Wetland Classification and Mapping of Seward, Alaska
SEWARD WETLAND ECOSYSTEMS
Riparian Ecosystem Wetlands
|The Resurrection River at the three bridges in Seward.||
The same wetland, outlined in blue, on a satellite image taken in 2003.
|Riparian Ecosystem wetlands are
rivers and streams and their associated valleys. Stream channels
not technically jurisdictional under section 404 of the Clean Water Act (the
section regulating activities in wetlands) because they do not normally support
vegetation. They are covered under another section of the act, and are frequently
regulated, especially if they contain anadromous fish, so they are included
on this wetland map.
River terraces are also included on the map. Areas named as river terraces are not jurisdictional wetlands, but may become so after flooding alters a channel course. Terraces are relatively wet, often with a water table within two or three feet of the surface, but not within a foot (or six inches when sandy), a wetland requirement.
Typical floodplain development processes do not apply in the valley-confined, aggradational environment of Seward area rivers and streams. Retreating glaciers provide a vast supply of materials, especially gravels. High rainfall events are common.
A high rainfall event (15" in 24 hours during early October, 1986) combined with shallow-rooted spruce and steep mountainsides multiplies flood flow potentials. Waterlogged forests slide into stream valleys as debris avalanches. Debris avalanches dam streams. These debris dams are eventually breached by floodwaters, and flows much larger than the precipitation-driven flood already in progress are released.
An Alaskan record flood of 2200 cfs per square mile of catchment area was produced by such an event on Godwin Creek during the 1986 flood event. The USGS estimated that Godwin Creek could have been flowing as high as 30,000 cfs during this event (about the same as the Kenai River in Soldotna at moderate flood stage). That flood transported boulders up to 8 feet in diameter, possibly in suspension (Jones and Zenone, 1988).
These large flows carry large quantities of readily available gravel, which drops out as the flood subsides. The gravel deposits elevate the streambed (aggradation), sometimes by a meter or more, often causing the stream to shift course to a lower position.
This aggradation process results in a braided river channel, and is common on glacier-fed streams. In 2005, a single rain-on-snow event at Mount Rainier, Washington resulted in thirteen feet of bed aggradation at one site on the braided stream draining the Van Trump Glacier. The average aggradation of that streambed was 5.6 feet following the event (Beason, et. al., 2006).
Seward area streams are confined to relatively narrow valleys. The streams emerge from their valleys to flow across broad alluvial fans. The point where a stream leaves its side-valley is a very narrow spot at the top of the broad fan. If the stream leaves its channel at this spot, as it does during a flood, it can potentially flow anywhere down the fan. The fans of the many Seward area streams and rivers converge.
This means that the alluvial surface from Bear Lake to Resurrection Bay, including the alluvial fans of Fourth of July, Lowell and Spruce Creeks, could potentially support a stream channel anywhere.
The Snow River valley experiences glacial outburst floods, or jökulhlaups, every two or three years. The river rises about seven to ten feet during one of these events. These regular floods result in more subdued terraces, and a blurrier distinction between lower terraces and higher floodplain wetlands, than on lower Salmon Creek and the Resurrection River.
Seward area Riparian Ecosystem wetlands are classified into three basic types based on Rosgen's stream classification (Rosgen and Silvey 1996). Two of the types refer to a couple of smaller streams: Lost Creek, and an unnamed creek draining into Sawmill Creek. The remaining Seward streams and rivers make up two thirds of all the area mapped: braided rivers with their associated terraces and floodplain wetlands.
RC refers to streams showing normal floodplain features such as point bars and cut banks (Lost Creek). RB refers to moderately entrenched, riffle dominated streams with narrow fringe wetlands (unnamed creek). RD streams are braided glacial outwash streams.
The braided streams can further be divided into groups based on the dominant particle size class of the bed material. In Seward this is frequently gravel. Most RD streams around Seward are classified as RD4, braided streams dominated by gravel, with some sand and cobbles. Much of the Fourth of July Creek bed is cobble, so it is classified as RD3.
The RD4 streams are further classified into three basic groups: channel and side channel; floodplain wetlands; and terraces. The channels contain the active channel, and its smaller side channels, which may have discontinuous flow. Floodplain wetlands are backwater areas where floodwaters are stored, but that are wet most of the year. Floodplain wetlands are also fed by hyporheic water- the water within the streambed, and connected to its main flow. The hyporheic zone can extend for hundreds of feet from the channel. Terraces are relatively well-drained, but the water table is still usually within two or three feet of the surface.
Floodplain wetlands and terraces are further divided. Floodplain wetlands are divided into types based on the depth to the water table. Terraces are divided based their relative elevation above the active channel. There are two subdivsions, T1 and T2; RD4T2 terraces are higher than T1 terraces.
Floodplain divisions are numbered 1-4, from open water to forest. RD4F1 wetlands are gravel bed braided river floodplain wetlands with standing water, F2 indicates a water table near the surface, often dominated by sedges; F3 is typically dominated by shrubs and F4 wetlands are forested.
Rivers are extremely dynamic systems in the Seward area. Channels frequently shift, especially on lower Salmon Creek and the Resurrection River. Mapping these wetlands sometimes feels like an exercise in futility. What is mapped as a floodplain wetland or terrace today could become something completely different next month. The map is useful for its delineation of the general boundary of the Riparian Wetland system. Although many floodplain wetlands and terraces will change character over the course of a few years, the relative proportions of each type will probably remain fairly constant (currently almost exactly 1:1) within the general boundary.
Floodplain wetlands exist along a gradient from open water to forest. Pond lily and water horsetail are common in open water floodplain wetlands (RD4F1). Sedges, especially Sitka sedge tend to dominate slightly higher floodplain wetlands with a water table at or very near the surface (RD4F2). Bluejoint reedgrass and field horsetail are also common in these wetlands. Shrubs dominate the next higher floodplain wetlands (RD4F3), especially Sitka alder but sometimes Barclay or Sitka willow, with bluejoint reedgrass and field horsetail. Forested floodplain wetlands can have a deeper water table and are typically dominated by Sitka spruce and occasionally black cottonwood, almost always with Sitka alder.
Terrace plant communities are successional. Sitka alder and Sitka willow quickly dominate lower, younger river terraces (RD4T1). Young black cottonwood becomes more prevalent with the alder on older terraces.
Younger upper terraces (RD4T2) are dominated by dense, tall Sitka alder with field horsetail. Young Sitka spruce and black cottonwood are evident. Older terrace surfaces support spruce forests with black cottonwood and a fading alder understory. Terraces are more pronounced in the Resurrection River delta, and subdued along the Snow River.
NWI and HGM
HGM: Middle Gradient Lotic River Fringe Wetlands.
NWI: Stream-adjacent wetlands are mostly permanently flooded PSS/EMJ Palustrine shrub/scrub and emergent wetlands.
HGM: Streams are middle and high gradient Lotic Streams and Rivers.
NWI: R3? (Braided streams are..., the other streams are upper perennial (R3) with cobble-gravel beds.)
|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 15:27