The headwaters of Mill Creek are located in the Blue Mountains, Umatilla County, Oregon. The river flows north from the mountains and then west through the City of Walla Walla, Walla Walla County, Washington, providing the city with drinking and irrigation water. The headwaters of Mill Creek are in a portion of the Umatilla national Forest set aside as a watershed for the City of Walla Walla. The Mill Creek watershed is home to elk, cougar, deer, and bear.
Human settlement dates back at least a century.
Mill Creek has been heavily altered due to the human impact on the surrounding environment. The human influence on the Mill Creek drainage basin is predominantly due to agriculture, grazing, forestry, and urbanization. Homes have been built along the banks of Mill Creek, most densely concentrated near Kooskooskie, Washington. This project aims to determine some effects that the greater Walla Walla community has on Mill Creek and to see how different land uses each impact water quality of Mill Creek. Where the river runs into Walla Walla, it is diverted by a levee and channeled by concrete corridors through the city, in some instances underneath roadways, in other cases under direct sun with no riparian vegetation.