Matanuska-Susitna Wetland Mapping

Cook Inlet Wetlands

Mike Gracz
13 January, 2010

TIDALLY-INFLUENCED DRAINGEWAY ECOSYSTEM Wetlands

A shaded relief map showing the tidally influenced wetlands in Goose Bay, on the western shore of Knik Arm..

Tidally-Influenced Drainageway wetlands occur along the shores of Knik Arm. Goose Bay and Palmer Hay Flats State Game Refuge together support nearly all the Tidally-Influenced Drainageway Ecosystem wetlands in the area mapped. These wetlands are influenced by the extreme tidal range of Knik Arm, mixing with large amounts of freshwater discharging from the glacial sediments comprising the shoreline.

Tidal ranges vary daily, monthly, annually and on an 18.6 year cycle. The highest tide recorded in Knik Arm was 34.86 feet at 4:12 AM on 6 November 2002, although during some years the tide never exceeds 33.5 feet (NOAA, 2007; referenced to Mean Lower Low Water). Therefore, during some years the tide barely influences a zone from about 33 feet to 35 feet. This two foot elevation distance can cover a large area over the gentle gradients encountered in the Palmer Hay Flats and Goose Bay Refuges.

Tidally-Influenced Drainageways support an extremely broad gradient from fresh to saltwater controlled by multiple processes. Salinity can be as low 4 PSU in upper Cook Inlet (KABATA, 2005).  Average seawater salinity is about 35 PSU (practical salinity units, or grams per kilogram of seawater). Freshwater from two sources dilutes salinity, and high tide residence time is of short duration.

The first source originates from glacially-fed river systems.  These systems input large amounts of freshwater during the growing season.  River water dilutes saltwater through sub-surface processes as well.  A zone of shallow groundwater lies adjacent to all stream and river systems, and can extend for more than a mile (the hyporrheic zone).  The hyporrheic zone is where river water moves below the surface, through adjacent sediments.  High flow in broad, low-gradient valleys with permeable sediments, as found in upper Knik arm, results in a wide hyporrheic zone. 

In addition to hyporrheic water, a large amount of groundwater is discharged from the permeable deposits underlying much of the area between Palmer and Houston (Jokela, et. al., 1991).  In Goose Bay this discharge appears to float a layer of peat about one meter thick, over a lens of water 4 meters deep. The surface elevation at the highest point is about 40 feet, well above the range of the highest recorded tide. The surface of the strongly gleyed mineral soil beneath the floating peat lies at about 30 feet, near the level of mean higher high water (29.16'), indicating almost monthly intrusion of saltwater. The water near the surface is dilute; the average specific conductance of five surface water measurements at the high point on the mat was 135.3 µS (25°C; Table 1), close to values commonly encountered in local streams.

Already diluted saltwater (by both surface and groundwater inputs) is further diluted during the ample time between tides.  The uppermost zone is inundated by only the highest tides of the 18.6 year cycle.  Additionally, the highest tides often occur outside the growing season.  These factors of high tide range variation and timing, low initial salinity, high groundwater discharge and hyporrheic input of the large river systems combine to support plants not normally considered halophytic (salt-loving), including balsam poplar forest, over soils indicating periodic tidal inundation.

These unique wetlands are classified into the Tidally-Influenced Drainageway wetland ecosystem.  This system is divided into two major zones, one where the tide influence seems to dominate (TDW), and the other where freshwater influence seems dominant (DWT). The zones where tidal influence is more frequent, approximately monthy, are classified into the Tidal Wetlands Ecosystem.


Table 1. Wetland Indicators in Tidally-Influenced Drainageway Ecosystem Map components

Map Component

Peat Depth (cm)

Water Table (cm)

Redox features (cm) Saturation (cm) pH Specific Conductance µS/cm Plant Prevalence Index

DWT1

34 (1)
-12 (1)
34 (4)
-12 (1)
7.1(2)
434.4 (2)
1.00 (1)

DWT2

414(3)

2.7 (3)

414 (3) 0 (3) 5.8(3) 135.3 (3) 1.28 (2)

DWT3

433(2) 7 (2) 371 (1) 2 (2) 5.5(2) 130.2 (2) 1.89 (2)

TDW2

119 (2) -4.5 (2) 53 (1) -5 (2) 6.8 (2) 391.7 (2)

1.05 (2)

TDW3

70 (3) 52 (2) 9 (2) 9 (2) 6.8 (2) 399.0 (2)

2.04 (2)

TDW5A

14 (1) 188 (1) 8 (1) 188 (1)    

3.58 (1)

 

Explanation:

Peat depth is a minimum, because some sites had thicker peat deposits than the length of the auger used (usually 493 cm).

Water table depth is a one time measurement. At sites with seasonally variable water tables this measurement reflects both the conditions that year, and the time of year.

Redox features with low n and deep depths typically indicate deeper peat deposits, which mask redox indicators.

pH and specific conductance measured in surface water or a shallow pit with a YSI 63 meter calibrated each sample.

Plant Prevalence Index calculated based on Alaska indicator status downloaded from the USDA PLANTS database, which may use different values than the 1988 list.


COMMON SOILS COMMON PLANT COMMUNITIES

TYPIC CRYAQUENTS

HISTOSOLS

Buckbean

Barclay willow - bluejoint reedgrass

Water sedge - marsh fivefinger

Thinleaf alder / sweetgale / bluejoint reedgrass

Explanation:

Soils from 1998 Soil Survey of the Matanuska-SusitnaValley Area, Alaska, available online, follow the link in the table heading.

Plant communities based on Kenai Peninsula wetland mapping and soil survey, or other published accounts, especially Viereck, et.al., 1992.

 


Tidal-Drainageway Ecosystem Wetland Map Components:

Map unit names are made of combinations of map components. A suffix 'c' idicates a created wetland, and a 'd' indicates a highly disturbed wetland.

DWT/TDW1: Open water

DWT/TDW2: Water table near the surface most of the growing season, often dominated by sedges.

DWT/TDW3: Deeper, variable water table, often dominated by shrubs, especially sweetgale, barclay willow, and thinleaf alder.

DWT4: Dominated by bluejoint reedgrass. (only present as a component in a single complex polygon)

DWT/TDW5A: Forested, typically by black spruce.

DWTR: A complex of more than two non-consecutive components. Often a narrow water course fringed by shrubs and forest.

Table 3.Summary of Matantuska-Susitna Valley and Cook Inlet Discharge Slope Map Unit occurrence (percentages of all wetlands).
Map Unit MatSu N Cook Inlet N Mat Su Hectares Cook Inlet Hectares Mat Su % Polygons Cook Inlet % Polygons MatSu % Area Cook Inlet % Area
DWT1 16 16 17 17 0.24 0.07 0.04 0.01
DWT12 9 9 65 65 0.14 0.04 0.15 0.03
DWT1-3 5 5 87 87 0.08 0.02 0.20 0.05
DWT2 45 45 294 294 0.68 0.19 0.69 0.16
DWT21 34 34 286 286 0.51 0.14 0.67 0.15
DWT23 10 10 180 180 0.15 0.04 0.42 0.10
DWT23c 1 1 1.6 1.6 0.02 0.00 0.00 0.00
DWT25A 3 3 29 29 0.05 0.01 0.07 0.02
DWT2-5A 1 1 11 11 0.02 0.00 0.03 0.01
DWT2d 1 1 4.6 4.6 0.02 0.00 0.01 0.00
DWT3 19 19 195 195 0.29 0.08 0.46 0.10
DWT35A 7 7 62 62 0.11 0.03 0.14 0.03
DWT45A 1 1 2.4 2.4 0.02 0.00 0.01 0.00
DWT5A 19 19 142 142 0.29 0.08 0.33 0.08
DWT5A3 3 3 36 36 0.05 0.01 0.08 0.02
DWT5Ad 1 1 0.5 0.5 0.02 0.00 0.00 0.00
DWTR 1 1 1.8 1.8 0.02 0.00 0.00 0.00
TDW1 15 15 186 186 0.23 0.06 0.44 0.10
TDW12 1 1 34 34 0.02 0.00 0.08 0.02
TDW1-3 3 3 10 10 0.05 0.01 0.02 0.01
TDW1-5A 2 2 63 63 0.03 0.01 0.15 0.03
TDW2 19 19 368 368 0.29 0.08 0.86 0.20
TDW21 5 5 324 324 0.08 0.02 0.76 0.17
TDW23 9 9 547 547 0.14 0.04 1.28 0.29
TDW2-5A 2 2 6.1 6.1 0.03 0.01 0.01 0.00
TDW3 8 8 225 225 0.12 0.03 0.53 0.12
TDW32 6 6 255 255 0.09 0.02 0.60 0.14
TDW35A 6 6 59 59 0.09 0.03 0.14 0.02
TDW3c 1 1 0.5 0.5 0.02 0.00 0.00 0.00
TDW5A 4 4 108 108 0.06 0.02 0.25 0.06
TDW5A3 2 2 30 30 0.03 0.01 0.07 0.02

 

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Contact:
Mike Gracz
Kenai Watershed Forum 
PO Box 15301
Fritz Creek, AK  99603
907-235-2218
13 January, 2010
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