Level IV: (3c) Prairie Terraces
The Prairie Terraces ecoregion includes all terraces of the Willamette River upstream of the Portland/Vancouver Basin. It is dissected by low-gradient, meandering streams and rivers. Elevation varies from 160 to 500 feet (50 to 150 m).
The region’s broad fluvial terraces once supported oak savanna and prairies, featuring Garry oak, camas, sedges, tufted hairgrass, fescue, and California oatgrass. Wetter areas supported Oregon ash, Douglas-fir, bigleaf maple, black cottonwood, and an understory of poison-oak, hazel, and Indian plum, with some Ponderosa pine to the south. Today, only relict native prairie remains.
The poorly drained soils derived from glacial lake deposits are extensively farmed for grass seed and small grains, as grasses tolerate poor drainage and poor rooting conditions better than other crops. Historically, seasonal wetlands and ponds were common, but many streams are now channelized, and the wetlands have been reclaimed for grain crops. In addition to agriculture, the Prairie Terraces have experienced the brunt of urban and suburban development along the Interstate 5 corridor.
Further Reading: “Wildlife Conservation in the Willamette Valley’s Remnant Prairies and Oak Habitats: A Research Synthesis,” by David G. Vesely and Daniel K. Rosenberg (Oregon Wildlife Institute, Corvallis), 2010.