Wetland Classification and Mapping of Seward, Alaska



Map Unit Descriptions


Ecosystem: Riparian


Map Units: RD4F34; RD4F43


Seward Area Extent:

RD4F34: 6 wetland polygons; 57.5 acres

RD4F43: 2 wetland polygon; 58.8 acres

An RD4F34 along Nash Road.  It is fed by groundwater, hyporheic water from Salmon Creek, and by recent flooding.  The increased water level following recent floods will probably kill the forest, and perhaps many of the alders, changing the character of this wetland.

Wetland Indicators

Type: Floodplain wetland.

Average depth to water table: 18 cm

Organic layer thickness: 3 cm

Average depth to redoximorphic features: 11 cm

Common Soils: Typic Cryaquents

Common Plant communities:


Sitka alder / field horsetail (Alnus viridis ssp. sinuata / Equisetum arvense)

Sitka alder / bluejoint reedgrass (Alnus viridis ssp. sinuata / Calamagrostis canadensis)


Sitka spruce / Sitka alder (Picea sitchensis / Alnus viridis ssp. sinuata)

Sitka spruce - black cottonwood / Sitka alder (Picea sitchensis  - Populus balsamifera / Alnus viridis ssp. sinuata)


NWI: PSS1C (F3); PFO4(1)B (F4)

HGM: Bidirectional, non-tidal Floodplain flat

These floodplain wetland complexes are a combination of the shrubby (RD4F3) and forested components (RD4F4).  When the shrubby component is dominant the wetland is named RD4F34, and when the forested component dominates it is named RD4F43.  These wetlands are fed by a combination of river floodwaters and hyporheic water.  One small RD4F43 adjacent to the Snow River is probably also fed by groundwater discharge from an adjacent bedrock knob. 

One RD4F43 wetland lies in the Resurrection River floodplain, the other nearby, but along Jap Creek.  They are both separated from a channel by a natural levee. 

Two small wetlands mapped as RD4F34 lie on the Snow River floodplain.  The other four all lie along lower Salmon Creek.  Three of them were formerly one larger wetland that has been divided by the Seward Highway and Nash Road.  That wetland seems to be changing as Salmon Creek flows closer to the highway following recent floods.

Floodwaters are slowed and stored in these wetlands.  These particular floodplain wetlands probably support more flood storage than open water and sedge dominated floodplain wetlands, because the water table is lower.  A lower water table has more storage capacity in the soil above.  The lower it is before a flood, the more pore space available to store floodwater.

Braided stream systems carry large amounts of material during frequent floods.  As floodwaters subside the material is deposited, resulting in streambed aggradation; as much as several feet during a larger event.  When the bed aggrades, it becomes higher than the surrounding valley, so the stream channel often shifts to a lower position.  Because of this process, which dominates most of the valley floors and alluvial fans in the Seward area, prediction of where a stream channel might be following a flood event is probably impossible.  Terraces and floodplain wetlands are affected.  These areas are expected to change character following floods.

Floodplain regulation

From The Kenai Borough website:

"The Kenai Peninsula Borough manages a Floodplain Ordinance that addresses proper development to reduce flood risks and lessen the economic losses caused by flood events. The ordinance provides building standards for construction projects within the floodplain to ensure the availability of flood insurance through the National Flood Insurance Program. These building requirements also are intended to minimize or prevent damage when flood events occur. The ordinance requires floodplain development permits for all projects in floodplains."


Do I Need a Permit?

 Introduction and Key to Plant Communities  

Introduction and Key to Ecosystems

    Kenai Hydric Soils    Map Unit Summary    Methods    Glossary



Contact: Mike Gracz
Kenai Watershed Forum 
Homer Field Office
Old Town Professional Center
3430 Main Street Suite B1
Homer, AK  99603

12 February 2007 15:27