Wetland Classification and Mapping of Seward, Alaska
SEWARD WETLAND ECOSYSTEMS
Kettle Ecosystem Wetlands
A Kettle peatland on a bedrock bench above the Snow River.
The same peatland, outlined in blue, on a satellite image taken in 2003.
Ecosystem wetlands around Seward are peatlands formed in ice-scoured
bedrock. They are especially common on the large rock drumlin north
and east of town, and on the benches and slopes of the area between Bear
lake and the Snow River. Bear Lake itself is classified as a large
Unlike Depression Ecosystem wetlands, Kettles are connected by other wetlands to a navigable waterbody. Sometimes this connection is too narrow to map, but could be seen detected on the aerial photography or during a field visit.
Like Depression Ecosystem wetlands Kettles occupy the low spots between ridges on bedrock knobs. These knobs were ice scoured during the last glaciation. The bedrock core of the knobs is arranged in layers tilted roughly perpendicular to the ground surface, and weaker layers erode more easily. More resistant layers remain as small ridges between the wetlands, lying in the eroded valleys between the resistant layers. Some layers eroded less uniformly leaving behind disconnected Depression Ecosystem wetlands. In many cases the weaker layer eroded uniformly enough to support connecting Kettle Ecosystem wetlands.
The weaker layers are frequently good conductors of groundwater, and springs often originate at their bases. Springs like these add to the water contributed by hyporrehic and flood flows from the Resurrection River and Salmon Creek to form the extensive wetland complex along Nash Road.
Kettle Ecosystem wetlands are the second most common wetlands, after Riparian Ecosystem wetlands, mapped in the Seward area. We named 751 acres Kettles, of the 4522 acres mapped (16%).
Depth to water table controls plant species distribution in Kettle Ecosystem wetlands. Where open water is sufficiently deep, the floating aquatic pond lily (Nuphar polysepala) is common. Where water is shallower, a variety of plants emerge above the surface, especially: water horsetail (Equisetum fluviatle), Buckbean (Menyanthes trifoliata), marsh cinquefoil (Comarum palustre), and tall cottongrass (Eriophorum angustifolium).
Where the water table is very near the surface, sedges generally dominate. The most common are: fewflower sedge (Carex pauciflora), tall cottongrass (Eriophorum angustifolium), and tufted bulrush (Tricophorum caespitosum). Sitka sedge (Carex sitchensis) and dwarf birch (Betula nana) are sometimes encountered.
Where the water table is a little deeper, and not often at the surface, shrubs tend to dominate, although fewflower sedge is particularly abundant around Seward in these Kettles. Dwarf birch, crowberry (Empetrum nigrum) and Sitka alder (Alnus viridis ssp. sinuata) are all common. Occasionally Barclay willow (Salix barclayi) and bluejoint reedgrass (Calamagrostis canadensis) are encountered.
Where the water table is deeper yet, but still qualifies the site as a wetland, woodland and stunted forest are common. Mountain hemlock (Tsuga mertensiana) and Sitka spruce (Picea sichensis) dominate, usually over early blueberry (Vaccinium ovalifolium) or rusty menziesia (Menziesia ferruginea). Sitka alder and field horsetail (Equisetum arvense) are frequently encountered.
NWI and HGM
Kettle Ecosystems fit into the US Fish and Wildlife Service's National Wetland Inventory palustrine category. They contain a variety of plant dominants from herbaceous emergents (PEM) to shrubs (PSS) and forest (PFO), with hydrologic regimes ranging from saturated through permanently, semi-permanently, and seasonally flooded (PSSB, PEMH, PEMF and PEMC, respectively).
The Hydro-Geomorphic Model (Tiner, 2003) would classify most Seward area Kettle Ecosystems as Terrene Basin groundwater-dominated throughflow wetlands. When the Kettle is at the edge of a central lake (greater than 20 acres and deeper than 6.6 feet) it is named a Lentic Fringe throughflow wetland. Some may also form the headwaters to small streams, in which case the modifier "headwater" is appended.
Common Kettle Plant Communities:
Sitka sedge (Carex sitchensis)
Fewflower sedge - tall cottongrass (Carex pauciflora - Eriophorum angustifolium)
Tufted bulrush - tall cottongrass (Tricophorum caespitosum - Eripohorum angustifolium)
Tall cottongrass (Eriophorum angustifolium)
Fewflower sedge - dwarf birch (Carex pauciflora - Betula nana)
Fewflower sedge - crowberry (Carex pauciflora - Empetrum nigrum)
Sitka alder - bluejoint reedgrass (Alnus viridis ssp. sinuata / Calamagrostis canadensis)
Mountain hemlock / early blueberry (Tsuga mertensiana / Vaccinium ovalifolium)
Mountain hemlock / early blueberry - rusty menziesia (Tsuga mertensiana / Vaccinium ovalifolium - Menziesia ferruginea)
Summary of Kettle Ecosystem Map Components:
K1- Open water. Floating or emergent vegetation.
K2 - Water table at or near the surface. Sedges and/or dwarf birch dominated.
K3 - Water table does not reach the surface. Shrubs or bluejoint grass (Calamagrostis canadensis) dominant.
K4 - Redoximorphic features or deep peat the wetland indicator. Woodland or forest.
Kettle map component combinations used around Seward: K12, K13, K1-3, K1-4, K21, K2-4, K23, K31, K32, K34, K43
|Contact: Mike Gracz Kenai Watershed Forum Homer Field Office Old Town Professional Center 3430 Main Street Suite B1 Homer, AK 99603 907-235-2218||
11 December 2006 15:28