Water Quality Improvement Project
Okanogan Basin Area:


The Okanogan River originates in the Cascade Mountains north of the international border between British Columbia (BC) and Washington State. On the BC side the Okanogan River is made up of a series of lakes and a free-flowing river coming from Lake Osoyoos, which straddles the boundary. Once it crosses the border into Washington it flows 78 miles to its confluence with the Columbia River.

The Okanogan River’s primary tributary is the Similkameen River, which enters the Okanogan River just downstream of Oroville, Washington. The Similkameen River normally contributes three-quarters of the combined flow in the Okanogan River. About 20 small tributary streams also drain the 2,600 square miles of the Washington portion of the basin. The basin is lightly populated. Agriculture, forestry, mining, and recreation are the major land-use activities in the Okanogan watershed.

Water quality issues

The Okanogan River and several tributaries are listed on Washington State's 303(d) list of impaired waters because they do not meet the EPA human health criteria for DDT and PCBs in edible fish tissue, as well as for non-attainment of Washington's chronic criteria for DDT in water. When water bodies are placed on the 303(d) list, the Clean Water Act requires that the state develop a Total Maximum Daily Load (TMDL) for that waterbody. The TMDL determines the extent of the pollutant problem in the water, and establishes the maximum load of that pollutant that the water body can accept without violating the state's surface water quality standards. The TMDL is accompanied by a implementation strategy or plan, worked out between the state and local citizens, governments, and special interest groups, to help the move the water body to clean water.

Why this matters

DDT is a pesticide that was widely used from the mid-1940s until 1958, when the U.S. Department of Agriculture (USDA) began a program to phase it out for its insect control programs due to concerns about its persistence in the environment and toxicity to non-target organisms, such as aquatic life. In 1972, the U.S. Environmental Protection Agency (EPA) banned DDT for all uses except for emergencies.

Despite the ban, DDT persists in the environment. Soils containing DDT can end up in streams and other surface water bodies through erosion. As DDT is transported to streams, it is absorbed by the plant and animal life within the stream system and then moves up the food chain. DDT biomagnifies at higher levels in each step of the food chain (plants, aquatic biota, fish, humans). DDT can also be found in ground water, as water infiltrates through contaminated soils into aquifers.

Pesticides and polychlorinated biphenyls (PCBs) can be toxic to fish and wildlife that use a contaminated water body.

In the past, PCBs were used as coolants and lubricants in electrical equipment such as transformers and capacitors. The United States banned the manufacture of PCBs in 1977 because they build up in the environment and can be harmful to humans and wildlife. PCB exposure can occur if you:

  • Eat food, including fish, meat, and dairy products that is contaminated by PCBs.
  • Drink PCB-contaminated water.
  • Breathe air near hazardous waste sites that contain PCBs.

Because DDT, PCB and other legacy chemicals are found everywhere now – cities, farms, forest roads, etc. - control of stormwater runoff or movement of soils by wind or vehicles is paramount in controlling spread of the chemicals.

The Okanogan watershed was one of Washington State’s first Total Maximum Daily Load (TMDL) efforts. Studies on DDT, PCB, arsenic, mercury, and other pollutants started as early as 1980. Because these pollutants are so hard to remove from the environment, Ecology has developed containment strategies as well through TMDLs.

What has been done

A TMDL project was conducted for the portion of the river that flows through Washington State. It was determined that the source of these contaminants appeared to be from historic agricultural and industrial activities common throughout the Okanogan River watershed.

The chemical characteristics of DDT and PCBs cause them to be classified as persistent, bioaccumulative toxins. They are a legacy of past activities, because the United States and Canada banned their use over 25 years ago. Due to their legacy nature mitigation of the DDT and PCB contamination, both directly and indirectly, has already occurred in the Okanogan Basin. Direct actions include banning these materials from use; indirect actions include irrigation improvements that have reduced the loss of agricultural topsoil that potentially could carry pesticide residues to the Okanogan River and associated waterbodies. The DDT and PCBs in the lower Okanogan River Basin TMDL continue to promote community actions that address DDT and PCB contamination, along with new actions of source identification and development of management strategies to address any sources of contamination subsequently revealed.

Status of the project

EPA approved the TMDL for the Lower Okanogan River in February 2005. The project is currently in implementation. Preliminary monitoring in 2009 and 2011 indicated that DDT levels were slightly reduced. Ecology will continue to monitor until 2015, then revise the implementation plan if needed.

Technical information

Unless otherwise specified, the following documents are Ecology publications.

DDT and PCBs Total Maximum Daily Load: Water Quality Improvement Report

Lower Okanogan DDT PCB Detailed Implementation Plan

Quality Assurance Project Plan: Okanogan River DDT and PCB Total Maximum Daily Load Effectiveness Monitoring

Lower Okanogan River Basin DDT and PCB Total Maximum Daily Load: Water Quality Effectiveness Monitoring Report

Related information

Okanogan Pesticide Collection Partnership: Keeping Pollution Out of the Water (Ecology water quality story)

WRIA 49: Okanogan Watershed Information (Environmental Assessment Program website)


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Last updated June 2015
  Water resource inventory area (WRIA) 49 map, Washington State.


WRIA: #49 (Okanogan)
County: Okanogan

Water-body Name:
Okanogan River, Lower


# of TMDLs: 10

TMDL approved by EPA
Has an implementation plan

Contact Info:
Mark Peterschmidt
Phone: 509-454-7843
Email: Mark.Peterschmidt@ecy.wa.gov

Central Region
Department of Ecology
15 West Yakima Ave - Suite 200
Yakima, WA 98902-3463