Half the lakes and rivers in southern Minnesota are too polluted much of the time for safe swimming and fishing, according to a new state survey that could intensify efforts to protect Minnesota's surface waters.

The finding emerged from a five-year assessment of Minnesota's watersheds by the Minnesota Pollution Control Agency (MPCA), which concluded that the problems are worsening and will require 20 to 30 years to address.

"This is further indication that our water challenges continue to increase," said Shannon Lott- hammer, who heads the MPCA's environmental analysis division.

Gov. Mark Dayton, appearing at the news conference where the study was released, said it underscores the need for water-quality legislation that now faces a tough battle at the Capitol. Dayton is fighting for a law that would require farmers to plant 50-foot-wide strips of natural vegetation between streams and their fields to slow the runoff of fertilizers implicated in the water problems in farm areas.

He said water quality policy should reflect science, not who can shout the loudest at the Legislature. "We can't allow it to get worse," Dayton said.

The state's leading farm groups have opposed Dayton's bill, arguing that their members have already taken steps to control farm runoff and that the new rules are excessive and confusing.

Lotthammer and MPCA Commissioner John Linc Stine said the findings also underscore the need to slow the flow of water from urban areas, where heavy drainage from storm sewers is speeding up streams and rivers and destabilizing their banks, creating sediment pollution.

Lotthammer said that in the past five years researchers also have seen an increase in subsurface drainage systems in southern Minnesota farm fields. Also known as "tiling," the systems carry excess water, sometimes loaded with nitrogen, phosphorus and pesticides, into watershed ditches and streams.

Stine said reducing the flow of pollutants now is better than cleaning up the water later.

"Prevention is the most affordable cure that we have," Stine said.

Wednesday's report marks the midpoint in a 10-year MPCA plan to study impairment in all of Minnesota's 81 watersheds. The report's broad strokes show mostly clean water in the northeast, emerging problems in the center of the state, and widespread degradation in the south and pockets of the northwest.

In many watersheds where farming dominates land use, just half or fewer of the lakes fully support swimming — largely because of excess phosphorus. The chemical, which comes from fertilizer and wastewater treatment plants, creates harmful, sometimes toxic, algae growth that makes lakes "green and slimy with algae" for at least part of the summer, the report said.

In the southeast, southwest and across a wide band stretching from the Twin Cities to extreme western Minnesota, less than 40 percent — and many times less than 20 percent — of streams and rivers fully support swimming and recreation, the report said. In those cases, the main problem is elevated levels of E. coli bacteria from human sewage and animal manure.

But trout streams in the southeast, while showing poor conditions for swimming, are better at supporting fish and other aquatic life, the report said.

Almost all the water bodies examined — 490 stream sections and 1,214 lakes — contain fish tainted by mercury.

In the Twin Cities, the agency studied 22 streams, finding that most were damaged by loss of habitat, low oxygen and excess nutrients from runoff.

The report provided a few close-ups, including a look into northern Crow Wing County, an area of intense interest for the Dayton administration because of rapid deforestation to accommodate potato fields and other row crops. Mayo Lake, located just south of Pequot Lakes, is part of the watershed and has been polluted by excess phosphorus. The report attributes the pollution to shoreland degradation, sewage leaking from private lakeshore septic systems and phosphorus runoff from hundreds of acres of upstream land.

An appendix says large losses of forest acres in watersheds can damage fish and other aquatic life. The environmental changes brought about by the conversion of pine plantations to potato farms prompted Department of Natural Resources Commissioner Tom Landwehr in February to temporarily halt the deforestation for a study of its environmental effects.

Steve Morse, executive director of the nonprofit Minnesota Environmental Partnership, said "megadoses of reality" are fueling a burst of discussion this year about water pollution in Minnesota. He credited state agencies for "putting the science out there" and leaders like Dayton for speaking openly about the problems.

But Minnesota still lags in solutions, he said.

"The situation is getting worse, not better," Morse said.

Tony Kennedy • 612-673-4213