Wetland Classification and Mapping of Seward, Alaska
Mike Gracz
Doug Van Patten
SOILS IN THE SEWARD PROJECT AREA
DOUGLAS J. VAN PATTEN
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The primary objective of this project was to identify, describe, and map the wetlands of the Seward area. For jurisdictional purposes, wetlands meet requirements established for three parameters: vegetation, hydrology, and soils. A hydric soil is defined as a soil that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions (lacking oxygen) in the upper part (USDA Soil Conservation Service, 1994). These soils often support distinctively colored features that indicate they are wet, even when examined during the dry season. Published soil surveys commonly provide comprehensive data
on hydric soils and the hydrology of an area. A survey such as Soil Survey of Western However, there is no soil survey available for the Seward area. The soils data presented here are not as comprehensive as the data normally presented in an official Natural Resources Conservation Service Soil Survey report. In this report, a Typical Pedon is described for each of the dominant soil subgroups observed. A Typical Pedon is a description of the soil profile at a particular wetland and is intended to characterize soils at similar wetlands. Similar subgroups are linked in the Typical Pedon description. Unlike in an Official Soil Series Description, the Typical Pedon does not provide the full range of characteristics for similar pedons that occur throughout the area, because we were mapping only wetlands around Seward, so only looked at soils found in wetlands (with a couple of minor exceptions). Soil Classification The system of soil classification used by the National Cooperative Soil Survey has six categories (USDA, 1999). Beginning with the broadest, these categories are the order, suborder, great group, subgroup, family, and series. Classification is based on field observations or from laboratory measurements. The series level is the most detailed category. Establishment of a new official soil series requires a lengthy and rigorous data acquisition and correlation process. In this report, I classify soils to the subgroup level. This level is sufficient to describe the soil properties necessary to define hydric soils. I describe 11 wetland soil subgroups and, for comparison, three that are not hydric. Each part
of a subgroup name describes a certain soil characteristic. For example in the
subgroup name: Histic Cryaquept,
Histic represents
a soil with a thick organic mat (20-40 cm); Cry-
means that it is a very cold soil (<8 degrees C); -aqu signifies that it is a wet soil
(within 50 cm of the surface); and –ept means it formed in relatively young material. Soil Taxonomy (USDA,
1999) provides
thorough guidance on soil classification.
Seward area landscapes and associated soils
River and stream valley wetlands Around Seward, valley floors are occupied by alluvial fans, floodplains, and fan deltas. These surfaces are geologically young and frequently receive unconsolidated materials from the surrounding uplands during storm events. They are very dynamic landforms. Stream channels on alluvial fans and braided floodplains are prone to lateral migration during flooding (Jones and Zenone, 1988). The dominant wet soils on these surfaces are Typic Cryaquents, and Histic Cryaquepts. These are young soils derived from the surrounding dark-colored graywacke and phyllite rocks. They often have sandy and/or gravelly layers. They differ in that Histic Cryaquepts have a surface organic layer at least 20 cm thick, while Typic cryaquents have thin surface organic layers. One Aquic Cryorthent was identified. Aquic Cryorthents have aquic conditions within 20 inches of the mineral surface for some time in normal years but are not as wet as Typic Cryaquents Evaluation of the indicators of wetland soils and hydrology in Typic cryaquents is complicated by the seasonal fluctuation of the water table and shifting stream channels. During dry periods it is somewhat problematic to identify hydric soil indicators because the key features (colors indicating redoximorphic conditions) are masked by dark-colored parent materials or absent from the coarse textured layers. The average depth to the water table observed during mapping of these soils was 29 cm (11.4 inches).
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Peatlands Organic material accumulates in wet depressional areas of floodplains, glacial moraines, and on concave discharge slopes, forming small peatlands. The organic soils in these peatlands are classified in the Histosol soil order (at least 40 cm thick), and are differentiated within the order by the degree of decomposition of the organic material. Fibrists are the least decomposed. They have about three-fourths plant fibers after rubbing. Hemists are partly altered both physically and chemically. They are intermediate in degree of decomposition between Fibrists and Saprists. Saprists have the highest degree of decomposition. They have less than one-sixth plant fiber after rubbing. Histosols that contain layers of mineral soil in their profile are included in the Fluvaquentic subgroup. Fluvaquentic Cryofibrist is an example of an organic soil that receives periodic flood-deposited mineral material. The average depth to the water table observed during mapping of these soils was 13.5 cm (5.3 inches).
Overall, eight peatland soil subgroups were identified in this project. They are described on two separate subgroup description pages: Fluvaquentic Cryofibrists, and Typic Cryosaprists.
Non-wetland slopes Glaciated mountain slopes and drumlins flank the valleys and are the source of material that is transported to the valley floor during storm events. The dominant soils on the mountain slopes and drumlins are Spodosols. They form on stable surfaces with relatively good drainage.
Typic Humicryods in an example. Care must be taken not to attribute the colors of these soils with reduction and oxidation features found in hydric soils. While the colors are the same, the processes are different. Other non-hydric soils that occur in the area are Typic Dystrocryepts, which occupy drier parts of alluvial fans and Typic Cryofluvents, which are on alluvial fans and floodplain terraces that have infrequent flood events.
Loess Windborne deposits, or loess, probably play a minor role in the soil forming processes around Seward. Fine sand and silt on broad braided floodplains can provide source material. Of particular interest is an approximately 2 cm layer of very fine sandy loam that is found in virtually all of the Histosols I described above the valley floor. It is (thought to be) a layer of volcanic ash. This layer was identified on all aspects and elevations to 1,000 ft and was observed at depths between 13 and 41 cm. We are currently awaiting lab analysis.
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03 May 2007 19:08 |
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