change.  The fibric soil series Salamatof was mapped more extensively on the Kenai lowland in the soil surveys published during the early 1970’s.  Fibric soils are typified by un-decomposed organic matter.  Many of the areas previously mapped as Salamatof are now being mapped as hemic Doroshin and/or Starichkof series soils.  Hemic soils are typified by partly decomposed organic material.  We do not know if this is because soil scientists mapping during the 1950’s and 1960’s made more errant judgment calls based on limited data, or if there was actually more fibric material present then.  If more fibric material was present, this would indicate that current peatlands may be entering a climate regime more conducive to decomposition. 

Another trend, in mineral Island Series soils, discussed under the Depression Ecosystem wetlands description further suggests that viewing Kenai Lowland peatlands as static is not appropriate.

Locally, peatlands are coming under increasing pressure and scrutiny.  Road and home building, groundwater withdrawal, logging practices and recreational activities are all having an increased impact.  This impact will have an effect on carbon and flood water storage dynamics, wildlife and fisheries habitat, and scenic values.  Globally and regionally it is important to understand the role of Kenai Peninsula peatlands.

Glaciation

Glaciers are responsible for the shape of the Kenai Lowlands landscape, and glacial till controls the distribution of wetlands.   Knowledge of local glacial events and processes is important to both develop and understand a locally relevant wetland classification system.  There is more to be learned about Kenai Lowlands glacial history and what is currently known was described before good maps and dating techniques became established.  The glaciation names in Karlstrom's pioneering paper on the Quaternary geology of this area (1964) are used below only because they facilitate describing surfaces found on the Kenai Lowlands.  These surfaces have since been found to be inconsistent throughout Cook Inlet, and the dates proposed for many ice advances are suspect.  The names are useful, however, because they describe most lowlands surfaces relative to each other, and because no other consistent names are available.  Karlstrom provided a good general framework , but much more detail is becoming available.

The author is extremely grateful to Dr. Richard Reger for his patient Quaternary geology tutelage.  Reger is currently re-mapping lowland glacial deposits.

To understand local glacial landforms and processes one must understand glacial chronology.  Glaciers did not come as one large ice-age event, then recede.  Many glaciation events occurred within larger glacial periods.  The events can be classified into three tiers: glacial periods, glaciations, and glacial advances.  The broadest are the periods.  The most recent glacial period is named the Wisconsin.  This period is recognized across North America.  On the Kenai lowlands, within the Wisconsin period there were at least three major glaciations.  Within the last of those glaciations, the Naptowne, there were at least four major advances, the first of which is named the Moosehorn advance (figure 2.).

Each successive glacial period, glaciation, and glacial advance was less extensive than the previous one.  We know this because if the last event was the most extensive it would have obliterated the evidence of all previous events.  Evidence is left behind in the form of moraines and associated deposits known collectively as glacial till.

The first glacial periods covered all of the Kenai.  Only during the last period (or two?) did glaciers fail to cover the entire lowlands and start to leave behind glacial till of differing ages.  The distribution of glacial till is key to understanding the distribution of wetlands.  Where dense, unsorted till is left behind on slopes, a water table is perched, supporting mineral soil wetlands.  Where fine-grained silts were deposited at the bottom of glacial lakes and drainage channels, peatlands form.

The Naptowne glaciation left behind much of the till we see on the lowlands today.  Before the Naptowne, three glaciations left behind significant areas more or less covered by till.  From oldest to youngest, these glaciations are the Caribou Hills, Eklutna, and Knik glaciations.  The Knik is probably an early Wisconsin-period glaciation, and the Caribou Hills till probably pre-dates the Wisconsin.  These earlier glaciations are discussed below. 

The oldest glacial surfaces on the lowlands are covered with till from the Caribou Hills glaciation.  These surfaces lie above about 600 meters elevation.  The Caribou Hills glaciation covered all of the lowlands. However, at lower elevations Caribou Hills till has been obliterated by subsequent, less extensive glaciations.  Caribou Hills till is now only present in a few locales at the highest elevations in the project area.

The next oldest till was laid down during the Eklutna glaciation.  That till lies between about 500 and 600 meters elevation on relatively flat plateaus and terraces.  The Eklutna glaciation was the first to leave the highest elevations of the lowlands ice-free. 

Two significant features were created during the Eklutna glaciation, high elevation plateaus, and the interlobate moraine.  Eklutna plateaus are high enough to hold snow patches late into the growing season.  Slowly melting, late-lying snow on top of a relatively dense (but discontinuous) till, maintains a water table near the surface during the growing season, supporting Late Snow Plateau Ecosystem wetlands.  The Eklutna probably also initiated the formation of the interlobate moraine, the large lake-dotted area between Nikiski and Sterling, which has been re-worked by subsequent glaciations, and is described below, under landforms.

The next surface, the steep-sided Knik surface, was carved from the Caribou Hills, probably during the first glaciation of the Wisconsin period.  Knik surfaces lie between about 300 and 500 meters elevation.  Because they are steep-sided, till has eroded away in many areas.  Because the dense glacial till that perches a water table is often absent on Knik surfaces, they are typically well-drained and often only subtly terraced.  Knik surfaces support  uncommon Headwater Fen Ecosystem wetlands in headwater basins and narrow Riparian Ecosystem wetlands along streams.  The most extensive hayfields are found on this surface.

The next surface-- the Naptowne--  is the youngest.  Naptowne glaciation landforms dominate the Kenai Lowlands below 300 meters elevation.  The Naptowne has been divided into at least four glacial advances.  The Moosehorn advance was the earliest and left behind large and distinctive landforms.  These and other glacial landforms are not uniformly distributed.  They are segregated into physiographic provinces, creating areas where different wetland Ecosystems dominate. 

ADVANCE:

GLACIATION:

PERIOD:

Landforms

Landforms and ecosystems do not occur uniformly across the lowlands.  Karlstrom (1964) divides the lowlands into five physiographic provinces, based on dominant landforms.  We divide the lowland project area into five groups, but these groups are slightly different:

1. The "interlobate moraine" (the northern portion of Karlstrom's (1964) Nikishka Lowlands), primarily lying north of the Kenai River, between Cook Inlet and the Moose River, is dominated by Kettle and Depression Ecosystem wetlands. 

2. The terraced moraines on the west side of the Caribou Hills (most of Karlstrom's Ninilchik Lowlands) are dominated by Relict Glacial Drainageway and Lakebed Ecosystem wetlands and Discharge Slopes. 

3. The kettle-knob topography along the Old Sterling Highway southeast of Anchor Point (the extreme southwest corner of Karlstrom's Ninilchik Lowlands), and around Caribou Lake about 22 miles northeast of Homer (a subdivision of Karlstrom's Caribou Hills Uplands), is dominated by Kettles. 

4. The land terminating moraines east of Sterling and west of Tustumena Lake (part of Karlstrom's Nikishka Lowlands) are dominated by Depression Ecosystem wetlands. 

5. The Caribou Hills (corresponding almost exactly with Karlstrom's Caribou Hills Uplands) are dominated by Late Snow, Headwater Fen and Riparian Ecosystem wetlands (and pre-Naptowne till).

The first class of landforms on the Kenai Lowlands, the "interlobate moraine" between Nikiski and Sterling, consists of material re-worked, then deposited after a lowlands ice sheet ablated at the end of the Moosehorn advance.  Huge braided outwash streams buried massive stranded ice blocks, which melted under the outwash sediments, forming the current kettle-knob landscape, which supports large Kettle and Depression Ecosystem wetlands.  Many small basins are occupied by modern lakes, while others are covered by peatlands and surrounded by knobs and modified kames.  At the margins of the interlobate moraine, drainage development is suggestive of scabland topography (Karlstrom, 1964) rather than the kettle-knob topography present southeast of Anchor Point (the third group of landforms).  Scabland topography is generated when a large proglacial lake drains catastrophically.  However they formed, these poorly-integrated relict drainage channels now support Relict Glacial Drainageway peatlands.

The second class of landforms, the terraced moraines west of the Caribou Hills, support flat, linear Relict Glacial Lakebed Ecosystem and Relict Glacial Drainageway peatlands between terrace risers.  The moraines were covered by a large ice-dammed lake.  This lake left massive fine sand and silt deposits which now perch a water table, creating conditions conducive to extensive peatland formation on lower elevation terrace treads.  Upper elevation terraces consist of broader risers supporting Discharge Slope wetlands and narrower, less extensive peatlands on terrace treads (risers are the more or less vertically oriented features between the horizontal treads).

The third class, the kettle-kame landscapes southeast of Anchor Point along the Old Sterling Highway, and around Caribou Lake, originated when ice lobes became stagnated as they melted, leaving behind a kettle-kame landscape in front of a relict glacial lakebed.  Low-lying areas were left covered with silt, which perches a water table, and now are covered by lakes or peatlands-- mostly Kettle and Relict Glacial Lakebed Ecosystem wetlands.  After ice blocks melted, glacial till remained behind as small knobs.  The lower slopes of the knobs are occupied by forested Discharge Slope Ecosystem wetlands.

The large land-terminating 'Moosehorn' moraines east of Sterling, and west of Tustumena Lake, the fourth class of landforms of the Lowlands Project Area, support Depression Ecosystem wetlands.  The moraines east of Sterling formed at the beginning of the Naptowne glaciation as a large lobe from the northern Harding Ice Field pushed across Skilak and Hidden Lakes, terminating just east of the Moose River.  This large terminal moraine complex probably formed both during a series of recessional and surging events, and from material deposited in crevasses.  Steep-sided parallel ridges isolate depressions between them.  The depressions are preserved because they are young and erosion has been insufficient to subdue them.  Even younger "Killey" advance moraines are prominent between Tustumena Lake and Kasilof and also contain many Depression Ecosystem wetlands.  They were deposited later in Naptowne time by the Tustumena glacier advancing across what is now Tustumena Lake.

The fifth landform class is composed of high plateaus in the Caribou Hills discontinuously covered by 'Eklutna' till.  In areas where the till remains, it perches a water table forming Late Snow Plateaus, which are uniformly covered with a diverse willow plant community.  Headwater Fens lie at the edges of the Late Snow Plateaus, in the headwater basins of first-order streams, often on a 'Knik' surface.  This creates a counter-intuitive wetland distribution: wetlands above (on Late Snow Plateaus) and below (on dense, Naptowne till) with uplands in between (on the steep, well-drained Knik surface, from which most of the till has eroded).

Ecosystems

Kenai lowland wetlands are grouped into ten Ecosystems, as described above.  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 map-able units for predicting wetland functions, as part of a hydro-geomorphic classification tailored to local landforms. 

The ten ecosystems on the Kenai lowlands are: Relict Glacial Lakebeds, Relict Glacial Drainageways, Discharge Slopes, Kettles, Tidal flats, Riparian corridors, Floating Islands, Late Snow Plateaus, Headwater Fens and Depressions.  Plant communities and processes are distinct within each of these ecosystems, although overlap of communities and processes occurs between them.  Sometimes the boundaries are blurry, as where a relict glacial lakebed grades to a relict drainageway, then to a modern stream channel emptying into a lake with a tidal outlet to Cook Inlet.  Classification is by necessity simplification.  The ecosystems, mapping units and plant communities described here all represent centroids, distinctions are blurry towards the margins.  Hopefully the classification framework presented here will help focus discussion about Kenai wetland functions and processes.

Tidal and Floating Island Ecosystems are not as strongly influenced by glacial activity as those just described. Tidal ecosystems are influenced by the tidal cycles of Cook Inlet and Kachemak Bay.  Only five Floating Islands were mapped, all generated when human activity dammed a wetland, creating a lake.  Remnants of the former wetland are left as islands, as large as 2 hectares, floating on the lake. 

Riparian Ecosystems are of two types, glacially fed, containing large buffering lakes, and those not fed by glaciers.  The Riparian Ecosystems not fed by glaciers are frequently present as modern under-fit streams, occupying valleys carved by large volumes of ancient glacial meltwater.  Those fed by modern glaciers (the Kenai and Kasilof Rivers) shift from rivers with stable channels and low meander-width ratios (Rosgen, 1996) that cross moraines, to rivers that meander across large, flat relict lakebed/estuarine terraces near their mouths.

Ecosystem Descriptions

The descriptions of each ecosystem repeat a common format, they first 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, 1997).  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 idealized cross-sectional diagrams and a summary table that links plant community names to their descriptions.  A summary of the map components and units found in each ecosystem ends the descriptions.

Ecosystem

Acres

% wetland area

Number of features

33

0.01

5

2684

0.8

265

4887

1.4

25

7189

2.1

305

Depression

11,205

3.2

2081

Drainageway

43,139

12.3

2133

Kettle

48,138

13.8

3519

Riparian

51,376

14.7

1813

Discharge Slope

78,477

22.4

3162

Lakebed

82,910

23.7

2853

Other*

20,003

5.8

339

 Introduction and Key to Plant Communities  

Introduction and Key to Ecosystems

    Kenai Hydric Soils    Map Unit Summary    Methods    Glossary

03 May 2007 17:59