The Charles River is of great recreational and ecological value to the Boston metropolitan region and the Commonwealth of Massachusetts. It is also the focus of the U.S. Environmental Protection Agency (USEPA) Region I, Clean Charles 2005 Task Force. The main goal of the Task Force is to make the Charles River 'fishable and swimmable' by the year 2005. Achieving 'fishable and swimmable' conditions will require continued progress in addressing a range of environmental conditions now degrading water quality, including the infiltration of saltwater from Boston Harbor into the freshwater Charles River.To better understand the pattern of saltwater intrusion, the U.S. Geological Survey (USGS), in cooperation with the U.S. Environmental Protection Agency (USEPA), Massachusetts Department of Environmental Management (MADEM), and New England Interstate Water Pollution Control Commission (NEIWPCC), collected data on the spatial distribution, temporal variability, and chemistry of the saltwater that entered the lower Charles River from June 1998 to July 1999. The purpose of this investigation is to extend and complement a regional-scale study of Charles River water quality conducted in 1996 (T. Faber, U.S. Environmental Protection Agency, written commun., 1997), and the ongoing water monitoring activities of the Massachusetts Water Resources Authority (MWRA) and the Charles River Watershed Association (CRWA). The data collected by this investigation supports the Clean Charles 2005 Task Force by providing detailed information concerning a major factor limiting 'fishable and swimmable' conditions in the lower Charles River. Finally, the study will be used to assist current planning efforts of the Metropolitan District Commission (MDC) to restore the historic parklands of the lower Charles River.The 'Basin' is the local term for the reach of the Charles River that begins at the Watertown Dam in Watertown, Mass., and extends about 8 mi through suburban and urban areas to Boston Harbor. Discharge to the harbor is controlled by the 'new' Charles River Dam in Boston (fig. 1). The Basin was created by construction of the 'old' Charles River Dam in 1908 to solve Boston's sanitary problems. Prior to the building of the old Charles River Dam, the lower Charles River was a tidal estuary in which the water levels rose and fell twice daily with the tidal cycle. Low tide would expose untreated sewage that was discharged directly into the river. Exposed sewage created noxious odors and served as a breeding ground for mosquitoes that caused sporadic epidemics of malaria and yellow fever (Jobin, 1998). Damming of the river interrupted the normal tidal cycle and flooded the estuary by creating a freshwater pool (the Basin) that had a constant water elevation of about 0.8 meters (m) above mean sea level. Flooding of the estuary initially improved sanitary conditions and the Basin became a source of enjoyment for the local population and the focus of a large waterfront park in Boston and Cambridge (Jobin, 1998).Although the infiltration of saltwater from the harbor into the Basin was anticipated when the old Charles River Dam was built, neither the magnitude nor the consequences of the infiltration was considered. By 1975, the Metropolitan District Commission (MDC) determined that harbor water covered about 80 percent of the river bottom within the Basin and composed about 50 percent of its depth. The MDC also concluded that fish kills and odors in the spring of 1975 were likely the result of the sulfide-rich saltwater mixing with the overlying freshwater (Metropolitan District Commission, 1975).Saltwater from Boston Harbor that enters the Basin is known as the 'salt wedge' because of the shape it assumes as it moves upstream. Freshwater discharge from upstream pushes against the intruding harbor water until the density differences cause stratification to occur; the freshwater then overrides the denser harbor water (Fischer and others, 1979). The depth from