The Montana Department of Environmental Quality announced at an open-house meeting in Divide on Tuesday that it is working toward designating the Big Hole River as impaired because of excessive algae growth driven by nutrient pollution, a formal move that would acknowledge a long-running problem on one of the state’s most prized trout streams.

Andy Ulven, who leads DEQ’s water quality division, told Montana Free Press on Tuesday that a eutrophication listing would “formalize that there is an issue on the mainstem” of the Big Hole. He said the agency’s proposal is still in draft stage and that he does not anticipate a final designation for the river until 2027 at the earliest.

The term DEQ is using in its discussion of the Big Hole is eutrophication, which describes the link between algae growth and excess nutrients such as nitrogen and phosphorus. When algae proliferate in a river or lake, dissolved oxygen falls, imperiling fish and the aquatic life they feed on.

Algal growth is also unpopular with members of the recreating public because it can make wade-fishing a slipperier — and therefore more perilous — endeavor. It drives down biodiversity in the macroinvertebrate population, which has repercussions for fish and other species higher up the aquatic food web, DEQ said.

The Big Hole, a mellow freestone river, is critical to southwest Montana’s outdoor recreation and agriculture economies. Its designation as impaired would set in motion a regulatory process requiring the state to develop a plan to reduce nutrient loading — a step that could affect farms, ranches, and communities along the river’s length.

The DEQ’s announcement in Divide marks the latest chapter in a years-long debate over nuisance algae growth in the Big Hole, which has drawn attention from anglers, conservation groups, and agricultural water users. The impairment designation, once finalized, would impose a formal water-quality target that the river must meet, potentially triggering new restrictions on discharges and runoff.

Ulven said the agency’s proposal is still in draft form and that the public will have opportunities to comment before any final listing is adopted.