Cedonia is a small unincorporated community in Stevens County, Washington, United States. Formerly a town, it lies in the valley of the Columbia River, on the western slope of hills known as Summit Mines, at a river section known as Lake Roosevelt, a reservoir created by Grand Coulee Dam. Washington State Route 25 is the primary transportatio…Cedonia is a small unincorporated community in Stevens County, Washington, United States. Formerly a town, it lies in the valley of the Columbia River, on the western slope of hills known as Summit Mines, at a river section known as Lake Roosevelt, a reservoir created by Grand Coulee Dam. Washington State Route 25 is the primary transportation arterial in the community.