A study conducted by The US Geological Survey found that many rivers in the US have high levels of fish with both male and female characteristics. The highest amount of feminized fish exists in the Mississippi river, where approximately 70 percent of male smallmouth bass have developed female traits. Possible causes for this phenomenon include hormone-disrupting chemicals that leak into the environment, such as estrogen from female contraceptives, pesticides, household cleaning compounds, and pharmaceuticals. In contrast, scientists did not find a single feminized fish in the Yukon river basin, which is located in Alaska.