Have you ever seen parts of your favorite lake or river turn into pea soup during the summer? Or maybe it looks like a can of green paint spilled? When temperatures climb, conditions are ripe for algae blooms to form on lakes or slow moving portions of rivers.
Why do the blooms form?
Blooms thrive in shallow, warm, non-moving bodies of water. High levels of nutrients like nitrogen and phosphorous, warm water temperatures and high light levels - or a combination of all three factors - may stimulate the rapid reproduction of algae until it dominates the local aquatic ecosystem, forming an algal bloom.
Blooms vary in appearance and can appear as foam, scum, or mats on the surface of the water. These blooms may manifest in a variety of colors, including blue-green, bright green, brown, or red.
While not common on the main channel, algal blooms do occur on the lower St. Croix River and Lake St. Croix, especially from late summer to mid-fall. Typically these algae remain suspended in the water column, but surface scums have been reported from some locations.
How can I help prevent harmful algal blooms?
Tips for staying safe
HUMANS: numbness of lips, tingling in fingers and toes, dizziness, headache, rash or skin irritation, abdominal pain, diarrhea, and vomiting.
PETS: weakness, staggering, convulsions, difficulty in breathing, and vomiting.
Algae Alert Network
Interested in helping the National Park Service monitor algal blooms? Join the Algae Alert Network and contribute to citizen science and public safety. Assist in monitoring the St. Croix River (from Wild River State Park to Stillwater) for algal blooms and collect samples between July and September.
For more information or to sign up, contact Jonathan Moore at (715) 491-6839 or e-mail us
Want more information about blue-green algae? This fact sheet will help.
Did You Know?
Before the invention of refrigerators, people harvested ice from rivers and lakes in the winter and stored the ice, covered in sawdust, in buildings. An ice house, storing ice from the river, once stood near the site of the park headquarters for St. Croix NSR, in St. Croix Falls, Wisconsin.