Two beaches on the lower reaches of the St. Croix River have been temporarily closed again due to blue-green algae blooms. This type of algae can produce harmful toxins in the water, making it unsafe for swimming.

Washington County Parks announced on June 11 that the beaches at St. Croix Bluffs and Point Douglas parks are closed until further notice. Both beaches were also closed due to algae last year, though the algae did not begin appearing until August and September. Algae has forced the closure of other beaches along the same stretch of river numerous times over the past several years.

The beach closures are signs of a serious problem facing the lower St. Croix: excessive nutrients in the water. The river has changed significantly since European settlement, and even more in the last several decades, and is now more vulnerable to a natural nuisance.

Blue-green algae are also known as cyanobacteria, ancient organisms that are found all over Earth. Cyanobacteria are generally not harmful unless their population explodes in what is called a bloom. Blooms are more common in water with a lot of nutrients such as phosphorus in it, as well as when water is warmer and precipitation patterns are affected by climate change.

In a bloom, the chemicals cyanobacteria produce as waste can reach high enough levels that they will sicken people and animals, causing irritating rashes at best and damaging kidney and neurological functions at worst. It has also a fatal threat to pet dogs, and deaths have occurred across Minnesota. The Washington County Parks Department closes beaches any time suspected cyanobacteria is observed, as a precaution.

Phosphorus gets into the river from natural sources, such as erosion and vegetation. But since the 1940s, more and more phosphorus has been getting into the river from farm fields and urban runoff. The lower St. Croix was the subject of study for several years that culminated in a 2012 report and plan to reduce nutrients that was approved by the U.S. Environmental Protection Agency.

The goal agreed on by Minnesota and Wisconsin officials is to return the river to the 1940s levels of phosphorus, before large-scale agriculture and development began affecting its water quality. That level is measured as 360 metric tons of phosphorus per year — one inspiration for St. Croix 360’s name.

The region draining toward the river has since seen significant efforts to slow the flow of nutrients from the land into the water since the goal was set. This has included farmers using conservation practices, cities and citizens installing rain gardens, organic farmers producing local food, agencies repairing gullies and restoring stream channels, communities passing regulations to prevent pollution from large-scale livestock operations, neighbors banding together to block dangerous proposals, and much more.