Wetland Classification and Mapping
of Seward, Alaska
Map Unit Descriptions
Map Component: RD4T1
Seward Area Extent: 20 wetland polygons; 73.2 acres
Willows and alder on an RD4T1 lower terrace of the Resurrection River along the Seward Highway.
Type: Floodplain wetland
Depth to water table: 13 cm at the one site measured
Organic layer thickness: 41 cm at the one site measured
Average depth to redoximorphic features: not measured
Common Plant communities:
Black cottonwood / Sitka alder (Populus balsamifera / Alnus viridis ssp. sinuata)
Sitka willow (Salix sitchensis)
HGM: Bidirectional, non-tidal Floodplain flat
RD4T1 are lower-level terraces of the large glacier-fed braided streams and rivers around Seward. Bed material is dominated by gravel, with some sand and cobbles. Vegetation is typically sparse willow and/ or alders. These areas appear dry on the surface but typically support a shallow water table. 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 (lower than about a foot from the surface).
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 RD4T1 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.
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."
|Contact: Mike Gracz Kenai Watershed Forum Homer Field Office Old Town Professional Center 3430 Main Street Suite B1 Homer, AK 99603 907-235-2218||
12 February 2007 15:25