Protecting the quality of lakes and rivers is an important societal concern, but the costs associated with ground-based sampling means that only a small fraction of this resource is monitored by conventional field methods. Satellite imagery now can measure key water quality constituents on virtually all lakes at low cost, greatly supplementing the information provided by ground monitoring programs.

With support from several state and federal agencies and the University of Minnesota we have been measuring lake water clarity, a key water quality indicator, on lakes for almost two decades. Using recent and archived Landsat satellite imagery we have mapped water clarity in more than 10,000 Minnesota lakes for seven time periods since 1975. 

Landsat Classification of Water Clarity

We documented strong relationships between the spectral-radiometric responses of Landsat sensors and ground-based observations of water clarity by Secchi disk, and our technique has proven to be an accurate, economical method to monitor lakes over large geographic areas and time. Water clarity of Minnesota lakes can be explored in the LakeBrowser, a web-based mapping tool that enables searches and display of historical results for individual lakes.

MN Lake Browser

New satellites like Landsat 8 and Sentinel-2 have sensors with enhanced capabilities that allow the retrieval of other key metrics of water quality from satellite imagery. Measurable metrics include chlorophyll (a measure of algal abundance), suspended solids, turbidity, and colored dissolved organic matter (CDOM). The new satellites have more spectral bands, more frequent temporal coverage, and other improvements compared to previous Landsat sensors. Together, the parameters they can measure control many important biological, chemical and physical features of lakes.

We recently mapped CDOM levels in Minnesota lakes using 2015 Landsat 8 imagery. With NSF funding, we are expanding this mapping across the Northern Lakes and Forest and North Central Hardwood Forest ecoregions, which span Minnesota, Wisconsin, and Michigan. We will use these results to develop a deeper understanding of the factors that control CDOM levels both spatially and temporally in Minnesota’s lakes. Similarly, with funding from the Legislative-Citizen Commission on Minnesota Resources, we currently are mapping chlorophyll and suspended solids concentrations in lakes across Minnesota. Results from both studies will be provided on this site and in the Lake Browser, as they become available.

map of CDOM levels in MN
Colored Dissolved Organic Matter classification. Landsat OLI images from 2015 were used to map CDOM levels of Minnesota lakes. Click on the image to go to a full resolution, scalable version. See poster and Earth Observatory article for further explanation.

With the expanded availability and analysis of imagery, we expect remote sensing of water quality to become a widely used technique over the next decade that will greatly expand our knowledge base on the status of water quality in Minnesota lakes.