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That's because freshwater ecosystems are so closely linked to human activity. Industry and agriculture are concentrated alongside flowing waters, and sooner or later the residue of virtually everything we do winds up running down the nearest creek—if we haven't dried up the creek first. In the southwestern U.S., as in other arid parts of the world, wildlife must compete for water with a burgeoning human population. Neither the Rio Grande nor the mighty Colorado is more than a trickle at its mouth today.

But it is the American Southeast that stands out as a world center of freshwater-species diversity, especially the southern Appalachian Mountains. Carved up into countless hills and hollows that are aglimmer with springs, riffles, rapids, smooth glides, and pools, the highly eroded mountains provided the isolated niches in which freshwater creatures could evolve into a multitude of forms. They also escaped the Ice Age glaciers that bulldozed much of the continent farther north. The result: The Southeast holds the grandest array of freshwater mussels on Earth; North America's premier collection of freshwater snails, crayfish, and turtles; and nearly 700 of the approximately 1,000 species and subspecies of U.S. freshwater fish.

Like most freshwater fish, those of the Southeast tend to be small and subdued in coloring—for most of the year. If you dunk your head in during spring or summer, though, when the males assume breeding hues, you might think you were near a coral reef. Christmas darters look like swimming red-garlanded trees; holiday darters and lipstick darters are striped and flecked in turquoise and orange. Male lollypop darters have knobs along the top of their dorsal fin that swell large and bright yellow—presumably to mimic eggs and inspire females to lay some. Behaviors can be equally striking. Male madtoms—finger-length catfish with barbels extending like whiskers from around their mouths—take eggs into their mouths to clean them. Some male darters do that by fanning water over the eggs, which also supplies the eggs with oxygen. The Conasauga logperch, barely five inches long, uses its snout like a crowbar to flip pebbles in search of food.

With so many streams drowned beneath reservoirs or smothered by sediments from human activities or laden with harmful chemicals, nearly a third of the Southeast's fish are at risk of vanishing, many within a matter of years. CFI isn't the only outfit working to preserve them. The Tennessee Aquarium in Chattanooga, other private facilities, and state and federal wildlife agencies have efforts under way as well. It's mostly thankless work. A group of independent scientists, the Southeastern Fishes Council, put together a list they call the desperate dozen—"the 12 fish most likely to become extinct soon," said Anna George, chief research scientist at the Tennessee Aquarium. "The public has never heard of most of them."

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