Transport of dissolved and suspended material by the Potomac River at Chain Bridge, at Washington, D.C., water years 1978-81

Abstract

The measuring station Potomac River at Chain Bridge at Washington, D.C., is located at the upstream end of the tidal Potomac River. Water-quality data were collected intensively at this site from December 1977 through September 1981 as part of a study of the tidal Potomac River and Estuary. Analysis of water-discharge data from the long-term gage at Little Falls, just up stream from Chain Bridge, shows that streamflow for the 1979-81 water years had characteristics similar to the 51-year average discharge (1931-81). Loads were computed for various forms of phosphorus and nitrogen, major cations and anions, silica, biochemical oxygen demand, chlorophyll a and pheophytin, and suspended sediment. Load duration curves for the 1979-81 water years show that 50 percent of the time, water passing Chain Bridge carried at least 28 metric tons per day of total nitrogen, 1.0 metric tons per day of total phosphorus, 70 metric tons per day of silica, and 270 metric tons per day of suspended sediment. No consistent seasonal change in constituent concentrations was observed; however, a seasonal trend in loads due to seasonal changes in runoff was noted. Some storm runoff events transported as much dissolved and suspended material as is transported during an entire lowflow year. INTRODUCTION Background The non-Federal Advisory Committee on Water Data for Public Use recommended that the U.S. Geological Survey conduct a pilot interdisciplinary estuarine study patterned after the successful U.S. Geological Survey RQA (river quality assessment) study of the Willamette River (Rickert and Hines, 1975). The U.S. Geological Survey responded to the committee's recommendation by starting in August 1977 an estuarine RQA; that is, the PES (Potomac Estuary Study) was initiated to investigate physical, chemical, and biological conditions in the tidal Potomac River and Estuary located in Washington, B.C., Maryland, and Virginia (fig. 1). The chemical and hydrodynamic phenomena that existed in tidal rivers and estuaries prompted the U.S. Geological Survey to augment the RQA study group with chemical, geochemical, geological, biological, and hydrodynamic research teams. The RQA data collection on the tidal Potomac River and Estuary was completed in September 1981. Four hydrologic data reports (Smith and Herndon, 1979, 1980a, 1980b, 1980c) contain data for longitudinal surveys of the tidal river and estuary conducted between August 1977 and September 1978; one hydrologic data report (Blanchard, 1983) contains data at Chain Bridge; three hydrologic data reports (Blanchard and Hahl, 1981; Blanchard, Coupe, and Woodward 1982; and Blanchard and Coupe, 1982) cover periodic and synoptic data collection at approximately 23 stations for the period October 1978 through September 198