The decades-long effort to preserve Flathead Lake’s crystal-clear water continues to be a success, according to a new report from the Flathead Lake Biological Station.  

The lake has shown “no consistent signs of long-term or even recent deterioration of the lake’s world-class water quality,” according to the Biological Station’s annual State of the Lake report released in June. 

Since 1977, the University of Montana field station on Flathead Lake has tracked key metrics that directly impact the lake’s health, water clarity and aquatic life. The recently released report examined water samples from summer 2025 and found that all the data fell well within the historical norms for Flathead Lake.  

The markers include water clarity, the abundance of planktonic algae, the concentration of dissolved oxygen and the concentration of phosphorous. Scientists monitor these specific trends because they can cause changes in the lake that are obvious to the average water recreationist: cloudy water, algal blooms and decreasing fish populations.  

While the measurements for these markers did shift in a direction that would suggest declining lake health from 2024 to 2025, the report indicates that this movement is normal and represents typical annual variance. The long-term trends reveal that Flathead Lake’s water quality has been consistent since the Biological Station first started testing water samples nearly five decades ago. 

When the Biological Station first started their monitoring program in the late 1970s, samples showed the early warning signs of declining water quality. Human activity resulted in the increase of nutrients and sediment through shore erosion and untreated sewage, as well as the introduction of non-native species, according to the Biological Station report. 

The early results from water sampling spurred federal and state agencies to classify Flathead Lake as “impaired” and prompted the nation’s first ban of phosphate detergents, as well as the implementation of advanced wastewater treatment strategies.  

The June report shows that ensuing efforts to preserve the lake’s ecosystem have thus far been successful, according to the Biological Station. 

In 2025, Flathead Lake’s water clarity, measured by lowering a disk into the water until it is no longer visible, decreased slightly from the previous year but was still above the long-term average of 11.2 meters, according to the report. Water clarity, one of Flathead Lake’s defining characteristics, varies throughout the year, and measurements are recorded in the summer after the conclusion of the spring river runoff into the lake.  

The abundance of planktonic algae, which the Biological Stations tracks by measuring the concentration of chlorophyll in the water, increased relative to last year but still falls well within the historical trends. Clearer lakes, like Flathead Lake, have lower quantities of chlorophyll, while lakes with more chlorophyll are greener. 

The concentration of deep-water oxygen decreased to 85% in 2025 compared to around 87% in the previous year. The data reflects the percentage of the maximum oxygen concentration that water is able to hold at its given temperature, and high values are better for aquatic organisms like the various species of fish found in Flathead Lake. While there was a decrease from the previous year, the Biological Station has observed a statistical increase in deep-water oxygen concentration since 1992, indicating a decrease in oxygen-consuming organic matter in the lake’s bottom waters.   

The levels of phosphorous in Flathead Lake showed no statistical increase from the previous year and remained lower than the long-term average. High phosphorous concentrations are known to cause algal blooms that degrade water quality, can be toxic, and can deplete deep water oxygen levels. Wastewater, livestock runoff and agricultural fertilizer use are all common causes of phosphorous increases.   

The Biological Station also monitors the Mysis shrimp population in Flathead Lake, due to their impact on aquatic life, and projects a slow increase in the species over the long term.  

The non-native freshwater shrimp from the Midwest wreaked havoc on Northwest Montana’s lakes in the 1980s after they were introduced as a food source for kokanee salmon. But the salmon and shrimp feed on the same zooplankton, and the kokanee salmon population crashed. At the same time, fellow midwestern transplants lake trout and lake whitefish thrived on the new food source at the expense of native species like westslope cutthroats and bull trout.  

To view the full 2025 State of the Lake Report, visit flbs.umt.edu/media/vmrhhm0e/stateoflakereport2025.pdf.

Reporter Elsa Ericksen can be reached at 406-758-4459 or [email protected]. If you value local journalism, pledge your support at dailyinterlake.com/support.