Water Quality Improvement Projects
Mill Creek Area:


Mill Creek is a tributary of the Walla Walla River in southeast Washington State. It is a perennial stream that originates in the Blue Mountains. The stream flows into Oregon, then back into Washington to join the Walla Walla River. Lower Mill Creek splits into several branches (distributaries) at the eastern end of the city of Walla Walla. The reach of Mill Creek that flows through the city was highly modified by the U.S. Army Corps of Engineers in 1948 to control flooding. They widened the channel substantially and reinforced it with weirs and concrete flumes. This reach has very low flows during the summer months because the majority of water is diverted down Yellowhawk and Garrison Creeks to provide irrigation water. Human modifications to channel form and stream flow in lower Mill Creek are major causes of degraded water quality conditions. Washington-designated beneficial uses of lower Mill Creek are salmon spawning and rearing, and secondary contact recreation.

Conditions in the upper Mill Creek watershed are vastly different. The headwaters are managed as a municipal watershed to protect the city of Walla Walla’s water supply. This portion of the watershed is nearly pristine and provides critical habitat for summer steelhead and bull trout, which are both listed as threatened under the federal Endangered Species Act. The beneficial uses for upper Mill Creek are char spawning and rearing, and extraordinary primary contact recreation. (See study area map)

Water quality issues

Mill Creek was listed on Washington State's water quality assessment 303(d) list of impaired water bodies because it did not meet state standards for ammonia-n and chlorine levels. The federal Clean Water Act requires Ecology to develop a water cleanup plan known as a Total Maximum Daily Load (TMDL) or other appropriate water quality improvement plan for each water body on the 303(d) list. Each TMDL is reviewed and approved by the Environmental Protection Agency (EPA). Separate TMDLs were developed to control ammonia-n and chlorine. Mill Creek was also listed on the 303(d) list for temperature, dissolved oxygen, pH, fecal coliform, chlorinated pesticides, and polychlorinated biphenyls (PCBs). Plans to control these pollutants were included in TMDLs developed for the entire Walla Walla Watershed. See the Technical Information section for additional information.

Mill Creek Channel, Eastern Washington State.  Photo courtesy of Michael Kuttel, WA Department of Ecology.

Status of the projects


Receiving water studies showed that the Walla Walla wastewater treatment plant (WWTP) effluent had minimal effect on water quality in Mill Creek. Ecology developed a TMDL for ammonia-N. The TMDL study determined an ammonia-N loading capacity (wasteload allocation, WLA) for the December through April period, when the WWTP discharges to Mill Creek. These rates kept the instream ammonia-N concentration at a level consistent with the state water quality standard. There was no WLA for May through November, since the WWTP discharges to an irrigation system rather than Mill Creek during this time period. Ecology stated that the proposed new permit conditions reduced instream residual chlorine problems and may improve the dissolved oxygen profile below the treatment plant. Ecology submitted the TMDL to EPA in March 1992. EPA approved the TMDL in February 1993.



Based on an Ecology study, it was determined that the Walla Walla WWTP was the sole source of chlorine into Mill Creek. Ecology developed a TMDL to control levels of residual chlorine in Mill Creek downstream from the WWTP. Ecology submitted the TMDL to EPA for approval in March 1992. EPA approved the TMDL in July 1997. As of August 2005, the Walla Walla WWTP plant has switched to UV disinfection and chlorine is only used for disinfection on an as needed basis. The WWTP discharges to Mill Creek from December through April, and to an irrigation district from May through November.

Why this matters

Technical information

Related information


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Last updated March 2016
  Water resource inventory area (WRIA) 32 map, Washington State.


WRIA: #32 (Walla Walla)
Walla Walla

Water-body Name:
Mill Creek


# of TMDLs:
Ammonia-N - 1
Chlorine - 1

Approved by EPA

Contact Info:
David T. Knight
Phone: 509-329-3590
Email: David.T.Knight@ecy.wa.gov

Eastern Region
Department of Ecology
4601 N. Monroe
Spokane, WA 99205