In Colorado's rivers and streams, what scientists have discovered in the white sucker fish worries even veteran scientists.
"I've done a lot of studies throughout my career which extends back to 1973," research associate John Woodling told NBC's Tom Costello. "This is the very first time that what I've found scared me."
"This fish has characteristics of both male and female," said Dr. David O. Norris of the University of Colorado, Boulder.
In three Colorado rivers, all downstream from sewage treatment plants, scientists have found many fish showing the effects of this unintentional hormone therapy.
In the Boulder Creek, female white suckers outnumbered males five to one. And 50 percent of the males also had female sex tissue.
Researchers believe the cause is too much estrogen, a natural female hormone passed through every sewer system. Adding to the problem is the fact that certain chemical compounds in detergents and soaps can mimic estrogen in the waterways.
Scientists believe that estrogen mimickers are caused by chemicals called nonylphenols found in paints, rubber, cosmetics and plastics. They are considered a possible cause of kidney, eye, liver and reproductive problems.
Banned in much of Europe and under review in Canada, nonylphenols are still common in America, where they flow out of sewage plants and into rivers across the country.
Recently, government researchers found natural estrogens and estrogen mimickers in 80 percent of the streams tested in 30 states.
"We would be ingesting those chemicals, would absorb them, and they would add to whatever natural hormones we already have in the body," explained Dr. Norris.
No one is certain what the long-term impact will be on humans.
Scientists now have begun researching what this means for the nation's drinking water.