Pedogenic processes and soil-landscape relationships in North Cascades National Park, Washington

Thunder Creek watershed (TCW) in the northern Cascade Range, Washington State provides a complex environment for studying soil formation. Approximately 15,000 cal. yrs B.P, the Cordilleran Ice Sheet (CIS) overrode the watershed during the last glacial maximum and deposited glacial drift that provides the substratum for soil formation today. Since then, the watershed has been blanketed by tephra from Mount Mazama, Glacier Peak, and Mount St. Helens. Soils with the gross morphology of Spodosols (Podzols) and Andisols (Andosols) occur in a complex landscape mosaic in TCW and along the entire Washington Cascade Range. The pedologic conundrum addressed in this paper is whether true Spodosols are forming in these areas, or are they mimics of Spodosols? Based on field morphology and lab analysis, soils in TCW are andic subgroups of Spodosols and Andisols in approximately equal abundance, with lesser areas of Inceptisols (Cambisols) and some Entisols (Gleysols). Field morphology and electron microprobe analysis of glass shards shows that the major volume of tephra, about 8 cm compacted thickness, is predominantly from the eruption of Mount Mazama 7600 cal. yrs BP. Tephra from this eruption has been mixed into the upper 50 cm of glacial drift and colluvium. Fallout from other eruptions in TCW had individual compacted thicknesses less than 2 cm. We conclude that true Spodosols are widespread in TCW; they have albic and spodic horizons. No pseudo-albic horizons of white, unweathered tephra layers were found in TCW. The cold, wet, and dominantly conifer-covered landscape supports podzolization in tephra-rich materials on parts of landforms that have been stable for several thousand years. Andisols are forming in drier microclimates, on gravitationally active landscapes, and under herbaceous vegetation. Inceptisols dominate landforms controlled by water erosion and deposition, where tephra has not been preserved in the soil, and on active avalanche chutes. Entisols were found only on active or recent floodplains. The distribution of each soil order in TCW corresponds to the preservation of tephra, vegetation type, and stability of the landform.