A lake re-emerges: Analysis of contaminants in the Semá:th X̱ó:tsa (Sumas Lake) region following the BC floods of 2021

Citation: Ross, P.S., Walters, K.E., Yunker, M. and B. Lo. 2022. A lake re-emerges: Analysis of contaminants in the Semá:th X̱ó:tsa (Sumas Lake) region following the BC floods of 2021. Raincoast Conservation Foundation. Sidney BC Canada. ISBN 978-1-9993892-6-0 www.raincoast.org/reports/flood-water/

November 2022 | ISBN 978-1-9993892-6-0
Written by Peter S. Ross, Kristen Walters, Mark Yunker, and Bonnie Lo

Executive summary

The catastrophic floods of late 2021 in southern British Columbia (Canada) and neighbouring Washington State (USA) destroyed homes, farms and businesses, with excess water spilling debris, animal carcasses and diesel fuel into historically productive fish habitat. 

We assembled a team to assess water quality in the former Semá:th X̱ó:tsa (Sumas Lake) area of the Fraser Valley over a seven-week period after the floods. We collected water samples from 11 surface water sites and four groundwater sites for comprehensive contaminant analysis and a subsequent risk-based evaluation. We measured 379 analytes (chemical components and bacteria), including 262 anthropogenic contaminants. We examined excess nutrients, metals, fecal coliform, hydrocarbons, pesticides, pharmaceuticals and personal care products, perfluorinated compounds, sucralose and tire-related chemicals. 

We detected an average of 87 analytes at each location, of which 20 were anthropogenic chemicals not found in nature. 

We compared our results to the strongest Environmental Quality Guidelines (EQGs) available from Canadian jurisdictions. EGQs are benchmarks used to assess the quality of aquatic environments; they are based on the toxicological risks of specific substances to aquatic life. Only 86 of the analytes we measured (23%) had Environmental Quality Guidelines, meaning that risks to fish for 77% of our analytes are not clear.

There were 59 exceedances of EQGs among our 29 surface water samples during our study, suggesting that fish habitat in the Semá:th X̱ó:tsa region was heavily degraded by multiple contaminants. We identified excessive nutrients, metals, hydrocarbons, and pesticides as being the primary pollutants of concern, underscoring the impacts of domestic and agricultural practices on the fish habitat that permeates the area. 

While no EQGs are available to interpret the 177 new and emerging contaminants in our study, the widespread detection of cocaine, painkillers, and pesticides raises fundamental questions about the health of an area that is home to both fish and people.

Our findings paint a disturbing picture of habitat quality for salmon and other fish in this area of the Fraser Valley, and highlight our collective failure to monitor and protect these waters today and for future generations.


The catastrophic floods of late 2021 in British Columbia wreaked havoc on homes, farms and businesses, flushing manure, fertilizer, fuels, pesticides and other contaminants into fish habitat. The Semá:th X̱ó:tsa area was heavily impacted, with floodwaters temporarily restoring the lake that had existed prior to its deliberate drainage in the early 20th century.

The lack of pre-flood baseline data prevents us from adequately documenting the impacts of the floods on water quality in the region, but our analysis after the floods revealed several insights (below).

While traumatic and damaging to the affected communities, the catastrophic floods of 2021 nonetheless revealed an unfortunate truth about the waters of the Semá:th X̱ó:tsa area. The floods brought attention to a notable deficiency in environmental monitoring, removed a shroud of ignorance around water quality and exposed the extent of degraded fish habitat in the Fraser Valley.

  1. Water quality in the Sumas Lake area is poor, regardless of flooding, having been degraded by agricultural and domestic activities;
  2. Fish are exposed to high levels of metals, pesticides, and other contaminants of concern, and lower levels of life-giving dissolved oxygen;
  3. High nutrient levels, likely from agriculture and domestic sources, and slow moving waters from barriers and flood control structures, are likely contributing to low dissolved oxygen levels;
  4. The lack of riparian buffer zones along the edges of streams, rivers and canals in the Fraser Valley means that nutrients, pesticides and other contaminants wash easily into fish habitat during rainfall and flooding events;
  5. Salmon are exposed to various human activities and pollutant discharges along their migratory journey for which the cumulative effects are not well understood;
  6. Limited and inconsistent monitoring of water quality in BC constrains our ability to understand and manage risks associated with land use practices and chemical uses;
  7. The consequences of climate change on water quality and water quantity in fish habitat in BC are uncertain, and we are poorly equipped to predict, understand and protect fish habitat into the future;
  8. Sustainable land use that protects and restores fish habitat will benefit from innovative and shared approaches to investments in resilient infrastructure and nature-based solutions.

Report by sections


We are grateful for the important contributions of Myles Lamont (TerraFauna), Sofya Reger and Katerina Colbourne (Fisheries and Oceans Canada), Janice Kwo (Lower Fraser Fisheries Alliance), and Stephanie Braig to this study.

We thank Murray Ned and Ian Hamilton at the Lower Fraser Fisheries Alliance; Erin Weckworth, Dave Schaepe and Julian Yates at the Stó:lō Research and Resource Management Centre on behalf of the S’ólh Téméxw Stewardship Alliance (STSA); Dion Weisbrod and Danya Douglas – S’ólh Téméxw Guardians on behalf of the STSA; Amanda Gawor, Cheyenne Ned, and Richard Hall at Semá:th First Nation; Maggie Mazurkewich and Gillian Fuss at the Emergency Planning Secretariat; Jason Huang at Pacific Salmon Foundation; Emma Webster, Shirley Wang, and Bryan Jackson at the BC Ministry of Forests, Lands and Natural Resource Operations (now the BC Ministry of Land, Water and Resource Stewardship); Liz Freyman, Diane Sutherland, Colleen Loguisto and Cindy Meayes at the BC Ministry of Environment and Climate Change Strategy; Neil Dangerfield at Fisheries and Oceans Canada; and Darren Scott and Amy Peters at the City of Abbotsford.

This study was made possible by the contributions of the Lower Fraser Fisheries Alliance, the S’ólh Téméxw Stewardship Alliance, Sumas First Nation, Pacific Salmon Foundation, Fisheries and Oceans Canada, and the British Columbia Ministry of Environment and Climate Change Strategy. 

We thank the excellent analytical teams at CARO Analytical Services (Richmond BC) and SGS AXYS (Sidney BC).

We thank Mike Pearson, Misty MacDuffee, Dave Scott and Chris Genovali for constructive reviews of content in this report.