Matanuska-Susitna Wetland Mapping

Cook Inlet Wetlands

RIPARIAN ECOSYSTEM Wetlands

Wetlands overlain on an aerial photo showing major creeks and the Little Susitna River, near Wasilla.

Riparian Ecosystem wetlands are rivers and streams and their adjacent valley bottoms. Adjacent valley bottoms often support uplands. Although they are not considered wetlands for regulatory purposes, these locations will eventually flood as the channel migrates across the valley floor.

Rosgen's stream classification system is employed, with some modification. In Rosgen's system streams are classified using channel and valley forms, among other characters. In the area mapped stream reaches are largely E streams, those formed on sediments laid down by other, larger processes. In the Mat-Su Valley these processes are associated with the maximum extent of glacial ice during the last advance. E streams are highly sinuous, pool dominated reaches in underfit valleys. E stream reaches in the valley have been classified into three types: l, s and b. In Cook Inlet we find many linear E stream reaches (sinuaosity < 1.5 and often 1.2); they are named Rel. Sinuous E reaches are named Res (all of Rosgen's E streams are highly sinuous), and bankfull reaches are named Reb. Bankfull streams are dammed by either beavers, roads or under-sized culverts.

Meadow Creek east of Big Lake.

Wasilla Creek has many B stream reaches, where a riffle and pool structure dominates. Vine, or Phantom Creek exhibits B stream character, but is an unclassified different type because it does not connect at the surface to Cook Inlet. It is the creek that heads in a wetland south of Memory Lake, and disappears into gravels just west of Vine Road.

The Little Susitna River is a C stream, with cut banks and point bars. This is the classical model of a steam with a well-developed floodplain. The bankfull condition on these streams corresponds to the 1.5-2 year flooding event. This level is identified from the elevation of floodplain bars and islands. Because they are common, and carry a relatively high flow, events corresponding to the 1.5-2 year flood level transport most of the stream energy over the mid-long term, reulting in the characteristic point-bar, cut bank, gravel bar morphology often observed. A huge flood can alter the postion of these features.

The Little Susitna River, a C stream, near the Sushana Drive bridge.

The classical C-stream model is used to develop floodplain maps, but breaks down on different stream types, especially on D streams. D streams are large braided channels, fed by glacial discharge in our area. The Matanuska and Knik River, and many of the stream channels in the valles around Seward fit this category. Two basic types of D streams are named in the Cook Inlet region, D3 and D4. D3 reaches are dominated by cobbles; Fourth of July Creek, near Seward is the only D3 reach. D4 reaches are dominated by gravels. The remaingn D streams in the area are in this category, their names are prefixed by RD4. RD4 reaches are further divided. Wetlands formed in temporarily abondoned areas of the channel are RD4F wetlands, and they are subdivided into the numeric classes used elsewhere in this classification- i.e. RD4F2 is a floodplain wetland with a water table near the surface most of the season, usually dominated by sedges.

RD4T wetlands are terraces in D4 channels. These wetlands are subdivided into RD4T1 and RD4T2. Terraces with the T1 suffix are first level terraces, and T2 terraces are higher, and often forested. These higher RD4T2 units do not normally meet wetland determination criteria. They are included in the wetland map because RD4 channels are extremly dynamic. Mapping often seems a futile excersise in these systems, because channel position will likely change within a few years, incorporating former RD4F wetlands into the new channel, creating new RD4T1 and -T2 features high, and relatively dry- at least until the next high-flow event. RD4T2 terraces persist for a century of two, but the presence of trees collapsing into the current channel indicates that they will eventually be consumed by the continually accumulating bed material as it is transported from the mountains.

RD reaches are aggradational environments- extremly so around the Seward area. The rivers are not eroding into a channel, but depositing material along their courses. The "floodplain" is continually building up as each high flow event deposits more material carried from the glaciers and surrounding mountains. The asphalt plant in Seward can continually mine gravel from a single location, as annual high flow events replenish the hole created by each year's extraction. In such a situation, the term "floodplain" represents a radically different conceptual model than the traditional cut-bank, point bar process indicates. As such, a 150-year flood carries different consequences.

Japanese Creek in Seward, a D reach, showing a small channel threading through the large amount of material transported during frequent high flow events.

Explanation:

All data from wetlands throughout the Cook Inlet Lowlands, not just from the Matanuska-Susitna Valley.

Numbers in paraentheses indicate number of samples.

Peat depth is a minimum, because some sites had thicker peat deposits than the length of the auger used (between 160 - 493 cm).

Water table depth is a one time measurement. At sites with seasonally variable water tables this measurement reflects both the conditions that year, and the time of year. If no water table was encountered, no value was recorded; use number of samples to aid interpretation. Deeper average water tables idicate higher variability.

Redox features with deep depths typically indicate deeper peat deposits, which mask redox indicators so the depth corresponds to the peat thickness.

pH and specific conductance measured in surface water or a shallow pit with a YSI 63 meter calibrated each sample.

Plant Prevalence Index calculated based on Alaska indicator status downloaded from the USDA PLANTS database, which may use different values than the 1988 list.

Riparian Ecosystem Wetland Map Components:

Summary of Riparian Ecosystem Map units:

AMT- Abandoned meander terraces and channels.  Limited to a few reaches along the Kenai and Kasilof Rivers.

RA- Steep, high-energy cascade with few pools.

RAA- Waterfall

RB- Higher gradient (>2%); riffle-dominated.

RC- Floodplain developed.  Point bars.  Riffle/pool morphology.

RDA- Multiple braided, low gradient, pool-dominated channels on glacial deposits supporting peat development.

RD3C- Braided river channels with cobble-sized particles dominant

RD4C- Braided river channels with gravel-sized particles dominant

RD4F1- Open-water floodplain wetland adjacent to a braided river dominated by gravel-sized particles.

RD4F2- Sedge-dominated floodplain wetland adjacent to a braided river dominated by gravel-sized particles.

RD4F3- Shrubby floodplain wetland adjacent to a braided river dominated by gravel-sized particles.

RD4T1- Low terrace, with seral vegetation (herbaceaous or willow and/or alder) adjacent to a braided river dominated by gravel-sized particles.

RD4T2- Higher, forested terrace adjacent to a braided river dominated by gravel-sized particles.

REs- Sinuous, low gradient, pool-dominated, on glacial deposits.

REb- Bank-full due to beaver dam, roads, logging debris or natural obstruction.  Low gradient, on glacial deposits.

REa- Stream surface not discernable on 1:25,000 B&W aerial photography.  usually low gradient, pool dominated, but occasionally would fit the B reach type.  On glacial deposits.

Rib- River island or bar- usually bare or early seral vegetation only.

Rt- Tidally influenced river or stream.  Usually too small to map but extends about a mile upstream on larger streams, and several miles on the Matanuska, Knik, Kasilof and Kenai Rivers.