Wetland Classification and Mapping
of Seward, Alaska
Map Unit Descriptions
Map Units: RD4F32; RD4F23
Seward Area Extent:
RD4F32: 6 wetland polygons; 66.0 acres
RD4F23: 5 wetland polygon; 34.2 acres
An RD4F23 along Nash Road. It is fed by groundwater, hyporheic water from Salmon Creek, and by occasional flooding.
Type: Floodplain wetland.
Depth to water table: 15 cm on 25 July 2006 at the single site measured
Organic layer thickness: 8 cm at the one site measured
Average depth to redoximorphic features: 0 cm at the one site measured
Common Soils: Typic Cryaquents
Common Plant communities:
Sitka sedge (Carex sitchensis)
Bluejoint reedgrass (Calamagrostis canadensis)
Sitka alder / field horsetail (Alnus viridis ssp. sinuata / Equisetum arvense)
Sitka alder / bluejoint reedgrass (Alnus viridis ssp. sinuata / Calamagrostis canadensis)
NWI: PEM2C (F2); PSS1C (F3)
HGM: Bidirectional, non-tidal Floodplain flat
These floodplain wetland complexes are a combination of the sedge-dominated (RD4F2) and shrubby or bluejoint reedgrass dominated components (RD4F3). When the shrubby component with a deeper water table is dominant the wetland is named RD4F32, and when the sedge component with a water table at or near the surface dominates it is named RD4F23. These wetlands are fed by a combination of river floodwaters, hyporheic water, and groundwater discharge from adjacent bedrock knobs.
All of these wetlands lie in a river floodplain, separated from a river channel by a natural levee. Five of the six wetlands mapped as RD4F23 lie on the lower Salmon River floodplain along Nash Road. The sixth has some tidal influence, and lies adjacent to an unnamed stream draining Mount Alice. Four of the five RD4F23 wetlands lie along Salmon Creek, the fifth lies along the Snow River.
Floodwaters are slowed and stored in these wetlands. These particular floodplain wetlands probably support more flood storage than open water floodplain wetlands, because the water table is lower. A lower water table has more storage capacity in the soil above. The lower it is, 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.
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 15:27