State and local health officials close Cow Creek Reservoir near Chinook -

State of Montana NewsRoom

Montana Government News


State and local health officials close Cow Creek Reservoir near Chinook

Friday, September 20, 2019/Categories: Department of Environmental Quality/Tags:

CHINOOK – Blaine County health officials reported Thursday that a test performed for cyanotoxins associated with a blue-green algae bloom at Cow Creek Reservoir has returned with high results. As a result, state and local health officials recommend the reservoir, southwest of Chinook, be closed to all recreation and that pets and livestock be kept away from these waters. Closure signs have been posted at access points.


On Sept. 12, the Blaine County sanitarian collected eight water samples from various places on the reservoir and from Cow Creek near the inlet and outlet. The samples were tested by the Environmental Protection Agency’s Region 8 lab in Denver with results returned on Sept. 18. One sample taken along the north shore midway between the dam and the inlet indicated high levels of microcystin – at or above 50 micrograms per liter (>50 ug/L). The EPA’s human health recreational advisory level is 8 ug/L.  


Microcystin is a known liver toxin and the Montana Department of Environmental Quality, the Department of Public Health & Human Services and the Blaine County Health Department support this temporary closure.


Officials note that conditions on the reservoir can change rapidly for better or worse depending on factors such as wind, weather and water temperature. The high level in the sample collected along the north shore may not be indicative that the entire reservoir is affected. However, officials urge caution and want to keep pets, livestock, children and adults out of Cow Creek Reservoir until further notice.


According to DEQ, these types of algal blooms are native to freshwater lakes and reservoirs in Montana. They tend to grow in warmer water and where there’s an increase in nitrogen and phosphorus runoff from a variety of human activities. They become increasingly common this time of year. The algae blooms float on the surface of the water and may look like pea soup, grass clippings, or green latex paint.


If you see what you think might be a harmful algae bloom, you can report it using the Montana Department of Public Health and Human Services website at:  The site also has a map of HAB reports and information on how to identify them.


If you suspect a toxic algae-related illness in a person or animal, including livestock, call Poison Control immediately at 1-800-222-1222 and/or contact Blaine County Sanitarian Ron Andersen at: 406-868-4957


Number of views (1200)/Comments (0)

Please login or register to post comments.