Wetland Classification and Mapping of Seward, Alaska

 

 

Map Unit Descriptions

 

Ecosystem: Riparian

 

Map Component: RD4T2

 

Seward Area Extent:  73 wetland polygons; 737.0 acres

A younger upper river terrace where cottonwood and spruce are beginning to overtop Sitka alder.

 

An older upper terrace with mature black cottonwood and Sitka spruce.

Wetland Indicators

Type: Typically not a jurisdictional wetland

Average depth to water table: at least 80 cm

Organic layer thickness: 2 cm

Average depth to redoximorphic features: 41 cm where encountered (half of sites visited)

Common Soils: Typic Cryofluvents, Typic Cryaquents, and Typic Humicryods at one site

Common Plant communities:

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

Black cottonwood / Sitka alder (Populus balsamifera / Alnus viridis ssp. sinuata)

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

Sitka spruce - black cottonwood / field horsetail (Picea sitchensis - Populus balsamifera / Equisetum arvense)

 

 

NWI: Not generally mapped. PFO4(1)B: if wet

HGM: Bidirectional, non-tidal Floodplain flat: if wet

The areas mapped as upper terraces (RD4T2) usually do not fit the definition of a jurisdictional wetland because the water table is frequently two or three feet below the surface.  These areas probably will become wetlands following some future flood, and, like floodplain wetlands, support important flood control services, so we included them as part of this wetland map. 

RD4T2 are higher-level terraces of the large glacier-fed braided streams and rivers around Seward.  Vegetation is typically Sitka alder (Alnus viridis ssp. sinuata) with invading Sitka spruce (Picea sitchensis) and black cottonwood (Populus balsamifera) forest. 

The glacier-fed braided streams around Seward generally support two terrace levels, a lower and upper.  The lower terraces tend to be sparsely vegetated, and support a shallow water table, while upper terraces support denser vegetation, anything from alders to mature spruce - cottonwood forests, and have a non-wetland water table (deeper than about a foot from the surface).  Wetland pockets typically occur within most upper terraces.  Small abandoned or active side-channels and low wet spots are frequently encountered.

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.  The area mapped as RD4T2 is expected to change following floods.

The Snow River is a glacier-fed braided stream with a gravel-dominated bed.  It floods every two to four years when meltwater that has accumulated in a glacier-dammed lake drains.  Bedload transport, deposition, and channel shifting processes occur there too, but on a more regular basis.  Perhaps because of this, terraces are more subdued and the transition between floodplain wetlands, lower and upper terraces is less pronounced than along the Resurrection River and Salmon Creek.

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."

 


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Contact: Mike Gracz
Kenai Watershed Forum 
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Old Town Professional Center
3430 Main Street Suite B1
Homer, AK  99603
907-235-2218

12 February 2007 15:25