Green Algae

Green algae coats rocks along the shore of the Gallatin River near Big Sky on Tuesday, Aug. 17, 2021.

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Neon-green algae has appeared along pockets of the Gallatin River for at least the fourth summer in a row, spurring concerns for juvenile fish populations and aquatic insects.

Upper Missouri Waterkeeper announced in a video update last week that algae was growing along the main stem of the Gallatin River from its confluence with the West Fork to the Deer Creek area.

Water quality specialists with the Montana Department of Environmental Quality on Wednesday visited the area to check for algae. They found moss — not algae — growing along the stretch of the Gallatin River identified by the nonprofit.

The agency experts did see algae growing in some pockets of the Gallatin River, mainly between the West Fork and Taylor Fork confluences, said Moira Davin, a spokesperson for the department. The algae was approaching nuisance levels in some areas.

“We can’t say yes or no — it was a full algae bloom,” Davin said. “We’re going back to do more testing …. Moss growing is not an indicator of water quality like algae would be.”

The stretch affected by algae this summer is shorter than in August, 2020, when the riverbeds along 22 miles of the Gallatin River and sections of the West Fork and Taylor Fork were caked in algae.

Cladophora — the non-toxic, filamentous algae that has reappeared in the Gallatin downstream of Big Sky every summer since 2018 — can impact aquatic life over time, according to Chace Bell, a DEQ water quality monitoring assessment specialist.

Water temperatures, air temperatures, water hardness, nutrient concentrations, streamflows, water clarity and sunlight all play a role in growth events. If the algal blooms happen often enough, the types of aquatic insects and the abundance of juvenile fish can change, Bell said.

Frequent blooms can cause a river system dominated by mayflies, caddisflies and stoneflies to become more scud-based, according to Michael Suplee, an aquatic biologist with DEQ. Over time, that causes the taxonomy of a river to shift.

Decaying filamentous algae disconnects with rocks in late summer, then travels downstream, piling in shallow areas, Suplee said. Juvenile fish tend to live in shallow spots along the shoreline. The decomposing algae absorbs the oxygen the young fish need.

Green Algae

Green algae coats rocks along the shore of the Gallatin River near Big Sky on Tuesday, Aug. 17, 2021.

So far, DEQ has not observed such changes in the Gallatin River, Davin said. Bell and Suplee couldn’t say with certainty how long it might take for changes to occur.

“It’s fair to say that over enough time and with enough density we could eventually see these kinds of impacts,” Bell said.

Kristin Gardner, executive director of the Gallatin River Task Force, said she saw nuisance algae growing along the Taylor Fork last Friday.

The task force has been monitoring water quality in and around the Gallatin River for almost two decades, collecting data to track nutrient levels, streamflow and aquatic insects.

Gardner has some theories about why algae this year hasn’t been as widespread, but she wants to wait until the data are processed.

“Once the field season is over we will take a step back and see what we have found,” she said.

Guy Alsentzer, executive director of Upper Missouri Waterkeeper, said smoke this summer could be limiting a widespread bloom by keeping some sunlight from reaching the riverbed. However, the algae is still indicative of a chronic, prolonged issue.

“The ‘new normal’ of the Gallatin River turning neon green downstream of Big Sky is not, in fact, normal,” Alsentzer said in a news release. “The Gallatin’s algal blooms are 100% preventable, if state agencies and local governments are willing to step up to protect it.”

DEQ requires three years of data to confirm that recurring algal blooms are ongoing and severe, Alsentzer said. That triggers a process where the agency can consider designating a river as impaired, which then allows the agency to create a pollution plan.

“After four consecutive years of algal blooms, science clearly indicates that the Gallatin is ‘impaired’ from the cumulative impacts of wastewater, and should be officially designated as such by the Department of Environmental Quality with a mandatory clean up plan,” he said in the release. “It’s time to take action, before it’s too late.”

There is sufficient data for DEQ to complete an assessment of the Gallatin River, Davin said. The agency would have to look at the data, go through various protocol and reach an outcome to declare the river impaired.

Upper Missouri Waterkeeper earlier this summer petitioned Montana Gov. Greg Gianforte to form a cold water fisheries task force so agencies can better coordinate when addressing issues within river systems.

“We need a task force to examine what’s working and what’s not working,” Alsentzer said. “We have the tools for addressing water pollution problems. Big Sky is one of the most affluent communities in the lower 48 states. There is no reason we shouldn’t scrutinize its efforts.”

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Helena Dore can be reached at or at 582-2628.

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