Water Quality Improvement Project
Similkameen River Area:


The Similkameen River begins in the Cascade Mountains between British Columbia (BC) and Washington State. The Pasayten River flows north out of Washington to join the Similkameen River in British Columbia. After crossing the border, the Similkameen River becomes a tributary of the Okanogan River at Oroville, Washington. Mining, forestry, agriculture, and recreation are the major land-use activities in the Similkameen watershed.

Water quality issues

The Similkameen River is listed on the Washington State 303(d) list because it does not meet the human health criteria for arsenic. The major source of arsenic appears to be a legacy resulting from historical mining activities, mostly in British Columbia between Hedley and the U.S. border. Washington State sources are Palmer Lake, likely from periodic flooding and sediment deposit by the Similkameen River, possible inputs from Sinlahekin Creek, and re-suspension of contaminated sediments.

Why this matters

Arsenic is a naturally-occurring element found in soil and minerals. It is also commonly found in products such as wood preservatives, pressure-treated lumber, pesticides, automobile batteries, and light-emitting diodes (LEDs). Arsenic cannot be destroyed in the environment - it can only change its form.

Stormwater runoff and industrial wastewater discharges carry arsenic into our downstream waters. All of us are exposed to arsenic through the air we breathe, the water we drink, and the food we eat. Higher exposure to arsenic can cause health problems, depending on the type, amount, and method of arsenic ingestion.

Status of the project

It was determined that the Similkameen River naturally exceeds the EPA arsenic criteria upstream of the areas disturbed by mining near Hedley, BC. Under these circumstances, natural conditions constitute the water quality criteria. Because arsenic levels naturally exceed criteria, the loading capacity for the river is set equal to the natural background concentration of arsenic.

The Department of Ecology developed a total maximum daily load (TMDL) study and report for the Similkameen River from the international border between Washington State and British Columbia (river mile 27.1) to the river’s mouth near Oroville, Washington, including Palmer Lake outlet and other tributaries to the river within Washington. An arsenic monitoring plan was suggested for the Similkameen River. The plan recommends: 1) periodical review of Canadian federal/provincial water quality monitoring data for the Similkameen; 2) renewing arsenic monitoring of the river in Washington if and when cleanups begin; 3) analyzing inorganic arsenic in Similkameen River and Palmer Lake fish to assess human health risk; and 4) conducting a study of arsenic sources and cycling in Palmer Lake.

After responding to public comments, the TMDL report was submitted to EPA for approval. EPA approved the TMDL on February 17, 2004.

Technical information

Unless otherwise noted, the following documents are Ecology publications.

Lower Similkameen River Arsenic Detailed Implementation Plan

Lower Similkameen River Arsenic Total Maximum Daily Load: Submittal Report

Quality Assurance Project Plan - Similkameen River and Palmer Lake Investigation of Arsenic in Fish Tissue

A Total Maximum Daily Load Evaluation for Arsenic in the Similkameen River

Related information

Similkameen River and Palmer Lake Investigation of Arsenic in Fish Tissue

Effects of Small-Scale Gold Dredging on Arsenic, Copper, Lead, and Zinc Concentrations in the Similkameen River

WRIA 49: Okanogan Watershed Information (Water web site)


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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:
Similkameen River, Lower


# of TMDLs: 2

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
1250 West Alder Street
Union Gap, WA 98903-0009