Benton County, Oregon is about 40 miles southwest of the capital city of Salem and about 45 miles northwest of the city of Eugene.

Several rivers and numerous streams flow through Benton County. Its eastern border is the Willamette River, and Marys River watershed lies mostly within the boundaries of the County. Other rivers located partially within Benton County include The Long Tom, The Luckiamute, and The Alsea. The Alsea Basin is geographically separated from the rest of the county by the Coast Range. Benton County is home to Marys Peak, the tallest mountain in the Coast Range. 

Approximately 15,000 years ago the great ice dam of Missoula Lake thawed and refroze repeatedly causing 2,000 years of cataclysmic floods throughout what is now known as the Columbia River Gorge and the Willamette Valley. Those flooding events deposited rich sediments, making the Willamette Valley the most fertile and productive area in Oregon.