MONROE (AP) — As Canada geese bank westward above the green and blue expanse of Muddy Creek Wetland, wood ducks, mallards, pintails and northern shovelers paddle on the shallow ponds below.

The 108 acres near Monroe used to be a rye grass farm and was a cattle ranch before that.

Now low soil berms hold rainwater in ponds and indigenous prairie grasses poke up through the mud.

It’s the work of two wildlife biologists, Chris Kiilsgaard and Jeff Reams who have pooled their money and experience to return this patch of Willamette Valley to its former function.

They’re avid environmentalists, but hope to make some money as well.

If done right, it could pay off in millions of dollars. Muddy Creek is part of Oregon’s growing bank of mitigation banks, wetlands that developers and land managers can buy into when they can’t avoid damaging wetlands themselves.

Regulated by Oregon’s Department of State Lands and the U.S. Army Corps of Engineers, mitigation banks work on the premise that landowners who benefit the public should be paid.

Muddy Creek is the 14th of the state’s wetland banks with another seven under review. They’re among about 500 across the country, said Dana Field, mitigation specialist at the Department of State Lands.

The banks are part of an emerging worldwide market that buys and sells environmental enhancements. In the United States it is a $3 billion a year enterprise, said Ricardo Bayon, who heads the Ecosystem Marketplace, which provides information and analysis on the trend.

These transactions recognize that nature provides not only goods such as lumber and coal that people are accustomed to paying for but services as well, Bayon said.

“All of nature’s services, which include clean air, clean water, carbon sequestration, climate regulation, all of that has never been part of the economic equation and we’ve been using it unsustainably,” Bayon said.

Companies that curb their emissions of greenhouse gases can get paid by companies that don’t. Landowners who enhance their property to create habitat for endangered species can profit in the same way.

While the rest of the world leads the United States in the buying and selling of carbon credits, the United States leads in wetlands mitigation banking, Bayon said.

Wetlands ease the impact of flooding, filter impurities from water and provide essential habitat to plants and animals.

Federal and state governments require developers and land managers who damage wetlands to either create replacements or pay someone else to do it.

It’s high risk, high reward, Ream said. A state evaluation has translated the potential value of their wetlands into 60 credits, which they can sell. So far they have permission to sell 4.9 credits.

Both once worked for the state Department of Fish & Wildlife. People without their technical knowledge have to hire consultants.

Among those who might buy credits: the Creswell School District, which has applied to fill wetlands for a soccer field and Walter Custom Homes for a housing development that includes wetlands.

Muddy Creek credits can only within its geographic region. What the credits are worth is partly driven by the marketplace. The West Eugene Wetlands, managed by the City of Eugene, gets $50,000 per credit these days, but credits in the Portland area go for as much as $165,000.

At the Eugene rate, he and Reams stand to make $3 million for their restoration work.

Rules require that funds be set aside to manage the wetlands in perpetuity. That means finding someone to manage the land once the credits are all sold. Groups such as the Nature Conservancy and other organizations have experience managing such projects, said Katherine McDonald, director of conservation for Nature Conservancy.