The Northwest basin desert is located within the Columbia river drainage basin but has a smaller area than the actual river basin. The desert portion of the river basin is defined by the xeric flora that grows within it and this desert basin, in modern times has commonly been an xeric region.

The Columbia Basin is a river drainage basin in the Pacific Northwest region of North America and in common usage, the term often refers to a smaller area, generally the portion of the drainage basin that lies within eastern Washington.

Still, when looking at the entire river drainage basin, it expands to include areas in Idaho, Oregon, Montana, Washington, Wyoming and into southern British Columbia.

Within the Columbia river drainage basin, there are some areas with much higher rainfall, such as is in the Northern Rockies ecoregion in Washington, Idaho and Montana.

These wet climate areas are due to prevailing maritime winds which are called the Pineapple express. Meteorologists calls these winds an Atmospheric river which builds up moisture in the tropical Pacific around Hawaii and then this river flows northeastward to the US and Canadian west coasts to dump its moisture in the form of heavy rainfall and snow.

Once the Atmospheric river arrives at the Pacific Northwest coast, a large portion of the rain clouds are funnelled around the Olympic Mountains and eastward into and through the Samish Sea. Then, this river continues eastward across the Northwest Rockies ecoregion, often as far as the Montana Rockies.

258,000 square miles

Columbia river, Snake river

Kootenay river, Pend Orille-Clark Fork-Flathead river, Spokane river, Okanogan river, Yakima river, John Day river, Deschutes river, Willamette river.

xeric desert