To celebrate their 24th wedding anniversary, Mike Link and Kate Crowley will go to the Lake Superior shoreline in Duluth on April 29 and start walking toward Wisconsin.

Then on into Michigan. Then on into Canada.

They'll keep walking, as close to the shoreline as Mother Nature and fence lines allow, planning to walk back into Duluth near Labor Day, ready to spend time with their grandchildren once again. If all goes well, Link, 64, and Crowley, 60, will be the oldest circumnavigators to have hiked around this broadest expanse of freshwater in the world.

Their effort, Full Circle Superior, is rooted in environmentalism, but they're just as fervent about inspiring aging Americans to move, move, move.

"Walking is the best exercise in the world," said Link, who fairly beams with every sentence. "We spend all this money on energy only to deprive ourselves of our own energy, our own fitness."

Move, move, move is also tied to dream, dream, dream.

The couple, from Willow River, Minn., believe that getting older shouldn't mean that our lives grow less adventurous, our aspirations less lofty. That philosophy explains how the idea of going full circle around Superior -- the 1,826 miles is akin to walking from Duluth to Miami -- came to be.

Several years ago, Link was contemplating retiring as director of the Audubon Center of the North Woods, where he'd worked since 1971. Crowley suggested that he literally walk away on his last day, striding across the grounds in Sandstone, Minn., to the highway, to the Willard Munger Bike Trail, and north to Willow River. Then the idea became: Why not walk to Duluth? Then: Why not around Lake Superior?

"When we have an idea, we have a hard time putting limits on it," Link said, laughing.

Full Circle Superior seeks to focus attention on the health of the lake that holds one-tenth of the world's fresh water, a statistic that ironically may work against it. "The lake is vulnerable because of its size," Link said. "People look at it and think, 'What could we do that could possibly harm it?' Large is sometimes the worst thing you can be."

They'll be recording data about vegetation, animals and geology six times daily, photographing each stream crossed and cataloguing invasive plant species. In doing so, they'll update the historical record and in some cases provide benchmark information for future research. The couple will be working with the Natural Resources Research Institute (NRRI) at the University of Minnesota Duluth; Michigan Tech University in Houghton, Mich.; Algoma University in Sault Ste. Marie, Ontario; SeaGrant and other research groups.

"I think it's really unique," said George Host, senior research assistant at the NRRI. "No one has gone around the lake in one season, collecting data and photographing in such an intensive period." For example, Link and Crowley will regularly take photos looking north, south, east and west from a single point.

"We've also asked them, when they cross streams, to note anything that could have an impact on water levels," Host said. "Many of these are trout streams where a drop in water level might impede fish migrations."

Data aside, he added, "there's just the spirit of the whole thing."

If any couple were destined to do this, it would be Link and Crowley. In addition to his 38 years with the Audubon Center, Link helped found the Friends of the Boundary Waters Wilderness and teaches at Hamline University's Masters of Environmental Education program. He's written 22 books about travel and nature. Crowley is a naturalist and writer who still works at the Audubon Center.

People have circled this greatest of the lakes in various ways. A lone man hiked it in 1985, as did relay teams from the Anishinabe and First Nation tribes in 2000. Others have kayaked, sailed and biked its shores. But perhaps no one has walked while hugging the shoreline as closely as the Full Circle plan calls for.

"As closely" is the operative term. Palisade cliffs line parts of the North Shore, but there are long stretches of public beaches along Michigan's southern coast. The pair have coordinated with Canadian authorities to deal with the remote nature of the Crown Lands along the northeastern corner of the lake.

"The good news is that there still are places on shore that you can't get to," Link said. An absence of roads and trails means that they'll have to kayak around part of that shoreline. They hope to cover 12 to 15 miles a day.

The environmental data will be input by Amanda Hakala, a naturalist driving the support vehicle. University of Alberta student Wei Hao has designed the databases, while environmental educator Todd Starling will coordinate the Canadian segment.

Along the way, Link and Crowley also hope to talk with people who live near Superior's shores, asking them to tell their stories, tapping "their hearts and minds," Link said. "You can really get a sense of place through people's stories."

Host said NRRI also is interested in how people regard their watery neighbor. "We'd like sort of a knowledge survey, learning what people understand about watersheds, especially storm water and its effects on erosion," he said. "What do people really know about that? That would be a neat data set to get."

Each day, Full Circle Superior will provide an online update at www.fullcirclesuperior.org. Lake Superior magazine will publish articles in its bimonthly issues, and a book is to follow.

Right now, Link and Crowley are raising money.

"We don't want to end up with a great hike and a big bill at the end, although we're going to do it anyway," Link said. "It's our life's dream."

Kim Ode • 612-673-7185