Map science

Mapping the Semá:th X̱ó:tsa (Sumas Lake) region following the BC floods of 2021

Our new map shows water quality sampling sites and the re-emergence of Sumas Lake after the 2021 floods.

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. 

The resulting report, A lake re-emerges: Analysis of contaminants in the Semá:th X̱ó:tsa (Sumas Lake) region following the BC floods of 2021, was published in November, 2022. The findings paint a disturbing picture of habitat quality for salmon and other fish in this area of the Fraser Valley, as well as highlight our collective failure to monitor and protect these waters today and for future generations.

We created this map to show where the 2021 floodwaters overlapped with the historic Semá:th X̱ó:tsa, along with the locations of sampling sites used for contaminant analysis. 


Gerle B. Analysis of contaminants in the Semá:th X̱ó:tsa (Sumas Lake) region following the BC floods of 2021. [web map]. Sidney (BC): Raincoast Conservation Foundation. 2023.

About the map

This map displays the Semá:th X̱ó:tsa region of British Columbia. Historically, this area contained Semá:th X̱ó:tsa, which greatly expanded during spring freshet or flooding events. The Semá:th X̱ó:tsa polygon was created by Riley Finn1 to fill in the gap from a 1984 study done by North and Teversham2. Due to the highly variable water levels of the lake throughout the year, this polygon is an estimate. Semá:th X̱ó:tsa was drained in the early 1920s and the land is now largely used for agriculture. The Barrowtown Pump Station prevents this lake from re-forming and is also displayed on this map. 

In late 2021, this region was significantly impacted by flooding and Semá:th X̱ó:tsa began to retake its historic shape. The layer displaying floodwater was created using Sentinel-2 Imagery from November 21, 2021 and is an approximation as the floodwaters were ever-changing.3 

After the floods, a team from Raincoast collected water samples from the 11 surface water sampling sites and two groundwater sampling sites seen on the map. These samples were analyzed and the results were compiled into a report

Using the map

The widgets in the corners allow you to view the legend, change which layers are visible, zoom in and out, zoom to your current location, search for an address or place, and change the basemap. Sampling sites will display more information when clicked on. 

References and sources

1 Finn RJR, Chalifour L, Gergel SE, Hinch S, Scott DC, Martin TG. Quantifying lost and inaccessible habitat for Pacific salmon in Canada’s Lower Fraser River. Freshwater Ecology. 2021;12(7). doi:10.1002/ecs2.3646 

2 North MEA, Teversham JM. The vegetation of the floodplains of the Lower Fraser, Serpentine and Nicomekl Rivers, 1859 to 1890. Syesis. 1984;17.

3 EO Browser,, Sinergise Ltd. Modified Copernicus Sentinel data [2021]/Sentinel Hub.


Thank you to Colin Richardson for creating and sharing the historic Semá:th X̱ó:tsa freshet layer and for the time and effort put into this project. Thank you to Riley Finn for creating and sharing the historic Semá:th X̱ó:tsa layer. 


Please feel welcome to embed, share, and remix this information, as long as you provide proper attribution, and with the exception for the layers that have their own licenses.