WATER SEEKS its own level and so, apparently, does uncertainty about how dirty Pittsburgh's waterways are. "People are always like, 'Is it safe to go in the river, will I get sick?'" reports Adam Amrhein, who mans the Kayak Pittsburgh rental stand beneath the Roberto Clemente Bridge. But even when Amrhein tells patrons there's a river-water advisory - thanks to sewage overflows during wet weather - nobody ever turns around and goes home.

Still, according to the health department Web site, all that means is river users should "minimize water contact," especially if they have weakened immune systems or open cuts or sores. Given the rivers' powers of dilution, some regard even that approach as needlessly cautious. "A healthy adult can swim in sewage-infested water," says John Lucadamo, program director for the non-profit Venture Outdoors.

Fish manage pretty well too - or at least much better than they used to. As recently as the mid-1960s, "The lower Monongahela didn't support any macrobiotic life," says local aquatic biologist Mike Koryak. But sewage treatment (dating from the late '50s), pollution controls, the decline of heavy industry and a drop in coal mining (and hence in acid-mine drainage) meant that by the time of a 1973 survey, says Koryak, the Mon welcomed 15 species of fish.