Seward Area Wetland Mapping

Wetland functions valued by society include: filtration for water quality, habitat for food and recreation, and water storage for flood control.  Before this project, Seward wetland maps were incomplete and the classification used to map them based on a system intended to describe wetlands nationwide. 

Seward area soils have never been classified or mapped.  Because soils are a key component to understanding wetlands, key wetland soils were described and classified to the subgroup level.

A locally relevant classification has been developed to map wetlands on the Kenai Peninsula Lowlands.  That classification was applied, with some modification, to map Seward area wetlands.  All the wetlands larger than about 3 acres in the Seward area, were mapped at a scale of 1:25,000, and about half of them were photographed during summer field visits in 2006. 

Methods

Mapping

 

Mapping Classification

More than just wetland boundaries are outlined; different types of wetlands are mapped separately.  In order to effectively manage wetlands, names locally relevant to the function of each type of wetland were created.  Recent research has suggested that names relevant to function are based on two things: the surrounding landscape and hydrology, how water moves across the landscape (Brinson, 1993; Brinson, et.al., 1995, Tiner, 1997). 

 

With this in mind, a mapping classification was developed in 2002, to map the Western Kenai Lowlands.  This classification relies on the knowledge we gained while mapping soils and describing plant communities over a 900,000 acre lowland project area.  The mapping classification uses a hydro-geomorphic approach tailored to the local landscape.  The geomorphic portion of the classification involved subjectively choosing and naming common ecosystems, which were assigned based on dominant landforms.  Examples of landforms/ecosystems include: Depression, Kettle, Headwater Fen, Discharge Slope and Riparian.  Seven of the ten ecosystems named for the Western Kenai Lowlands were used in the Seward area.  Seward area Wetland Ecosystem descriptions were written separately to reflect the very different geomorphic setting of Seward wetlands.

 

The hydro- portion of the hydro-geomorphic classification uses depth to the water table to further categorize Ecosystem names. For example, a small pond connected to a navigable water body is classified as a Kettle Ecosystem wetland with a water table above the surface.  It is named K1: “K” for Kettle Ecosystem and “1” indicating a water table above the surface.  A woodland Kettle wetland (deeper water table) is named K4.  Plant communities often indicate depth to water table. 

 

The mapping classification is open, that is, the number of map units is not set; they are made up of combinations of basic building blocks, the map components.  Map components are basic divisions within each ecosystem, and are used in combinations, following specific naming conventions, to devise map unit names.  The number of potential map unit names, using the map components and the naming rules, is much greater than the number actually encountered on the landscape, as some component combinations are unlikely.

 

    NAMING  CONVENTIONS:

The final computer mapping file of Seward area wetland polygons contains hyperlinks for each polygon to its corresponding map unit description.  Federal Geospatial Data Committee compliant metadata describe file creation methods and provide contact information.  You may download that zipped ArcView 8.x layer file here.

Wetland delineation and photo interpretation

We used techniques learned while mapping soils on Natural Resource Conservation Service’s (NRCS) Western Kenai Soil Survey.  Uncorrected stereo-paired 1:25,000 black and white aerial photography flown in August 1997 was used under a stereoscope, with acetate overlays and an ultra-fine point "Sharpie" marker to delineate initial wetland polygons.  Wetland polygons are relatively homogeneous areas that fit into the mapping classification.  A minimum polygon size of about 3 acres was used, although some smaller polygons were delineated.

 

After initial polygons were delineated on acetate overlays, the lines were transferred, using a 0.5 mm plastic pencil, onto frosted mylar overlain onto a geo-rectified film positive of the same black & white aerial photography.  Once the lines were transferred, they were re-traced onto a second frosted mylar sheet, creating a final, clean product ready for scanning.

 

The clean linework was shipped to Resource Data Incorporated, in Anchorage Alaska, where it was scanned, vectorized, and cleaned up.  The result was an ArcView 9.x (ESRI, Redlands, CA) Geographic Information System (GIS) shapefile.

 

The shapefile was overlain onto the digital version of the aerial photography in ArcView, where the polygons were assigned map units using the classification system outlined above.  Map unit assignments were made while consulting stereo-paired aerial photographs from 1997, and National Wetlands Inventory maps. 

 

Map units were named based on the 1997 condition of a site.  The actual wetland was expected to have changed, especially in the Riparian Wetland Ecosystem, due to the extraordinarily dynamic nature of glacier-fed braided river systems.

 

During the interpretation, an additional map unit suffixes was created to describe wetlands that were created by humans (-c).  For example, a created pond (K1) was named K1c. 

 

Field

During the summer of 2006 we visited and photographed about half of the polygons drawn in the office to describe plants and soils, and check for naming and delineation accuracy.  To aid ground-truthing, field images were prepared by overlaying numbered polygons onto digital imagery (the same 1997 imagery used to delineate the polygons).  The field images with numbered polygons were printed on matte photo paper at 1200 dpi at 1:12,000.  A corresponding data sheet was prepared to track changes and record photograph numbers. 

 

A map-able wetland unit is one where the vegetation pattern is relatively homogeneous on the ground and discernable on the 1:25,000 scale imagery.  Map units are frequently based on general hydrologic character (depth to ground water), which is typically reflected in vegetation type.  For example, sedge types frequently occur on areas where groundwater is at or very near the surface, and shrubby peatlands occupy areas of deeper groundwater.

 

Only wetland polygons were mapped.  Delineation did not formally follow US Army Corps of Engineers Wetland Delineation Manual procedures (Environmental Laboratory, 1987), but we mapped with those procedures in mind, including using many soil descriptions.  We filled out data forms included in the new Alaska Draft Regional Supplement at selected wetlands.

 

During a field visit, if the map unit name was accurate, it was recorded, otherwise it was re-named. A photograph was taken using a digital camera. 

 

If the linework was not accurate, and included more than one unique map-able unit which covered more than 10% of the polygon area, new lines were drawn on a field image to make necessary modifications. 

 

If the polygon was not a wetland a note was made to delete it.  If additional wetlands were found they were drawn on the field images.  Polygon numbers are tracked to avoid assigning duplicates.

Data Synthesis and Summary

After field visits, all data were entered into an MS access database, and error checked by printing the entered data, checking it against the raw data on the data sheets and re-entering any corrections. 

 

Lines drawn in the field on the printed aerial images were used along with notes on datasheets to correct linework, heads-up in ArcView, and re-number polygons, if needed. 

 

Digital photographs were downloaded and renamed to match polygon numbers, using lower case letter suffixes to identify multiple photographs of the same polygon.

 

New fields were added to the shapefile (420Kb, zipped) to provide hyperlinks to appropriate map unit descriptions and digital photgraphs. 

 

The photographs and descriptions are stored on a Kenai Peninsula Borough server and are available on the world-wide-web at www.kenaiwetlands.net.  American Geospatial Data Committee compliant shapefile metadata are available at http://www.kenaiwetlands.net/Homermetadata.htm.

 

The ecosystem, map unit and plant community descriptions were generated in HTML using our field notes, data and some Chugach National Forest plant community descriptions (DeVelice, et. al., 1999).  The descriptions contain links to plant lists, soil subgroup descriptions, literature, wetland plant indicator status and other useful information.  Summaries for key wetland indicators such as depth to redoximorphic features, water table depth, and thickness of the organic layer were generated from soil holes dug at the same or very similar map units.  For example the average depth to water table in the description for map units D23/D32 was calculated using depths measured at a hole in a D3 wetland and a D23 wetland.

 

Users may access the map over the internet, on the Kenai Peninsula Borough's website.  There, a user can manipulate a map containing satellite imagery to retrieve parcel ownership along with wetland and other information.

 

A user with ArcView software can download a shape- or layer file that contains URL fields linking each wetland polygon to a picture (for polygons we photographed) or a map unit description.

 

Plant Community Classification

We incorporated Chugach National Forest plant community descriptions where applicable (DeVelice, et. al., 1999).  Where communities were already described by us for the Western Kenai Lowlands project, we used that plant community classification.  Because the Seward classification and map uses this classification, the methods we used are described below. 

Field

Plant cover

Most of the plant cover data were obtained from the Natural Resource Conservation Service (NRCS).  Between 1997 and 2004 NRCS collected soils and vegetation data as part of their Western Kenai Soil Survey.  Data from 22 Hydro-Geomorphic Modeling (HGM) plots collected in 1997 along the lower Kenai River watershed were also used (Hall, et. al. 2002).  The authors collected data from 100 plots to augment soil survey data, while working for the Alaska Natural Heritage Program (NHP) during the summer of 2001; these methods are described below.

 

Ocular estimates of percent cover by species are recorded using a plot-less reconnaissance method.  Because plants cover the ground at different spatial scales, a homogeneous area was sampled with attention to these different scales.  For example, tree cover is more appropriately characterized using a larger plot, while forest floor herb cover can be adequately characterized with a smaller plot.  These different scales of occurrence are taken into account when the worker chooses an area to represent plant cover.  Unlike using a fixed sized plot, where an alder may or may not occur, the sampler can record alder cover over a larger area, and use a smaller area to represent trailing raspberry cover, as long as the entire area is relatively homogeneous. 

 

Plant cover is recorded to the nearest 1% for values between 1 and 7%; values greater than 7% (up to 15%) are recorded as 10%, then values are recorded to the nearest 10% up to 100%.  Care is taken to assure that total cover sums to at least 100%; if observation indicates that cover is obviously much greater than 100%, then the sum should reflect the plants in the plot.

 

Plant stratum and life form are recorded using the categories of: tall, medium, short and dwarf; and herb, grass, shrub and tree, respectively.  Tall trees are greater than 40 feet tall and medium trees greater than 15 feet.  A stunted tree category is also used for trees obviously suppressed or stunted, otherwise a regeneration category is utilized.  Shrubs are tall if greater than 10 feet tall, medium if greater than 3 feet, and low if greater than 8 inches.  Other shrubs are recorded as dwarf.  Herbs are tall if greater than 2 feet, medium if taller than 4 inches; if shorter, they are dwarf herbs.  Only two grass categories are used: tall if greater than 2 feet and medium if less.

 

These are the same protocols that NRCS biological technicians used when collecting the data we obtained from the Western Kenai Soil Survey. Plant names follow the 2000 version of the PLANTS database (http://plants.usda.gov).

 
Environmental data

We measured three of four environmental parameters at each site: 1) water table depth, or 2) depth to modern (versus relict) redoximorphic features; 3) pH and 4) depth of the organic horizon.  Water table, organic layer, and redox feature depths were all measured to the nearest centimeter using a metal tape.  PH was measured using a YSI 63 pH/conductivity meter.  The meter was 2 step calibrated (pH 4.04 and 6.86) daily, using the methods outlined in the meter’s manual (YSI, 1998).  When measuring pH in the field, the probe was placed directly into water in a hole dug below the water table and the value recorded when the reading stabilized for 30 seconds.

 

Each site location was marked on an aerial photograph.

Data Synthesis and Summary

Plant communities

The largest portion of data used in this analysis originated with the Natural Resource Conservation Service (NRCS) Western Kenai Soil Survey, on which the primary author of this project was instrumental in implementing plant community data collection techniques.  Various widely inclusive criteria were used to filter the entire soil survey dataset for plots that might be considered wetlands.  Wetland plots are those that meet the criteria outlined in the Army Corps of Engineers Wetland Delineation Manual (Environmental Laboratory, 1987).  Soils with modern redoximorphic features or a water table closer than 31 cm to the surface; organic horizons greater than 20 cm thick; plots in units mapped as aquic suborder soils, and sites subjectively determined to flood “commonly” to “frequently” were retained. 

 

Those data were evaluated for completeness, especially during years where non-botanists/ecologists collected plant data unsupervised.  Unreliable and incomplete data were discarded.  Reliable and complete data were printed and error checked against raw data, and corrections re-entered into the database. 

 

We used inclusive criteria- i.e. some of the plots we included in the summary analysis do not meet the wetland criteria established in the 1987 manual.  Retention of some plots that might not be considered wetlands is useful for bracketing the classification, but can lead to misleading determinations of how well any individual plant community might indicate wetland conditions. 

 

The best example of this pitfall is the Lutz spruce / Oakfern – Bluejoint community.  Field observations indicate that this plant community is most frequently found on uplands.  However, in this analysis, two of the three samples occupied by that community were found on marginally wet soils (with redoximorphic features within 16 cm of the surface), and all were found within a soils unit mapped as an aquic suborder.  A summary indicating that 2/3 of the samples containing this community are wet would be misleading, as the sample itself reflects only the wet end of the continuum the community spans. 

 

Therefore, plant community fidelity to areas considered to be wetlands using the techniques outlined in the 1987 manual is not perfect.  Some 'wetland' plant communities will be found on uplands, while some 'upland' communities will occur on jurisdictional wetlands.  As we used liberal criteria to avoid missing any communities that sometimes occur on wetlands, most of the errors should be of the first type, i.e. some of the communities described will occur on uplands.

 

Additional data were obtained from the HGM (Hydro-Geomorphic Modeling) effort conducted in the lower Kenai River watershed in 1997 by an interdisciplinary team funded by the US Environmental Protection Agency (EPA).  The HGM data (Hall, et. al., 2002) were evaluated for completeness and reliability, recoded to match the USDA PLANTS database (which NRCS and Heritage Program field crews used) and error checked, with corrections re-entered into the database.

 

To determine plant community dominants, these three plant cover data sets (NRCS, HGM and NHP) were combined and run through the computer program TWINSPAN (Two Way INdicator SPecies ANalysis; Hill, 1979) as part of the PCORD (McCune and Mefford, 1999) software package.  TWINSPAN is a polythetic, divisive matrix algebraic solution that arranges a matrix of items and their attributes, then divides the items into groups based on maximum differences of attribute presence and abundance.  It works well when, as in much plant ecological data, many of the matrix values are zeros (i.e. few plants occur in all plots).

 

TWINSPAN was run several times on varying subsets of the data (e.g. all the plots with spruce (Picea spp.) cover greater than 10% were run together), and iteratively, with outliers removed on successive runs.  Once the primary plants responsible for group divisions became stable, the data sheets were sorted into initial divisions defined by their occurrence (all the sitka alder plots, for example). 

 

Data sheets from each initial division were sorted into final groups within each division using our ecological knowledge and indicator plants identified by TWINSPAN.  These final groups are defined by the occurrence of co-dominant or sub-dominant plants (all the sitka alder plots with field horsetail (Equisetum arvense) for example); or the tufted bulrush (Tricophorum caespitosum) plots with significant dwarf birch (Betula nana).  The final groups became the named plant communities.

 

Communities are named systematically.  The plants in the tallest layers are named first, within a layer the most frequent plants are named first, and the more abundant plants that occur at the same frequency are named first.  Plants in different layers are separated by slashes, plants in the same layer by dashes.  The layer order proceeds with trees followed by shrubs followed by grasses and sedges followed by herbs.  When a tall sedge is significantly more abundant than a dwarf shrub, as in the Tufted bulrush / Sweetgale or Tufted bulrush / Dwarf birch communities, the sedge is named first.  One subset of communities with generally low vascular plant cover and high (sphagnum) moss cover is named with sphagnum moss (in the 'ground' layer) first.  Common names generally follow the USDA PLANTS database.

 

Frequency of occurrence and average cover of dominant plants (greater than 10 percent cover) were tallied.  Environmental data (depth to water and/or redoximorphic features; pH, and depth of organic horizon) were also summarized using average, minimum and maximum values found at the soils holes dug in each plant community.  The descriptions were written using field notes and sketches, the knowledge we gained working in these ecosystems, and the plant and environmental summaries described above. 

 

Dot maps of the sites visited in each plant community were also assembled (from NRCS and NHP aerial photo location marks) and included in each plant community description.

 

If subsequent field visits indicated new plant communities were needed, we queried the database to find any plots satisfying group membership (e.g. the Sweetgale- Dwarf birch / Water horsetail community was created by summarizing the plots with sweetgale (Myrica gale) and dwarf birch (Betula nana) cover > 10% that also contained water horsetail (Equisetum fluviatle)).  Summaries of the other communities were not re-adjusted to reflect any loss of data caused by the creation of new plant communities.  This loss probably would not have changed those summaries significantly. 

 

No attempt was made to shoehorn every plot into a final group (plant community).  Many plots are unique and form a diverse 'unclassified plots' group.

 

Ecosystem and Map Unit Descriptions

Ecosystem and map component descriptions were created using plant community summaries, along with field notes and observations, environmental data and photographs.

 

Each NRCS soils hole was assigned a wetland map unit based on its location in a wetland polygon.  The data collected at these holes were used to summarize wetland environmental information (depth to water table, organic layer thickness, and depth to redoximorphic features) for each map component.

 

Especially important to the ecosystem descriptions are field notes taken at “type localities” where the dominant environmental gradients in each system are well defined.  At these localities, plant and map component relationships to these gradients are described and are represented by artist drawings in the descriptions.

 

Findings

 

Wetland Occurrence and Frequency 

Four hundred and fifty-four wetlands covering 4522 acres of the Seward are were mapped (18% of the land area).  Two hundred and fifty-eight of them were visited and photographed, detailed plant and soils descriptions were obtained at 81 sites. 

 

Wetlands of some types occur more frequently than others (Table 1).  Types are classified at two levels: Ecosystems and map units.  Each Ecosystem contains several map units; map units are differentiated by vegetation, depth to the water table or physical characteristics. 

 

Full descriptions of each Ecosystem and its associated map units are available on the World Wide Web at www.kenaiwetlands.net.  Highlights are summarized below, with the most abundant map units in parentheses following a description of the Ecosystem they occur in.

 

Table 1.  Acres of wetlands in the Seward area by Ecosystem and Map Unit.

For full Ecosystem and Map Unit descriptions, see www.kenaiwetlands.net.

 

Wetland Type

Acres

Wetland Type

Acres

Wetland Type

Acres

Wetland Type

Acres

Riparian

3180

Kettle

751

Tidal

266

Headwater Fen

110

RD4C

1110

K1 515 T67 89 H34 36

RD4T2

737

K3 47 T76 48 H4 25
RD4F4 285 K34 36 T65 42 H23 16
RD4F3 251 K1-3 33 T6 28 H3 16
RD4F43 215 K4 25 T7 27 H32 7
RB 87 K32 24 T07 16 H23 4
RD4T1 73 K1c 21 T78 12 H1 3
RD4F32 66 K43 20 T87 3 H42 2
RD4F34 58 K2-4 9 T5 1 H1-3 2
RD4SC 56 K2 4 Depression 45 H43 1
RD4F12 38 K23 4 D3 10 H2 1
RD4F23 34 K21 3 D1 7 H31 0.5
RD4F1-3 32 K23c 3 D32 5 Discharge Slope

18

RD4F21 31 K31 2 D2 5 SPS 17
RD4F2 29 K13 1 D1-3 5 SA 1
RD4F1 23 K12 1 D12 4 Wetland/Upland Complex 7
RC 9 K1-4 1 D21 3    
RD4F1-4 7 Drainageway 167 D4 2    
RD4Fd 6 DW5A 148 D23 2    
RD4F2-4 5 DW3 19 D34 2    
RD4F1c 4     D31 0.5    
        D43 0.5    
         
    TOTAL WETLAND ACRES: 4522; 18%

 


Download final Seward area shapefile (420Kb- In ArcView9.x; you'll need to point to the layer's data source, under 'properties', 'source' to view the full legend).  Metadata.



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Contact: Mike Gracz
Kenai Watershed Forum 
PO Box 15301
Fritz Creek, AK  99603
907-235-2218

03 May 2007 19:08