You’re flowing EAST into the Atlantic ocean: this is the Atlantic Ocean watershed.
The OVERALL health score of the
Atlantic Ocean
basin
is Fair
Enough data? Partially Sufficient
This basin directs water into the Atlantic Ocean, including the St. Lawrence River and the Great Lakes system. It covers much of Quebec, the Maritime provinces, and parts of Ontario, playing a crucial role in Canada’s economy and transportation.
Did you know? The St. Lawrence River was the gateway to European exploration and settlement in Canada, with Indigenous peoples such as the Haudenosaunee and Algonquin using the river long before Jacques Cartier arrived in the 16th century. Today, it remains a key economic and cultural corridor.
NATIVE LANDS
Anishinabek (Algonquin, Chippewa, Mississauga, Nipissing, Ojibwe, Odawa), Ho-de-no-sau-nee-ga (Haudenosaunee Confederacy; Mohawk, Oneida, Onondaga, Cayuga, and Seneca), Nēhiyawak (Cree), Nitaskinan (Atikamekw / Nehirowisi Aski), Nitassinan (Innu), Skarù:ręʔ (Tuscarora), Wabanaki Confederacy (Mi’kmaq, Maliseet, Passamaquoddy, Penobscot, and Abenaki), Wyandot.
JURISDICTION
Newfoundland and Labrador, Nova Scotia, Prince Edward Island, New Brunswick, Quebec, Ontario
POPULATION CENTER(S)
Toronto, ON
WATERSHED SIZE
1.2 million km² (~12% of Canada)
POPULATION
~24 million
YEARLY FLOW
44,874 m³/s (that’s ~18 Olympic swimming pools filled per second!)
Summary of results
Very good | Good | Fair | Poor | Very poor | Data deficient | Not scored |
Divided into... | ||||||
|---|---|---|---|---|---|---|
How to interpret the scores
Very good | Good | Fair | Poor | Very poor | Data deficient | Not scored |
Our freshwater health scores are based on four key metrics: water quality, hydrology (flow), benthic macroinvertebrates, and fish populations. For each metric, we use standardized criteria—such as exceedance of federal and provincial water quality guidelines, long-term trends in river flow, tolerance values for invertebrates, and native fish species richness—to assess conditions across watersheds.
To make these scientific results easier to understand, we translate them into a simple 1 to 5 scale: 1 = VERY POOR and 5 = VERY GOOD
These scores provide an overall picture of freshwater health, helping both experts and the public quickly see where attention is needed.
It’s important to remember:
- We calculate scores at a national scale, using the best available data. While this provides a valuable snapshot, it may not capture every local nuance.
- For some regions and specific waterbody types (like northern areas or mud-bottom prairie rivers), certain metrics—like the Hilsenhoff Biotic Index (HBI) for invertebrates—may not fully reflect local conditions. We’ve flagged these cases.
- Data sufficiency is a key consideration: Some watersheds have long-term, robust data. Others may be classified as “Data Deficient” due to many reasons besides lack of data, including monitoring that is incompatible with the FHA, inaccessible data, and the Covid pandemic. Learn more on our data sources page.
For more detail on how each score is calculated, including thresholds, trend analyses, and limitations, please visit our About the Indicators page or consult our Tech Doc for the full methodology.
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Datapoints
This analysis includes all the data we could access—whether from open platforms, agency reports, or historical records. Some datasets were readily available, while others came from hardcopy reports and handwritten notes that we digitized and formatted. We’re grateful to all data providers for sharing their data. Even so, this is only part of the picture. Learn more about data accessibility challenges.
9,472,766
6,503,246
351,874
110,357
Comparing water quality across sub watersheds
We’ve pulled out a six of the 48 parameters that we display on each Watershed Report so that we can compare. Click through to see all individual water parameter scores.
Very good | Good | Fair | Poor | Very poor | Data deficient | Not scored |
By watershed | Chloride | Dissolved oxygen | Total Phosphorus | Ammonia | Lead | Glyphosate |
|---|---|---|---|---|---|---|
Learn more about our analysis
The overall water quality score is based on how frequently key water quality parameters exceed established guidelines. Individual parameters provide insight into specific concerns, but water quality is complex, and further investigation is encouraged before drawing conclusions about the factors influencing the scores. Here, we’re showing you the percentage of samples within the recommended thresholds over the past five years (2018–2023). These results are compared to the five years preceding (2014–2018) to assess trends. You can visit our glossary page in our Tech Doc for descriptions of each parameter and our Guidelines and Tolerances page to understand the thresholds used in this region.
How's it flowing?
Exploring how flow has changed over time
The Atlantic Ocean basin scored Good.
For some watersheds, there’s been only a minor change, while others have seen shifts including increases and decreases. The Atlantic Ocean watershed trend is: Minor increase in flow.
We wanted to dive deeper than the score into how flow has changed over time. We took the daily average flow for the Atlantic Ocean watershed from the past 30 years (1994-2023) and compared it to the previous 30 years (1964-1993).
Total daily flow
Atlantic Ocean basin’s average daily flow from 1994-2023 compared to 1964-1993. (N=131)
How much has it changed?
The magnitude of change (%) for Atlantic Ocean basin’s total daily flow – 1994-2013 compared to 1964-1993. (N=131))
about our analysis
We used flow data from HYDAT for this watershed, focusing on stations with long-term records—at least 50 years of data and a minimum of 6 months of valid daily flow data per year. This approach ensures inclusion of stations in colder regions, where rivers may freeze part of the year. We then calculated average daily flows across two 30-year periods: 1964–1993 and 1994–2023. By comparing these periods, we smoothed out short-term events like floods or droughts to better understand long-term changes in flow patterns. Learn more about our hydrology calculations in our Tech Doc.
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most common benthic invertebrate taxa observed
chironomidae
The score for benthic invertebrates for the Atlantic Ocean watershed is Fair.
Benthic (meaning bottom-dwelling) macroinvertebrates are small aquatic animals and the aquatic larval stages of insects. They include dragonfly and stonefly larvae, snails, worms, and beetles. These creatures make up the backbone of the ecosystem and, since they live there full time, are a great way to understand its health!
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The most frequently reported taxa is the chironomidae, with 35% of all samples containing them.
About chironomidae
Chironomidae, or non-biting midges, are the most common benthic invertebrates in much of Canadian freshwaters. With over 10,000 species worldwide, they are a diverse family of insects with the ability to live in environments ranging in temperature, oxygen levels, and salinity. This makes them a great indicator species, as their presence, absence, and abundance are all linked to changing environmental conditions.
about our analysis
We assessed benthic macroinvertebrate health using the Hilsenhoff Biotic Index (HBI), which measures how tolerant invertebrate communities are to pollution. We used data from CABIN, the Royal Ontario Museum, Conservation Authorities, and regional programs, focusing on samples identified to at least the family level. For most sub-watersheds, we calculated an average HBI score and assigned a health rating from 1 (very poor) to 5 (very good). However, in some regions—such as the North and certain Prairie rivers—HBI is not considered an appropriate tool due to ecological differences or data limitations. In these cases, we marked the sub-watershed as Not Scored. You can learn more about how we evaluated benthics on our Guidelines and Tolerances page and our Tech Doc.
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Most frequently reported fish:
American Eel
The fish score for the Atlantic Ocean is Data deficient.*
* Scores are based on the change in species richness (read more in ‘About our Analysis’ below). We plan to work with experts to make the fish measurement more useful in the future. Factors like understanding monitoring purpose, waterbody size, and more will help us avoid things like sampling bias affecting scores (for example, sampling for one fish or for a species of interest will skew scores).
Here are the most popular fish across all the sub-watersheds of the Atlantic Ocean! In most cases, these are the species of interest in their watershed. Dive into each subwatershed to read all reported fish species for the past 10 years.
By watershed | Most common fish |
|---|---|
Maritime Provinces | Atlantic Salmon |
St. Lawrence | American Eel |
about our analysis
The fish health score is based on trends in native fish species richness, which can reflect overall ecosystem health. Fish monitoring in Canada often relies on presence/absence data, limiting detailed community-level assessments. Still, tracking species richness over time provides valuable insights. In addition to the scores, we present the trend in native species richness over the past ten years (2014–2023) and compare it to the preceding decade (2004–2013) to assess changes. Watersheds are classified into three categories: Good (no significant decline), Fair (a significant decline in either median or total species richness), and Poor (a significant decline in both). Read more in our Tech Doc.
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Dive deeper
Other resources
We recognize that these Reports are a zoomed out way to understand watershed health. You may have additional questions about your local waterbody. We encourage you to find out more about the local conservation work and testing happening in your region. Some regions produce report cards for individual waterbodies, but often this is dependent on resources.
Many regions have opportunities to take action to collect water quality data and get involved in restoration and education opportunities. Learn more about how you might get involved.
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