​Assessing the current water clarity status of ~100,000 lakes across southern Canada: A remote sensing approach

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Graphical Abstract
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Figure 1. Boundary for lake interpolation and ecozones represented: PM = Pacific Maritime, MC = Montane Cordillera, BP = Boreal Plain, Pr = Prairie, WBS = Western Boreal Shield, EBS = Eastern Boreal Shield, MP = Mixedwood Plain, AM = Atlantic Maritime.
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Figure 7. Summer season water clarity classes by HII class. The first row (green dots) represents Very Low Clarity lakes. The second row (seagreen dots) represents Low Clarity lakes. The third row (turquoise dots) represents Clear Lakes. The fourth row (blue dots) represents Very Clear Lakes. (...)
Background

Canada contains more lakes than any other country, representing an incredible diversity of ecological and hydrological types. However, because of their sheer quantity and often remote locations, the vast majority of these lakes remain unmonitored by traditional ground-based methods. This lack of data creates a significant gap in our understanding of national water quality and lake health. We recognized that water clarity is a critical indicator of aquatic health, as it directly influences biological productivity, dissolved oxygen levels, and water temperature. To address this monitoring gap, we set out to move beyond localized studies and develop a comprehensive, national-scale assessment of lake status across the country.

Approach

We utilized satellite imagery from the Landsat 8 sensor to estimate water clarity for approximately 100,000 lakes across southern Canada. To handle this massive volume of data, we employed a cloud-computing platform that allowed for planetary-scale geospatial analysis. We applied a mathematical formula that translates the ratio of blue to red light reflected from the water into a measure of water clarity known as Secchi depth. In this study, we applied an established empirical algorithm to derive estimates of Secchi depth from Landsat 8 reflectance data, allowing us to estimate lake water clarity across southern Canada. To ensure accuracy and remove interference from clouds, haze, or wildfire smoke, we used a "median filtering" technique to calculate average clarity over a four-year summer window. This approach allowed us to assess how clarity is influenced by lake depth, surrounding land use, and geological features.

Key Findings

We identified clear spatial patterns in water quality driven by regional geography, physical lake characteristics, and human activity:
  • Regional and Geological Drivers: We discovered that lakes in the western mountains and the Eastern Boreal Shield are significantly clearer than those in the Prairies and Boreal Plains. This is largely tied to the underlying rock; lakes on hard, weather-resistant intrusive rocks are generally clearer than those sitting on easily erodible sedimentary formations.
  • Physical Lake Influences: Lake depth is a primary factor in water quality. We found that deep lakes are significantly clearer than shallow ones across most of the country, as shallow systems are more susceptible to wind-driven sediment mixing and internal nutrient loading.
  • Human and Natural Impacts: While areas with high human impact—such as urban centers and agricultural zones—tend to have lower clarity, we found that many low-clarity lakes also occur in remote areas. This suggests that natural processes, such as organic matter leaching from wetlands, are powerful drivers of lake status alongside human influence.

Impact

This research provides the first comprehensive look at water quality for nearly 100,000 lakes, the vast majority of which were previously unsampled. By demonstrating that satellite technology can effectively monitor massive freshwater networks, we have created a vital baseline for tracking environmental change over time. Our findings help identify which regions are most vulnerable to human impact or natural shifts, supporting more effective, large-scale watershed management. Ultimately, this work provides a essential tool for ensuring the long-term protection of Canada’s vital aquatic ecosystems.

Resources

Published Paper: Deutsch* ES, Fortin MJ, Cardille JA. Assessing the current water clarity status of~ 100,000 lakes across southern Canada: A remote sensing approach. Science of The Total Environment. 2022 Feb 17:153971. DOI : https://doi.org/10.1016/j.scitotenv.2022.153971
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Data Repository: HydroLakes Database