This landscape was maintained by the Native American inhabitants of the valley who set frequent fires which encouraged the open grasslands and killed young trees.

The American settlers of the region, since the 19th century, suppressed fires and converted much of the valley to agriculture, which has caused much of the former grassland and savanna to revert to closed-canopy forest.

Historically, the basin was characterized by Oregon white oak groves and Douglas-fir forests on the uplands; black cottonwood groves on riverbanks and islands; Oregon ash, red alder, and western redcedar in riparian areas; and prairie openings maintained by Native American burning, with camas, sedges, tufted hairgrass, fescue, and California oatgrass.

Numerous wetlands, oxbow lakes, and ponds can still be found, but today the region is dominated by urban and suburban development, pastures, cropland, and tree farms.

Historically, riparian gallery forests containing ash, black cottonwood, alder, and bigleaf maple grew on fertile, alluvial soils.

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.

The region covers 1,971 square miles (5,105 km2) in Oregon along the length of the valley and includes the Baskett Slough and Ankeny national wildlife refuges.

It contains rolling foothills with medium gradient, sinuous streams, and a few buttes and low mountains, rising to an elevation of approximately 1,500 feet (457 m).

Historically, the drier areas supported Oregon white oak and madrone woodlands and prairies, with California oatgrass, fescue, blue wildrye, brodiaea, and other prairie forbs; while the moister areas supported Douglas-fir forests, with sword fern, oceanspray, hazel, baldhip rose, poison oak, and wild blackberry.

Today, the valley foothills are characterized by rural residential development, pastures, timberland, vineyards, Christmas tree farms, and orchards.