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working for clean rivers

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Geology & Soils

Cascade silt loam covers most of the subwatershed, especially the upland hills. This soil is poorly drained, consisting of a dark brown silt loam surface layer and subsoil. Generally, in this area, fragipan (clay hardpan) exists at a depth of 20 to 30 inches below the surface, and basalt bedrock occurs at a depth of 40 to 60 inches. The hydrologic soil group classification for this soil is Type C, characterized as having a slow infiltration rate and high runoff potential ( BES, 1999).

At the lower elevations and on some of the hillside slopes and valleys, Wauld Very Gravelly Loam predominates. The hydrologic classification for this soil is Type B. These soils are well drained, with a moderate rate of permeability. A 6-inch layer of dark brown gravelly loam covers the surface. The 24-inch thick subsoil is gravelly clay loam. Depth to bedrock ranges from 20 to 30 inches ( BES, 1999).

A small northeastern portion of the watershed west of just east of St. Helens road contains Type D hydrologic soils that exhibit low infiltration rates.
Maps & Files

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