Four-fifths of the cropland that butts up against the streams and rivers of southern Minnesota is missing at least some of the legally required natural borders that are the first step in safeguarding waters that flow to the Mississippi River, Lake Pepin and eventually the Gulf of Mexico, according to the first detailed mapping of the region's rivers.

Overall, the southern third of the state earns a "C" because most of the waterways have modest protections, according to the Environmental Working Group a national watchdog group that conducts scientific research to promote environmental action.

But a set of precise aerial maps compiled by the group also reveal widespread violations and large disparities from one watershed to another.

Those borders of wild grasses, trees or shrubs — 50-foot buffer strips required by state law — are nature's way of filtering agricultural pollutants such as nitrogen and phosphorus, while also providing critical refuge for birds, bees, turtles, frogs and other wildlife. They are considered the first step in conservation in an area of the state where row crops take up more than half the landscape.

Minnesota's rule has been in place for years and is one of the few such laws in the country. The aerial photo maps created by EWG specifically for Minnesota are the first comprehensive look at how well it's being implemented at a time when the state and the nation are becoming increasingly concerned about agriculture's impact on water.

The Environmental Protection Agency estimates that agricultural runoff degrades more than 125,000 miles of rivers and streams across the country. Minnesota is spending millions in state tax dollars in a watershed-by-watershed effort to make major reductions in agricultural pollution by 2025.

But enforcement of the state's buffer rule has long been a sore point with environmental groups. They argue that the state and county governments, which are charged with implementing it, rarely use one of the few but very effective regulatory tools they have to protect vulnerable streams and rivers.

"Laws do work," said Kris Sigford, a water quality specialist with the Minnesota Center for Environmental Advocacy, a nonprofit environmental law firm. The fact that most of the rivers are partly protected is evidence of that, she said, adding, "But they are not self-implementing."

'Useful' information

The state's top environmental officials, who reviewed the findings, said EWG's methodology and results are accurate and show that many places need attention. In particular, they said, EWG found that nearly half of the missing ­buffers are along the small streams that form the headwaters of Minnesota's great rivers.

"It's very useful," said John Linc Stine, Commissioner of the Pollution Control Agency. "We are going to use this to inform local governments."

Local officials said that many farmers plant healthy buffers voluntarily, while others are more reluctant. Planting buffer strips can mean a difficult choice to take productive land out of production, especially in recent years as corn and soybean prices have spiked, said Tom Muller, a farmer who serves on the ­Cottonwood County soil and water board.

County officials say attention to the issue is on the rise, but acknowledge that many county boards are simply unaware of the rule or are reluctant to create controversy in small agricultural communities. But they also say the report illustrates the scope of problems on the landscape that have been frustrating them for years.

"I live a half mile from Watonwan River," said David Bucklin, a technician for the Cottonwood County Soil and Water Conservation District in southwest Minnesota. Decades ago a 3-mile segment was straightened, and now giant farm equipment plows right up to its banks. "It looks more like a ditch," he said.

A simple barrier

Buffers, which have been around as long as farming itself, are part of Minnesota's Shoreland Protection Act, the law that lays out protections for its thousands of lakes and thousands of miles of rivers and streams. Environmentalists say the rules applying to farmers are the least explicit and most lenient; some lakes require up to 350 feet of protective natural vegetation, while farm buffer strips are only 50 feet.

Still, buffer strips are as functional as they are simple. A stand of prairie grasses, shrubs or trees creates a barrier that stops soil from running off fields into the streams, where it clouds the water, killing fish and some plant life. Much of that chemical-laden sediment then flows into the Minnesota River and eventually dumps into Lake Pepin. But more important, excess phosphorus finds its way into the water when it attaches to the soil. And phosphorus, a fertilizer that causes excess plant growth and toxic algal blooms, is one of the key pollutants in the Mississippi River that has created a dead zone in the Gulf of Mexico the size of ­Connecticut.

"It's very effective for phosphorus," said Matt Helmers, a soil scientist who studies buffer strips at Iowa State ­University.

Craig Cox, a vice president at EWG, said the buffer analysis was a natural because new mapping and photography technology makes it possible, and because Minnesota is the only state in the Midwest to have such a law.

"In the heart of the Corn Belt, I don't know of any other state that has comparable protection," he said. But such laws and their enforcement are critical "to strike a balance between what taxpayers pay for and what should be the basic responsibilities that go with the rights of land ownership," he said.

'I didn't even know'

Drawing on federal land-use data as well as high resolution aerial photographs from 2012 and 2013, EWG found that only 18 percent of the waterways adjacent to cropland earned an "A," meaning that 100 percent of the acres within 50 feet of the water were covered by natural vegetation. About a third had less than 70 percent. But there were major differences among counties, watersheds and even the same stream.

"It's really a checkerboard, and kind of inexplicable," Cox said.

Some county environmental officials disputed EWG's grading system. Julie Conrad, land use planner for Blue Earth County, where three major rivers merge into the Minnesota River, said EWG judged their water protection efforts purely on agricultural land, and not on land with other owners or uses. The county's mapping shows that when considered as a whole, their waterways are quite well protected, she said.

Cox said the study focused on farms "because that's where the pollution comes from."

One problem, county officials said, is that in many places, farmers, county and zoning officials don't even know the law exists.

"I didn't know about that ordinance, even though I sit on the soil and water board," said Muller, who helps run a family farm of about 2,600 acres of corn and soybeans in Cottonwood County. He rents some cropland that borders a "nice creek" that is planted with natural grasses as part of a federal conservation program.

But there is resistance as well, Muller added. "It's kind of a nuisance thing," he said. "A lot of farmers, especially if they are renters, they say, 'Let's get as much out of this [land] as we can.' "

In addition, enforcement is a difficult and politically fraught problem, said several county environmental officials. It requires the backing and funding of a county board, which usually includes farmers or retired farmers for members.

"If this is ever going to be taken on on a larger basis, it has to come down from the state," said Bucklin.

State environmental officials say that putting their weight behind the law requires a delicate balancing act.

"We've been careful not to push the counties over the edge of the cliff in their work," said Rob Collett, water resource manager for the Department of Natural Resources's southern region. "But I do think we could do a better job on seeking enforcement and follow-up."

When counties do take action, they can be very effective. Conrad said that in 2012, the county launched a new effort to increase protections "because our rivers are highly valued." They identified 336 landowners who need to install or improve their buffers, and so far 227 have started putting that land into conservation ­programs.

But now the hard part starts, she said — persuading landowners who are reluctant. "The backing of the county board is essential," she said. "And the county attorney's office has to be willing to prosecute it, too."

Josephine Marcotty • 612-673-7394