More than a decade of effort to clean up wastewater -- plus millions of dollars in construction and new technology -- have made the lower reach of the Minnesota River a much healthier place for aquatic life, state officials announced Monday.

Tests conducted in August, when the river was at its lowest point in years, showed that even under stressed conditions the water now has enough oxygen to support fish, mussels, plants and other life, according to officials from the Minnesota Pollution Control Agency (PCA).

It proves, they said, that long-term efforts to reduce the nutrient phosphorus, which suppresses oxygen in water, were a good investment.

"This happy discovery really emphasizes that environmental advances are long-term and the resources dedicated are worth it," said PCA Commissioner John Linc Stine.

If further testing shows the oxygen levels persist even at lower flows, then the last 20 miles of the river will no longer be considered "impaired" for low oxygen, a pollution designation it's held since 1992.

The Minnesota River, the state's second largest, cuts through the lower half of Minnesota and has long been one of its most polluted bodies of water. Partly, that's the nature of its geography -- it is a young river that has not yet cut down to bedrock and is full of sediment sloughing from its sandy banks -- but it's also been polluted by sewage, industrial effluent and runoff from feedlots and farm fields as it flows through the heart of Minnesota's agricultural region.

In the last 20 miles before it reaches the Mississippi, however, one of the river's primary problems has been phosphorus. The nutrient, coming from sewage and wastewater treatment plants in communities southwest of Minneapolis, can generate massive growth of algae, which can be toxic to people and animals. When the algae dies, the bacteria that eat it also consume dissolved oxygen in the water, making the water uninhabitable for other forms of life.

"It's like removing all the air from a room," said Glenn Skuta, manager of water quality for the PCA.

Phosphorous down by half

In 2004, the state embarked on a long-term effort encouraging treatment plants to reduce phosphorus discharges. Over time, the PCA gradually required 100 communities and industrial plants to find ways to reduce phosphorus. For some small communities, that meant installing new treatment systems. Some built holding ponds to control the timing of discharges. For others, like the Blue Lake treatment plant in Shakopee, which cleans 28 million gallons of water a day, it meant huge investments in technology. The plant uses bacteria to remove phosphorus, then sells it for fertilizer.

Today, the river has only half as much phosphorus as it did in the early 1990s.

"It's great to see results that are making a difference in the river," said Susan Haigh, chair of the Metropolitan Council, which operates the Blue Lake and six other treatment plants.

Most of the pollution from industry, treatment plants and storm water has been resolved, largely as a result of the 40-year-old federal Clean Water Act. The river is healthier than it has been in decades, and state wildlife officials say they are starting to see the return of sturgeon, muskies, paddlefish and other game fish along some of its stretches.

What next?

But much of the river's length is still significantly impaired by sediment, phosphorus and bacteria, largely because of intense agriculture that lines its banks -- an increasingly contentious problem because farmers and small agricultural operations are exempt from the Clean Water Act.

"The 800-pound gorilla is agricultural runoff," said Kris Sigford, a water quality expert with the Minnesota Center for Environmental Advocacy, a nonprofit environmental law firm.

The U.S. Department of Agriculture this year launched a new pilot program in Minnesota to test whether voluntary conservation incentives for farmers will make a difference in how they manage their land. At this point, Stine said, that's the state's strategy, too.

"It will remain to be seen if voluntary measures alone can do the job," he said. But the stakes downstream are big, he added.

"We are the source of all the water that leaves Minnesota," he said. "We are responsible for the runoff. People downstream, they look upstream."

Josephine Marcotty • 612-673-7394