BC lake · 1,727 km² · Peace Arm watershed

Williston Lake

Its median summer total phosphorus in 2025 was 3,160 µg/L, from 1 reading.

The readings

126 summer readings on record from 1998 to 2025, at 6 sampling sites.

	Median, latest 10 sampled years	Compared with BC lakes
Total phosphorus	3,160 µg/L 2025, 1 reading	Too few readings (1) to compare with other lakes.
Chlorophyll a	1.2 µg/L 1999, 63 readings	Lower than 66% of the 156 BC lakes with at least 5 readings (lower is clearer).

Total phosphorus the nutrient that most often feeds algae in BC lakes

Chlorophyll a the green pigment in algae: a direct measure of how much is growing

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).

	Index	Class
From chlorophyll a (1.2 µg/L, 1999)	32	oligotrophic

Left out for too few readings (under 5 in the latest 10 sampled years): total phosphorus.

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

Year	Total phosphorus	Chlorophyll a
2025	3,160 µg/L (1)	–
1999	7.0 µg/L (61)	1.2 µg/L (63)
1998	4.0 µg/L (1)	–

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

Where it was sampled

E234188 WILLISTON RESERVOIR 2.5 KM NORTH OF BENNETT DAM 37 readings
E234186 WILLISTON RESERVOIR IN PEACE REACH; NORTH OF POINT CK 32 readings
E234187 WILLISTON RESERVOIR AT JUNCTION; MIDCHANNEL NORTH OF FINLAY BAY 29 readings
E234185 WILLISTON RESERVOIR IN SOUTH PARSNIP BASIN; 4 KM N OF SCOTT CK. 26 readings
E234184 WILLISTON RESERVOIR IN FINLAY REACH BETWEEN COLLINS AND MICA CKS. 18 readings
E338076 MACKENZIE (PE000327) INLET TO THE WETLAND 4 readings

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.