The once-pristine headwaters of the Mississippi River are being threatened by steady population growth, more intensive farming and the loss of thousands of acres of wetlands and forests, a trend that has prompted an ambitious new plan to protect the river by preserving natural land in its watershed.

The idea is to stem the rising flow of nitrates, phosphorus, road salt and other pollutants found each year in the upper reaches of the great river, which provides drinking water for nearly half Minnesota's population.

The Nature Conservancy and Ecolab Inc., one of the state's largest employers, outlined the plan Thursday, warning that the Mississippi could otherwise become as polluted and unusable as the Minnesota River and some of the other murkiest waterways in the state. They are asking the state and local municipalities to protect and restore 208,000 acres of natural land within the headwaters at a cost of up to $600 million.

The study acknowledges it's a hefty price but estimates that if those key areas alone could be preserved — a fraction of the 13 million acre basin — it would save property owners and cities in central Minnesota over $130 million annually in drinking-water treatment costs, replacement of contaminated wells, lost tourism spending and reduced property values for lakeshore homes and other real estate.

If nothing is done, the study argues, taxpayers will eventually be on the hook for another multibillion-dollar cleanup project, like the one now focused on the Minnesota River.

Prevention is the more reasonable and cost effective way of dealing with a problem that will only get worse, said Doug Baker, CEO of Ecolab.

"We have a chance not to make the same mistake twice," Baker said in an interview Wednesday.

During the four-year period 2008 to 2012, Minnesota lost wetlands at the highest rate in the country and cut down forests at a rate that was second only to Georgia, according to a study from the University of Wisconsin.

Much of the land-use change occurred in the center of the state, where the Mississippi River forms. The result is more sewage, manure and fertilizers being produced and released in an area with fewer bogs and swamps to filter them, the study shows.

The region's loss of forested areas over the past decade was particularly "eye opening," said Doug Shaw, assistant state director for the Nature Conservancy.

"It's alarming because we know that's the heart of our water supply," Shaw said. "And for what that region means for wildlife — that's the start of the Mississippi flyway," a major migration route for North American birds.

To analyze the challenge, Ecolab and the Nature Conservancy hired consulting firm McKinsey & Co. The study draws on past research from the University of Minnesota and other schools around the Midwest as well as the Minnesota Pollution Control Agency and U.S. Department of Agriculture.

It estimated the loss of property values by studying the value of cabins and houses on lakes such as Red Rock Lake in Benton County, where toxic algae blooms have made the water unsafe for swimming. Lakefront houses in Minnesota lose a half percent of value for every foot of clarity lost in a lake, the study shows.

Baker said he believes the state should prioritize its water quality funding on prevention projects, before the much costlier efforts to restore waters that are already severely impaired.

Ecolab, a maker of cleaning and sanitizing chemicals and water filtration products, has partnered with the Nature Conservancy on other studies and water restoration efforts.

The state could use the conservation programs it already has in place to target the highest priority lands in the Mississippi headwaters, Baker said. The programs are all voluntary, where landowners agree to sell the development rights on a lot, but continue to own the land and pay taxes on it.

"It's a sensible plan," he said. "We're not trying to boil the ocean. We're just trying to target the areas that will make the biggest difference."