The ecoregion extends across a wide swath of the Columbia River Basin from The Dalles, Oregon to Lewiston, Idaho to Okanogan, Washington near the Canada–U.S.

The arid sagebrush steppe and grasslands of the region are flanked by moister, predominantly forested, mountainous ecoregions on all sides.

In uncultivated areas, moisture levels are generally high enough to support grasslands of bluebunch wheatgrass, Sandberg bluegrass, and Idaho fescue, without associated sagebrush, which is more common in the Pleistocene Lake Basins.

The region covers 3,712 square miles (9,614 km2) in Eastern Oregon, including the Umatilla Indian Reservation and agricultural areas south of the Columbia River.

In Oregon, the flood waters accumulated from the eastern entrance of the Columbia River Gorge upstream to the Wallula Gap to form ancient Lake Condon.

Major irrigation projects provide Columbia River water to the region, allowing the conversion of large areas into agriculture.

Where present, native vegetation consists of needle-and-thread, Indian ricegrass, bluebunch wheatgrass, Sandberg bluegrass, and basin big sagebrush.

[2][4] The Dissected Loess Uplands ecoregion consists of disjunct rolling hills and flat plateau remnants cut by the Lower Snake and Clearwater Canyons.

The Dissected Loess Uplands covers 224 square miles (580 km2) in western Idaho and larger areas in southeastern Washington, south of the Snake River between Dayton and Lewiston, including part of the Nez Perce Indian Reservation.

The region covers 109 square miles (282 km2) in Oregon and much larger areas in Washington, extending from the Horse Heaven Hills in the south to the Kittitas Valley in the north, including parts of Yakama Indian Reservation, Hanford Reach National Monument, Wenas Wildlife Area, and the Yakima Training Center.

The region covers 453 square miles (1,173 km2) along the Idaho border, including parts of the St. Joe National Forest and the Coeur d'Alene Indian Reservation, and larger areas in Whitman and Spokane Counties, Washington.

The region covers 146 square miles (378 km2) in Oregon and a contiguous area in Washington, in a narrow band from Pendleton to Dayton, including part of the Umatilla Indian Reservation.

Riparian vegetation in narrow reaches is often limited to a band of white alder at the water line, with some mockorange, western clematis, and chokecherry; broader floodplains and gravel bars are dominated by introduced species, such as reed canarygrass, sweetclover, and teasel.

The region covers 674 square miles (1,746 km2) in Oregon, along the lower reaches of the Deschutes and John Day rivers and their tributaries.

Canyon depths exceed 1,400 feet (430 m) and create drier conditions than in neighboring regions; mean annual precipitation is only 12 to 25 inches (300 to 640 mm).

Outside of towns and transportation corridors, the canyons provide good habitat for bighorn sheep and game birds, with grasslands of bluebunch wheatgrass, Idaho fescue, bluegrass, and Wyoming big sagebrush.

The region covers 80 square miles (207 km2) in Idaho and slightly larger areas in Washington, along the lower reaches of the Snake and Clearwater rivers and their tributaries.