Changes in streamflow and water quality in selected nontidal sites in the Chesapeake Bay Basin, 1985-2003

Abstract

Water-quality and streamflow data from 33 sites in nontidal portions of the Chesapeake Bay Basin were analyzed to document annual nutrient and sediment loads and trends for 1985 through 2003 as part of an annual evaluation of water-quality conditions by the Chesapeake Bay Program. As part of this study, different trend tests and methodologies were evaluated for future use in assessment of the effectiveness of management actions in reducing nutrients and sediments to the Chesapeake Bay. Trends in streamflow were tested at multiple time scales (daily, monthly, seasonal, and annual), resulting in only one significant trend (annual flow for Choptank River near Greensboro, Md.). Data summaries for observed concentrations indicate higher ranges in total-nitrogen concentrations in the northern five major river basins in Pennsylvania, Maryland, and Virginia compared to the southern five basins in Virginia. Similar comparisons showed no distinct differences for total phosphorus. Flow-weighted concentration is useful in evaluating changes through time for the Susquehanna, Potomac, and James Rivers. Results indicate the Potomac River had the highest flow-weighted concentrations (2.5 milligrams per liter) for total nitrogen, and the Potomac and James Rivers averaged about the same (0.15 milligram per liter) for total-phosphorus concentrations. Flow-weighted concentrations were lowest in the Susquehanna River for phosphorus and sediment because of the trapping efficiency of three large reservoirs upstream from the sampling point. Annual loads were estimated by use of the U.S. Geological Surveys ESTIMATOR model. Annual nutrient and sediment loads in 2003 were the second highest total nitrogen, total phosphorus, and sediment loads for the River Input Monitoring sites since 1990. Trends in concentrations, when adjusted for flow, can be used as an indicator of human activity and management actions. The flow-adjusted trends indicated significant decreasing trends at approximately 55, 75, and 48 percent of the sites for total nitrogen, total phosphorus, and sediment, respectively. This suggests management actions are having some effect in reducing nutrients and sediments. Sampling protocols for the river inputs to the bay have targeted high flows. Because this sampling strategy creates the potential for bias in estimated loads and trends, calculations are limited to flow-adjusted loads and trends in this report.