Part 2: Superior - The greatest lake

Lake Superior practically insists on oceangoing metaphors. It's an inland sea, with a rockbound coast and a bottom dotted by shipwrecks. More than 40 percent of the nation's fresh surface water is in this lake, more than 10 percent of the world's. Drain the other Great Lakes and you could refill them from Superior, with enough left over for three extra Eries.

But despite such vastness, and in sharp contrast to the ocean's resilience, Superior and the other Great Lakes are remarkably vulnerable. All the rain on earth eventually reaches the sea, cleansing and replenishing. But Superior's watershed is a mere 49,000 square miles -- just half again its surface area.

Lake Winnipeg, for comparison, drains 386,000 square miles -- a territory 30 percent larger than the entire Great Lakes basin. Each year the basin is able to add a mere 1 percent to the Great Lakes' volume of 6 quadrillion gallons. About as much is thought to exit through evaporation and the St. Lawrence River. Hydrologists like to speculate on what portion of the lakes might be meltwater left by glaciers. Slow turnover means that whatever pollutants go into the Great Lakes stay put for a long time; this is hardly a place where "dilution is the solution to pollution."

Diverting water beyond the basin boundary would quickly exceed the replacement rate. So would accelerated evaporation caused by global warming. These things were not much on my mind a few weeks ago, sitting in a kayak in the Rossport Islands of Ontario, admiring the unbuilt shoreline, the straight-line horizon of water and sky, a fall of pink boulders under 40 feet of water clear as gin. Like their vastness, the beauty of these lakes works against our awareness that they're a virtually closed, very fragile system in slow but steady decline -- and that Superior, at the top of chain, is the most vulnerable of all.

RON MEADOR

ABOUT THIS SERIES

"With Water in Mind" is a long-term project by the Star Tribune's editorial page staff and online staff. With this Sunday's articles, we also offer online an engaging new photo gallery, complete with musical accompaniment, of historic water photos from the Minnesota Historical Society.

Coming topics will include:

• Agriculture and its effects on our rivers downstream to the Gulf of Mexico.

• Lake Pepin's pollution.

• Invasive species.

The stories that are part of this project and many additional features are collected at startribune.com/water.