Prospectus for an NSF-funded Hydrologic Observatory: The Potomac River Basin and Western Shore Chesapeake Bay Drainage The Potomac River has a long heritage of research in the hydrologic sciences, which is anchored by a dense network of USGS stream gaging stations. Included is the Potomac River station at Point of Rocks, which has the longest gaging record in the United States. The Potomac River observing network has been enhanced by the Potomac NAWQA study. The USGS research heritage in the Potomac River basin includes the Shenandoah River study sites of John Hack, which transformed the study of fluvial and hillslope geomorphology; Luna Leopold’s Watts Branch study site; and the Potomac Estuary Study of the early 1980s. Hydrologic extremes in the Potomac River basin have played a high profile role in the development of water resources in the United States. The record flood peaks in the Potomac River in 1889 and 1936 were major stimuli for fresh approaches to solving the nation’s flood hazard problems. Extreme floods with nominal recurrence intervals of 100 to 500 years have continued to occur somewhere in the area covered by the Potomac and nearby watersheds of the central Appalachian region with a frequency of about once every 10 to 15 years, and in addition to associated local hazards these events have significant impacts on regional sediment yields, water quality, and living resources in receiving waters. The severe drought of the early 1960s crystallized the debate over water supply in the humid regions of the United States, and this debate remains of current importance with concern over potential impacts of climate change coupled with growing demand associated with population growth and urban sprawl. The water supply for the Federal Government and the Washington D.C. metropolitan region is tied to the largely unregulated flow of the Potomac River. The municipal demand for water supply has risen to the point that extended droughts, such as those in the early 1960s and early 1930s, would overtax the supplies of the unregulated Potomac River. Innovative water management and forecasting tools have been developed for the Potomac River basin and these developments have reduced the pressures for construction of major dams and reservoirs. Innovative management tools rely on solid scientific understanding of the hydrology of the basin and there are gaps in this understanding that are directly linked to reliability of water supply. Of particular importance is groundwater recharge and its control of late summer and fall baseflow. Mean annual precipitation in the Potomac River basin ranges from a minimum of approximately 750 mm in portions of the Valley and Ridge physiographic province to maxima of more than 1250 mm in high elevation regions along both the eastern margin of the Valley and Ridge province (the Blue Ridge) and western margin of the Blue Ridge (in the Appalachian Plateau). The Valley and Ridge exhibits an even more pronounced minimum in cloud-to-ground lightning strikes, illustrating the striking controls that complex terrain exerts on precipitation systems in the region. The heterogeneities in rainfall distribution associated with complex terrain are poorly understood and of fundamental importance for all aspects of the hydrologic cycle. Urbanization has resulted in striking heterogeneities in hydrologic response in the region. Anderson’s classic study of urbanization and its impacts on hydrologic response was carried out in the northern Virginia suburbs of Washington D. C. The Baltimore Ecosystem Study provides an invaluable regional resource for urban hydrology studies, as do other ongoing projects in the counties surrounding Washington and along the I-270 corridor. Considerable attention has focused on water quality in the Anacostia River and its links to urban hydrology, and significant resources are being invested in restoration plans for the Anacostia. The “sediment yield problem”, which concerns the declining trend in the volume of soil eroded on hillslopes compared with the sediment transported by the river out of the basin, has a long history in the Potomac. Major studies in the Potomac include Grace Brush’s investigations of sedimentation rates in the Potomac estuary and their links to centuries of changing agricultural practice, Robert Meade’s studies of the sediment yield problem of major East Coast rivers using old (19th century) and recent suspended sediment observations from the Potomac, M. Gordon Wolman’s study of the cycle of erosion associated with urbanization, and Wark and Keller’s study of the variation of soil erosion with land use. Because sediment has been identified as one of the two (?) most important pollutants affecting the Chesapeake Bay, and because it serves as a carrier for a variety of associated contaminants, there are broad implications associated with the changing relationships between upland erosion, hydrologic response to climate patterns, and remobilization of sediment from storage at intermediate locations in the landscape. The problem is central to assessing the role of non-point source pollution on receiving waters, like the Chesapeake Bay. A major algal bloom in the Potomac River, virtually within view of the Capital, illustrated the coupled hydrologic and biogeochemical controls of water quality. The 1983 algal bloom in the upper Potomac estuary was likely due to release of phosphorus from benthic sediments in the upper estuary. The coupled transport of sediment and nutrients and the complex cycling of nitrogen and phosphorus were implicated as major players in the algal bloom. The transport of sediment and nutrients is dominated by major flood events. It was estimated that flooding from Hurricane Fran in 1996 transported sediment and nutrients to the Potomac estuary exceeding the mean load for 10 years. Because there is a long history of hydrologic investigations as well as a remarkably dense network of monitoring stations for both basic research and assessment of societal needs within the nominated study area, we believe the Potomac River Basin and other nearby watersheds draining to the western shore of Chesapeake Bay offer a valuable set of targets for establishing an NSF-funded Hydrologic Observatory. All five of the CUAHSI science questions are addressed by current and prospective research agendas. The inherent heterogeneities imposed by natural landscape patterns spanning five physiographic provinces are overlain with the historical legacy of four centuries of human disturbance, including a wave of deforestation, agricultural land use, and land abandonment leading to reforestation contemporaneous with some of the most rapid expansion of urban and suburban land use in the United States. The availability of new tools for spatially and temporally intensive characterization of atmospheric phenomena and land-surface patterns is anticipated to help us in developing a new understanding of the driving phenomena over a variety of spatial and temporal scales, which are expected to lead to much-improved modeling and predictive capability.