Matanuska-Susitna Wetland Mapping
TIDAL ECOSYSTEM Wetlands
A tidal gut along Turnagain Arm in Upper Cook Inlet, near Hope.
Tidal Ecosystem wetlands are influenced by periodic inundation by saltwater. Inundation varies daily, monthly, annually and on an 18.6 year cycle. These cycles create different zones of inundation, and at the head of Cook Inlet, with among the highest tides on Earth, the influence of each cycle can be observed. Tidal Ecosystem wetlands are those influenced at least by annual tides.
Vince and Snow (1984) describe distinct zones marked by both frequency and duration of annual tidal inundation at the mouth of the Susitna River. These zones, and their characteristic vegetation, are used to describe different Tidal Ecosystem wetland types. Some areas are inundated frequently and for long duration, other are influenced less frequently and retain saltwater for varying lengths of time. Tidal ecosystem wetlands can be difficult to classify using only aerial photography. Site specific ground-truthing is required for identification. Due to accessibility, and other priorities, minimal ground truthing was performed for these wetlands. Near-shore T6 and T5 components are easiest to identify.
Tidal Ecosystem wetlands are the only ecosystems mapped with combinations of other Wetland Ecosystems. For example a T9SA unit was mapped at the head of Kachemak Bay. It is a mixture of a Lyngbye sedge (T9) and an Alder Discharge Slope component (SA). In a few places the gradient is shallow enough that freshwater and tidal plants can mix. Along Knik Arm where the tidal range is most extreme, another Wetland Ecosystem was named- the Tidally-Influenced Drainageway Wetland Ecosystem where this mixture occupies a relatively broad gradient also influenced by abundant shallow groundwater sources.
Tidal Ecosystem wetlands along an idealized gradient
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.
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.
Tidal Ecosystem Wetland Map Components:
T0- Bare mud
T1- saltpannes. Sparse, low glasswort (Salicornia maritima) and pearlwort (Sagina maxima).
T2- Mud with creeping alkaligrass (Puccinellia phryganodes). Vince and Snow's (1984) "Outer Mudflats zone 1". Inundated 26-46 times per summer (mean=34).
T3- Bare ground with goosetongue (Plantago maritima) and seaside arrowgrass (Triglochin maritima). Vince and Snow's (1984) "Inner Mudflats zone 5". Inundated 6-13 times per summer (mean=8).
T4- Alkali grass (Puccinellia nootkaensis and P. Hultenii) dominates, usually with a beachrye (Leymus mollis ssp. mollis) component. Loosely follows Vince and Snow's (1984) "Outer Mudflats zone 2" which is inundated 10-20 times per summer (mean=15).
T5- Ramensk's sedge (Carex ramenskii) dominates with pools. Mare's tail (Hippuris), spikerush (Eleocharis spp.), saltmarsh starwort (Stellaria humifusa) found in and around the pools. Vince and Snow's (1984) "Outer Sedge Marsh zone 3". Inundated 0-5 times per summer (mean=3).
T6- Lyngbye's sedge (Carex lyngbyei) cover nearly continuous. Vince and Snow's (1984) "Outer Inner Sedge Masrh zone 7". Inundated 0-4 times per summer (mean=2). T6A is Lyngbye sedge mixed with alders.
T7- Beachrye (Leymus mollis ssp. mollis) diverse plant community on storm berms. Probably in the same position as Vince and Snow's (1984) "Riverbank Levee zone 6", which is innundated 0-2 times per summer (mean=1). Vince and Snow did not include beachrye in their Susitna Flats plant zonation work, except to mention that it becomes more abundant above about 12 cm "relative altitude" ("...relative to about 10 m above mean low water of spring tides").
T8- Pacific silverweed, largeflower speargrass (Poa eminens) and sometimes circumpolar reedgrass (Calamagrostis deschampsioides) dominate. A combination of Vince and Snow's (1984) "Riverbank Levee zone 6" and "Inner Mudflats zone 4". Inundated 0-2 (mean=1), and 8-13 (mean - 11) times per summer, respectively. On the Kenai, "Riverbank Levees" are typically dominated by beachrye (Leymus mollis ssp. mollis).
T9- Upper reaches of low gradient river mouths; dominated by manyflower sedge (Carex pluriflora). Vince and Snow's (1984) "Inner sedge marsh zone 8". Inundated 0-2 times per summer (mean=1).
TR- More than two non-consecutive units at a scale too small to map. Typically formed where gradients are steep, such as along larger tidal guts or at mouths of large streams.