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Oak/Conifer Foothills

↑About the Oak/Conifer Foothills

The Oak/Conifer Foothills are a region of the Eastern Cascades Slopes and Foothills straddling the border of Oregon and Washington. Where the Columbia river cuts through, the region begins east of Cook, WA and extends eastward to a bit past Lyle, WA and Rowena, OR. The region is mostly oriented north-south and is widest at its northern end.

The region has a warm-summer Mediterranean climate, with cool, wet winters, and warm, dry summers, but there is considerable variation by elevation and even by micro-site. Precipitation ranges from about 40-16 inches (1016-406mm) annually, with the most rain falling at high elevations in the west, and less at low elevations and in the east. The frost-free period ranges from about 90-140 days, but winter lows tend to be moderate and winter temperatures average above freezing even in the coldest months. Although valleys tend to be warm, some valleys can have colder nighttime low temperatures due to cold air drainage from nearby mountains. This phenomenon can lead to some valleys having an unusually large day-night temperature variation relative to what is typical at their elevation.

The topography here is variable, featuring foothills, low mountains, plateaus, and valleys. The degree of dissection of the plateaus is variable, with some portions having a well-developed drainage network and little flat ground, but others having broad expanses of relatively flatter terrain. On average this region is steeper and more rugged than areas to the east, but not as rugged and dissected as areas to the west. There are both permanent and intermittent or seasonal streams, all of which eventually drain into the Columbia river. The region is underlain almost exclusively by basalt, although there is a major difference in the rocks between most of the region, and the area along the Columbia river where much older formations are exposed. Soils here vary more based on location, but tend to be deep and loamy, with varying but often large degrees of rocky material mixed in. Soils that developed under grassland in valleys as well as on flatter uplands tend to be quite fertile, whereas the soils on steeper slopes tend to be poorly developed and more rocky.

This region was probably originally covered in a mosaic of coniferous forest, oak savanna, and grasslands, with forest cover tending to be densest in the west and at high elevations, savanna at intermediate sites, and grassland more dominant at lower elevations and in the east. Forested sites were dominated by Douglas fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla). Ponderosa pine (Pinus ponderosa) and Oregon white oak (Quercus garryana) became more dominant in the open savannas to the east. On wild sites, cover is more-or-less similar nowadays.

Nowadays there is a similar mosaic of land, but altered by a bit more active land use. The land here is mostly privately-owned. There is a mix of forestry, grazing, recreation, and orchards. In the valleys there is also some residential development and significant grain and hay farming. Overall this region is sparsely populated, with only a series of small towns throughout, most of which are unincorporated. Many of the towns here have lost population as employers such as orchards or lumber mills have closed, leading to some ghost towns. Much of this area is easily accessible relative to its low population and large portion of private land; I-84 cuts through the region along the Columbia river, and OR-35 and WA-141 form a major north-south corridor through the region. The southernmost portion of the region is less accessible as it tends not to line up with major roads. There is a small portion of public land in the form of state and local parks, mostly along or near the I-84 corridor.

At its broad northern end, there is a long border to the north with the Yakima Plateau and Slopes, except at the west of this region where it is bordered to the north by the Grand Fir Mixed Forest. In the south, it is also bordered by this same region to the west. However in the middle portion of this region, it is instead bordered to the west by the cooler, wetter, higher-elevation Western Cascades Montane Highlands. At its very southern end there is a small border to the south with the Ponderosa Pine/Bitterbrush Woodland. To the east, it is bordered by three different regions of the lower, drier, flatter Columbia Plateau: in the south, it is bordered to the east by the Umatilla Plateau. There is a small area along the Columbia river where it is bordered to the east by the Pleistocene Lake Basins. Then in the northeasternmost portion of this region, it is bordered to the east and south by the Yakima Folds.

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