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MARK STEIL: On the Minnesota-South Dakota border, near the town of Marietta, a light south wind ruffles the water of a marshy-looking lake. Shore birds pick in the muddy beach for a snack. Bees search for nectar in the stands of grass and flowers along the water's edge. A close look at the thin strip of dried mud flats surrounding the water reveals what makes this lake different. Like powdered sugar on a brownie, the dark mud is dusted by a layer of white.

GOODMAN LARSON: It's the saltiest body of water in Minnesota.

MARK STEIL: Avid bird watcher and all-around outdoor enthusiast Goodman Larson has spent a lifetime studying Salt Lake. He was born in Marietta, though lives now in the Twin Cities. But he still owns a farm near the lake and treasures the times he can visit. Salt Lake is only 4 feet deep. And Larson says it frequently dries up in drought cycles.

GOODMAN LARSON: I know for a fact, on dry years, it looked like the place is burning, with the wind blowing the white salt off Salt Lake.

MARK STEIL: Brine water lakes become more common the further west you go in the US. This Minnesota example is one third as salty as the ocean and supports an assortment of life not usually found in the state. Salt-tolerant grasses grow along the water's edge, and in the lake itself, shrimp.

GOODMAN LARSON: I remember taking a plankton net out in the water. And a plankton net at the end has a test tube. And so they concentrate the plankton in that test tube. And it was pink from the brine shrimp that are found here.

MARK STEIL: Larson's son, Ken, volunteers to taste the water of Salt Lake.

KEN LARSON: It just tastes like very soft water without that harder mineral content.

MARK STEIL: The salt in Salt Lake is a natural byproduct of the alkaline soil surrounding it. Brad Olson works for the Minnesota Department of Natural Resources. He says runoff from nearby farm fields carries all sorts of things into the lake, including the salt trapped in the soil.

BRAD OLSON: This particular basin doesn't have a natural outlet. So whatever is coming in stays here. And the only water that we're losing is through evaporation and filtration down into the soil.

MARK STEIL: But only the water evaporates. Any minerals in it, like salt, are left behind. It's this centuries-old process of runoff and evaporation that has concentrated salt in the lake. For Goodman and Ken Larson, the best thing about the area are the birds attracted by the brine-water food-- shore birds like avocets and willets, all sorts of ducks, including the canvasback, gadwall, and shoveler. Canada geese are regular visitors. And in the prairie grasses surrounding the lake, the Larsons peer through a spotting scope, hoping for a surprise.

GOODMAN LARSON: I think it looks like a dickcissel to me.

KEN LARSON: That's what I thought.

GOODMAN LARSON: Yeah. A dickcissel is a Western bird that you will find in Minnesota. And there, it's flying.

MARK STEIL: The lake is well-known among Minnesota birders as a great place to see prairie and saltwater birds not seen elsewhere in the state. But Ken Larson says that's only part of the attraction.

KEN LARSON: It's extremely charming. It's extremely picturesque. It's great for birding. But it's so beautiful and so quiet out here.

MARK STEIL: The busiest time for visitors at the lake is probably spring, when bird watchers armed with binoculars and spotting scopes turn out to see what sort of rare treasures the annual migration brings. The birders even help pay for a wood observation deck on the east side of the lake, all the better to view a body of water that's one of a kind in the land of 10,000 lakes. Mark Steil, Minnesota Public Radio.