PlaceDossier / BC places / Lakes

BC lake · 62 km² · Cowichan watershed

Cowichan Lake

Its median summer Secchi depth over 2016–2025 was 11.5 m, from 81 readings. Clearer than 95% of the 122 BC lakes with at least 5 readings.

The readings

214 summer readings on record from 2008 to 2026, at 4 sampling sites.

Median, latest 10 sampled yearsCompared with BC lakes
Secchi depth11.5 m
2016–2025, 81 readings		Clearer than 95% of the 122 BC lakes with at least 5 readings.
Total phosphorus2.0 µg/L
2017–2026, 28 readings		Lower than 97% of the 269 BC lakes with at least 5 readings (lower is clearer).
Chlorophyll a0.5 µg/L
2017–2026, 28 readings		Lower than 95% of the 156 BC lakes with at least 5 readings (lower is clearer).
Secchi depth how deep a black-and-white disc stays visible: deeper means clearer water
051015 2010201520202025 2008: median 8.5 m from 14 readings2010: median 9.5 m from 2 readings2013: median 11.4 m from 6 readings2015: median 16 m from 3 readings2016: median 14.2 m from 4 readings2017: median 15 m from 3 readings2018: median 10.4 m from 27 readings2019: median 10.1 m from 29 readings2020: median 14.8 m from 3 readings2021: median 14.8 m from 3 readings2022: median 16.1 m from 3 readings2023: median 16.8 m from 3 readings2024: median 12.5 m from 3 readings2025: median 15.3 m from 3 readingsm
Total phosphorus the nutrient that most often feeds algae in BC lakes
310 2010201520202025 2008: median 2.0 µg/L from 13 readings2013: median 9.1 µg/L from 6 readings2015: median 2.0 µg/L from 3 readings2016: median 2.1 µg/L from 3 readings2017: median 2.0 µg/L from 3 readings2018: median 2.9 µg/L from 3 readings2019: median 2.0 µg/L from 3 readings2020: median 2.0 µg/L from 3 readings2021: median 2.0 µg/L from 3 readings2022: median 2.0 µg/L from 3 readings2023: median 2.0 µg/L from 3 readings2024: median 2.2 µg/L from 3 readings2025: median 2.0 µg/L from 3 readings2026: median 2.0 µg/L from 1 readingµg/L
Chlorophyll a the green pigment in algae: a direct measure of how much is growing
0.313 2010201520202025 2008: median 0.6 µg/L from 15 readings2013: median 0.6 µg/L from 6 readings2015: median 0.2 µg/L from 3 readings2016: median 0.6 µg/L from 3 readings2017: median 0.2 µg/L from 3 readings2018: median 0.5 µg/L from 3 readings2019: median 0.7 µg/L from 3 readings2020: median 0.7 µg/L from 3 readings2021: median 0.5 µg/L from 3 readings2022: median 0.6 µg/L from 3 readings2023: median 0.8 µg/L from 3 readings2024: median 0.2 µg/L from 3 readings2025: median 0.4 µg/L from 3 readings2026: median 0.5 µg/L from 1 readingµg/L

Median each year (bigger dot, more readings)Middle half of BC lakesBC median

Source: BC Environmental Monitoring Data System (EnMoDS) results, Environmental Monitoring and Analysis Branch, Province of BC, records to September 19, 2026 (Open Government Licence – British Columbia); lake outlines and names: Freshwater Atlas lakes, Province of BC (Open Government Licence – British Columbia). Summaries by PlaceDossier.

Nutrient level Inferred

Reads as oligotrophic (few nutrients, little algae, usually clear).

IndexClass
From secchi depth (11.5 m, 2016–2025)25oligotrophic
From total phosphorus (2.0 µg/L, 2017–2026)14oligotrophic
From chlorophyll a (0.5 µg/L, 2017–2026)24oligotrophic

Inferred by us, not a provincial classification. Carlson's trophic state index (Carlson 1977, Limnology and Oceanography 22:361) turns each median into a score: 60 − 14.41 ln(Secchi m), 14.42 ln(phosphorus µg/L) + 4.15, 9.81 ln(chlorophyll µg/L) + 30.6. Classes after Carlson and Simpson (1996): under 40 oligotrophic, 40 to 50 mesotrophic, 50 to 70 eutrophic, 70 and over hypereutrophic. The index was built for summer readings from the middle of a lake; ours mix sites and months, so treat the class as a rough guide.

Year by year

YearSecchi depthTotal phosphorusChlorophyll a
2026–2.0 µg/L (1)0.5 µg/L (1)
202515.3 m (3)2.0 µg/L (3)0.4 µg/L (3)
202412.5 m (3)2.2 µg/L (3)0.2 µg/L (3)
202316.8 m (3)2.0 µg/L (3)0.8 µg/L (3)
202216.1 m (3)2.0 µg/L (3)0.6 µg/L (3)
202114.8 m (3)2.0 µg/L (3)0.5 µg/L (3)
202014.8 m (3)2.0 µg/L (3)0.7 µg/L (3)
201910.1 m (29)2.0 µg/L (3)0.7 µg/L (3)
201810.4 m (27)2.9 µg/L (3)0.5 µg/L (3)
201715 m (3)2.0 µg/L (3)0.2 µg/L (3)
201614.2 m (4)2.1 µg/L (3)0.6 µg/L (3)
201516 m (3)2.0 µg/L (3)0.2 µg/L (3)
201311.4 m (6)9.1 µg/L (6)0.6 µg/L (6)
20109.5 m (2)––
20088.5 m (14)2.0 µg/L (13)0.6 µg/L (15)

Median of each year's May to October readings; the number of readings is in brackets.

Source: BC Environmental Monitoring Data System (EnMoDS) results, Environmental Monitoring and Analysis Branch, Province of BC, records to September 19, 2026 (Open Government Licence – British Columbia); lake outlines and names: Freshwater Atlas lakes, Province of BC (Open Government Licence – British Columbia). Summaries by PlaceDossier.

Where it was sampled

  • E217508 COWICHAN LAKE LIMNOLOGY STATION # 2 128 readings
  • E217507 COWICHAN LAKE LIMNOLOGY STATION # 1 119 readings
  • E217509 COWICHAN LAKE LIMNOLOGY STATION # 3 97 readings
  • 0130180 COWICHAN LAKE - AT LOCKS 1 reading

Site names and IDs are as recorded in the provincial database; the IDs find each site's full record there.

What this record doesn't cover

Algae bloom reports and invasive mussel checks aren't in this record: the province's Algae Watch map and mussel monitoring aren't published as open data. For swimming, check the regional health authority's beach advisories; for blooms and mussels, ask the province's regional environment office or the local lake association.

Readings are from the province's monitoring database: samples taken by provincial staff, volunteer lake monitors, and companies and local governments that sample under their permits. We keep the ice-free season (May to October), and for phosphorus and chlorophyll only samples from the top 5 m. Each year's figure is the median of that year's readings; the summary is the median of every reading in the lake's latest 10 sampled years. A reading below the lab's detection limit counts at that limit. One site can be a bay rather than the open lake, so a lake's figures can differ from one end to the other.

Water quality changes with the season, the weather and the part of the lake. These are past readings, not a test of the water today: before you buy, ask the local government and the lake association what's known, and test a drinking-water intake yourself.