The fish, smallmouth and largemouth bass, are naturally males but for some reason are developing immature eggs inside their sex organs. Their discovery at such widely spread sites, including one just upstream from the Woodrow Wilson Bridge, seems to show that the Potomac's problem with "intersex" fish extends far beyond the West Virginia stream where they were first found in 2003.

The cause of the abnormalities is unknown, but scientists suspect a class of waterborne contaminants that can confuse animals' growth and reproductive systems. These pollutants are poorly understood, however, leaving many observers with questions about what the problems in fish mean for the Potomac and the millions of people who take their tap water from it.