The Boise River is experiencing a significant bloom of didymo, a slimy algae commonly known as “rock snot,” prompting officials in Ada County to investigate the outbreak affecting multiple recreational areas. The Idaho Department of Environmental Quality and City of Boise have identified the whitish-brown algae at four key locations along the river, raising concerns for residents and visitors who use the waterway for swimming, floating, and fishing.
Didymo, scientifically named Didymosphenia geminata, is a non-toxic freshwater algae that adheres to rocks and creates thick mats of gelatinous material. The bloom has been detected at Ann Morrison float take-out, Barber Park, Barber Pool, and the Beaver Dick fishing access area near Highway 21 and Diversion Dam. While the algae itself poses no health risk, the accumulation creates an unpleasant texture and appearance that can detract from recreational experiences.
Low Water Flows Blamed for Algae Surge
Unusually low river flows may be the primary culprit behind the outbreak. The Boise River is currently running at 634 cubic feet per second—the lowest observed flow rate for this time of year in three decades. When water moves more slowly through the channel, conditions become more favorable for algae to colonize rocks and expand unchecked. The slower-moving water also allows the whitish-brown slime to break loose and drift downstream through the popular float section, spreading the algae to additional areas.
Protecting Other Waterways
Authorities are advising Ada County residents and visitors to take preventive steps to avoid transporting didymo to other Idaho waterways. The DEQ recommends thoroughly cleaning boats, swimwear, waders, footwear, and any other gear that contacts the river before visiting a different body of water. Didymo can attach to equipment and be inadvertently carried to uninfected lakes and streams, where it may establish new blooms.
The outbreak underscores the challenges facing river managers in the Treasure Valley as water availability becomes increasingly constrained during summer months. The Boise River, a cornerstone of recreation and community life in Ada County, depends on adequate flow to maintain ecological balance and recreational quality.
What Comes Next
Residents seeking updates on water conditions and algae monitoring should contact the Idaho Department of Environmental Quality or the City of Boise Parks and Recreation department. As summer continues and water flows potentially decrease further, officials may issue additional advisories regarding river use. In the meantime, anyone planning to float the Boise River or fish at designated access areas should be prepared to encounter algae and follow cleaning protocols before moving to other waterways.
For those looking for alternative Ada County activities during the hot summer months, Meridian’s Generations Plaza hosts free Wednesday concerts through August 12, offering a family-friendly entertainment option. Additionally, Ada County’s Expo Park is preparing for its October opening following international design recognition, providing another future recreation destination for local residents.