Barb Liukkonen, Water Resources Education Coordinator, University of Minnesota Water Resources Center

Water clarity is a common indicator of water quality, recreational suitability and aesthetics. Citizen volunteers and agency staff regularly use Secchi disks to measure water clarity. However, with over 10,000 lakes in Minnesota, many of which aren’t easily accessible, many lakes are not monitored.

New technology and new interpretive tools allow researchers to use satellite images to assess water clarity for over 10,500 Minnesota lakes. The Landsat satellite orbits north to south and acquires images that are about 100 miles on each side. Sensors record reflected light and infrared energy as digital images. Analysts use these images to separate water features from land, and measure the intensity of reflected blue and red wavelengths from each lake. These measurements are calibrated with Secchi disk readings. Once a mathematical relationship is determined from 20-40 lakes, the clarity of all the lakes on a single Landsat image can be computed.

Lake clarity is affected primarily by three different constituents in the water:

1. algae suspended in the water column;
2. suspended sediment; or
3. dissolved natural organic matter, derived from the partial decay of terrestrial plants.

The cause of the “iced-tea” color in these lakes is often referred to as “tannic acid” or tannins. A more correct term is “humic materials,” but the result is the same – tea-colored water. They are common in northern Minnesota where there is forested watershed with many bogs and wetlands, and are usually low in nutrients with little suspended sediment or algae.

A map showing Minnesota lake water clarity has been developed, and will be available within a couple months, as either a 3′ x 4′ poster or an 8 ½” x 11″ fact sheet. Using a color scale, the map clearly illustrates differences in water clarity across the state, between ecoregions. When the map is available, we’ll let you know how you can get a copy.

Meanwhile, you can visit a web site that will have a PDF of the map and lots of information about water clarity and satellite imagery. And to learn more about the clarity of your favorite lake.

The satellite imaging work was carried out by the University of Minnesota Remote Sensing and Geospatial Lab with support from NASA. The poster was developed as a collaboration between researchers and staff at the University of Minnesota Remote Sensing Lab and Water Resources Center, DNR, PCA, Met Council and the Science Museum of Minnesota.

Posted in Lakes |