Washington ? Most of the nation’s rivers and streams – and the fish in them – are contaminated with pesticides linked to cancer, birth defects and neurological disorders, but not at levels that can harm humans.

Pesticides were found in almost all U.S. rivers and streams between 1992 and 2001, according to a study released Friday by the U.S. Geological Survey, although most drinking water supplies haven’t been affected.

“While the use of pesticides has resulted in a wide range of benefits to control weeds, insects and other pests, including increased food production and reduction of insect-borne disease, their use also raises questions about possible effects on the environment, including water quality,” said Robert Hirsch, the USGS associate director for water.

Pesticides were seldom found at concentrations likely to affect people, and they were less common in groundwater. But they were found in most fish.

Most frequently detected in agricultural streams were three herbicides used mainly on farms: atrazine, metolachlor and cyanazine.

Three other herbicides used commonly in cities – simazine, prometon and tebuthiuron – showed up more often in urban streams.

The USGS report is based on an analysis of data from 51 major river basins and aquifer systems nationally. It can be found at water.usgs.gov/nawqa.