Changes in Streamflow and Water Quality in Selected Nontidal Basins in the Chesapeake Bay Watershed, 1985-2004

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Abstract

As part of an annual evaluation of water-quality conditions by the Chesapeake Bay Program, water-quality and streamflow data from 32 sites in nontidal parts of the Chesapeake Bay watershed were analyzed to document annual nutrient and sediment trends for 1985 through 2004. This study also formalized different trend tests and methodologies used in assessing the effectiveness of man­agement actions in reducing nutrients and sediments to the Chesapeake Bay. Trends in streamflow were tested at multiple time scales (daily, seasonal, and annual), resulting in only one significant trend (annual-mean streamflow for the Choptank River near Greensboro, Md.). Total freshwater flow entering the bay for the July-August-September “summer” season 2004 was the highest ever estimated for that 3-month period (1937-2004). Observed (unbiased) concentration summaries indi­cate higher ranges in total-nitrogen concentrations in the northern major river basins, those in Pennsylvania, Maryland, and northern Virginia, compared to the more southern basins in Virginia. Almost half of the monitoring sites in the northern basins exhibited significant downward trends in total nitrogen with time. Comparisons with total phosphorus and sediment showed similar results to total nitrogen.

Monthly and annual loads were available for the River Input Monitoring Program sites from the U.S. Geological Survey. Although loads were significantly reduced from 2003, in 2004, the combined estimated total nitrogen loads were the third highest since 1990, whereas total phosphorus and sediment loads were the fifth highest. A flow-weighted concentration (FWC) is useful in evaluating changes through time. Combined annual mean total nitrogen FWC from the 9 River Input Monitoring Program sites indicated a downward tendency from 1985 through 1998 and an upward tendency since 2001. From 1990 to 2004, the mean concentrations of total nitrogen, total phosphorus, and sediment were 1.58, 0.085, and 51 milligrams per liter, respectively. Flow-weighted concentrations for phosphorus and sediment were lower in the Susquehanna River at Conowingo, Md., most likely due to the trapping efficiency of three large reservoirs upstream from the sampling point.

Trends in concentrations, not adjusted for flow, identified 10 statistically significant upward trends, and 50 statistically significant downward trends in concentration for the period 1985 through 2004. 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 downward trends at approx­imately 72, 81, and 43 percent of the sites for total nitrogen, total phosphorus, and sediment, respectively. This indicates that management actions are having some effect in reducing nutrients and sediments.