The first national assessment of the condition of wadeable streams in the conterminous USA

EPA and the States recently collaborated to conduct the first assessment of the condition of wadeable streams in the conterminous U.S. A common survey sampling design was used to select over 1,700 sites, which were then sampled following standard protocols. The design provided a statistically defensible approach for making inferences to all streams within the U.S. from the subsample selected. A suite of biological, chemical and physical indicators were selected to estimate both the biological quality and the relative ranking of a subset of stressors potentially impacting the biota. Less than 30% of length of wadeable streams in the conterminous U.S. were in least-disturbed condition based on a macroinvertebrate index of biological integrity, and almost 40% of stream length has lost more than 20% of expected taxa. Four chemical and four habitat indicators were also evaluated as potential stressors to macroinvertebrate quality. Total nitrogen, total phosphorus, riparian disturbance and excess fine sediments were the most widespread stressors, each of them exceeding thresholds of concern in over 25% of the stream length. A relative risk index showed that excess fine sediments were most strongly associated with poor biological condition, followed by nitrogen and phosphorus levels. EPA and the States plan to repeat this survey every 5 years rotating with surveys of lakes, rivers, wetlands, and coastal systems. The full wadeable stream report is available at www.epa.gov/owow/streamsurvey/