A new scientific report released by the U.S. Environmental Protection Agency (EPA) found that 46% of the nation’s river and stream miles suffer from phosphorus and nitrogen pollution. This situation creates a poor habitat environment that is not conducive for supporting healthy populations of aquatic life. These conditions are caused largely by excess fertilizers, wastewater, runoff from urban areas, and other sources, according to EPA.

The National Rivers and Streams Assessment 2008–2009 Report is based on an unprecedented national scale sampling effort undertaken by the EPA and its state and tribal partners, which covered 1924 river and stream sites across the country, encompassing everything from large rivers near urban areas to tiny undisturbed creeks in national parks. The report provides information on the biological condition of the nation’s rivers and streams, the key stressors that affect them, and how the condition of small streams has changed since 2004.

The report found that more than two out of five river and stream miles have phosphorous and nitrogen levels that are too high, which can result in algal blooms, low levels of oxygen, and harm aquatic life.

Results from the report show that 28% of the nation’s river and stream length is in good biological condition, 25% is in fair condition and 46% is in poor condition.