Elevated concentrations of nutrients are some of the most common stressors (contaminants) affecting rivers and streams throughout the United States. Problems associated with elevated nutrient concentrations in surface water are not new, but they are among the most persistent. According to the National Water Quality Inventory: 1996 Report to Congress by the U.S. Environmental Protection Agency (USEPA), 50 States, Tribes, and other jurisdictions surveyed water-quality conditions in 19 percent of the Nation's 3.6 million miles of rivers and streams and found overenrichment of nutrients to be the second-most common reason for impairment after the combined effects of suspended sediment and siltation (U.S. Environmental Protection Agency, 1996).