The [Mill Creek] Watershed area is protected. It is closed to the public allowing only limited access in coordination with the U.S. Forest Service. It is patrolled by a City Watershed Attendant and United States Forest Service personnel.
In addition to this primary source [Mill Creek Watershed], the City [of Walla Walla] also has seven deep (800-1400’) basalt wells used to supplement supply when stream levels decline in summer months.