Checking Lake Water Quality

Water Clarity on Avalanche Lake. Photo courtesy Flathead Lake Biological Station

Montana has 1,417 named lakes, reservoirs, and ponds that are 5 acres or greater covering about 577,000 acres. These waterbodies include various natural lakes – alpine lakes and closed basin lakes (i.e., lakes with no surface outlet), as well as large reclamation and/or hydropower reservoirs. Protecting and restoring lake water quality and designated uses (primary contact recreation, drinking water, aquatic life and agriculture) is part of Montana state law and the federal Clean Water Act.

Knowledge and understanding of lake water quality and the pollutants affecting it helps to preserve our lake systems. Overall, nutrients and salinity are the most widespread pollutants in Montana lakes. Other pollutants of concern are metals, sediment, polychlorinated biphenyls (PCBs), pesticides and herbicides. Contaminants of emerging concern (e.g., pharmaceuticals and personal care products) are beginning to be studied to determine their ecological risk in lake systems and to human health.

There are many measures of lake water quality, including clarity. Most people are concerned about how clear and clean lake water is, but clarity does not indicate lack of invisible toxic substances in the lake. Although clarity is often the most evident measure, other measures impact a lake’s suitability for recreational activities, drinking, aquatic life, and agricultural activities. Expected levels of clarity are based on a lake’s natural trophic state. Weather conditions, such as wind and rain, affect short term variability in clarity. Poor clarity may also indicate a nutrient overload because excess nutrients promote algal growth which decreases how far light can penetrate into the lake water.

Secchi disk. Photo courtesy Flathead Lake Biological Station

One easy way to check water clarity is to use a Secchi disk. Scientists and volunteer citizen scientists use Secchi disks to monitor their lakes for possible changes in water quality.

The Secchi Disk
Father Pietro Secchi, an astrophysicist who was the scientific advisor to the Pope, invented the Secchi disk. In 1865, Secchi used white disks to measure the clarity of the water in the Mediterranean Sea. Various sizes of disks have been used since that time, but the most frequently used disk is an 8-inch diameter metal disk painted in alternate black and white quadrants.

By using this remarkably simple instrument, one can determine the general clarity of a lake. The observer lowers the disk into the water until it is no longer visible. At that point, the depth is recorded.

By measuring Secchi disk depths regularly and over a period of years, historic changes in water quality can be determined and long-term trends established. Multiple measurements throughout the years tend to compensate for the variability caused by the observer and weather conditions. Examples of long-term Secchi depth data collection are shown in Figures below.

The annual average secchi depth data collected by the Flathead Lake Biological Station shows that the water clarity in Flathead Lake has improved slightly over time. Graphic courtesy Flathead Biological Station.


The monthly average Secchi depth data collected by the Flathead Lake Biological Station shows that there is great seasonality to the secchi depth in Flathead Lake throughout the year. For example, the graph depicts a large decline in clarity during the spring when there is peak flow in the river discharge from the spring snowmelt. Graphic courtesy Flathead Biological Station.


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