INTRODUCTION

The Charles River is of great recreational and ecologicalvalue to the Boston metropolitan region and the Commonwealth of Massachusetts.It is also the focus of the U.S. Environmental Protection Agency (USEPA) RegionI, Clean Charles 2005 Task Force. The main goal of the Task Force is to make theCharles River "fishable and swimmable" by the year 2005. Achieving"fishable and swimmable" conditions will require continued progress inaddressing a range of environmental conditions now degrading water quality,including the infiltration of saltwater from Boston Harbor into the freshwaterCharles River.

To better understand the pattern of saltwater intrusion, theU.S. Geological Survey (USGS), in cooperation with the U.S. EnvironmentalProtection 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 chemistryof the saltwater that entered the lower Charles River from June 1998 to July1999. The purpose of this investigation is to extend and complement aregional-scale study of Charles River water quality conducted in 1996 (T. Faber,U.S. Environmental Protection Agency, written commun., 1997), and the ongoingwater monitoring activities of the Massachusetts Water Resources Authority (MWRA)and the Charles River Watershed Association (CRWA). The data collected by thisinvestigation supports the Clean Charles 2005 Task Force by providing detailedinformation concerning a major factor limiting "fishable and swimmable"conditions in the lower Charles River. Finally, the study will be used to assistcurrent planning efforts of the Metropolitan District Commission (MDC) torestore the historic parklands of the lower Charles River.

The "Basin" is the local term for the reach of theCharles River that begins at the Watertown Dam in Watertown, Mass., and extendsabout 8 mi through suburban and urban areas to Boston Harbor. Discharge to theharbor is controlled by the "new" Charles River Dam in Boston (fig.1). The Basin was created by construction of the "old" Charles RiverDam in 1908 to solve Boston's sanitary problems. Prior to the building of theold Charles River Dam, the lower Charles River was a tidal estuary in which thewater levels rose and fell twice daily with the tidal cycle. Low tide wouldexpose untreated sewage that was discharged directly into the river. Exposedsewage created noxious odors and served as a breeding ground for mosquitoes thatcaused sporadic epidemics of malaria and yellow fever (Jobin, 1998). Damming ofthe river interrupted the normal tidal cycle and flooded the estuary by creatinga freshwater pool (the Basin) that had a constant water elevation of about 0.8meters (m) above mean sea level. Flooding of the estuary initially improvedsanitary conditions and the Basin became a source of enjoyment for the localpopulation and the focus of a large waterfront park in Boston and Cambridge (Jobin,1998).

Although the infiltration of saltwater from the harbor intothe Basin was anticipated when the old Charles River Dam was built, neither themagnitude nor the consequences of the infiltration was considered. By 1975, theMetropolitan District Commission (MDC) determined that harbor water coveredabout 80 percent of the river bottom within the Basin and composed about 50percent of its depth. The MDC also concluded that fish kills and odors in thespring of 1975 were likely the result of the sulfide-rich saltwater mixing withthe overlying freshwater (Metropolitan District Commission, 1975).

Saltwater from Boston Harbor that enters the Basin is known asthe "salt wedge" because of the shape it assumes as it moves upstream.Freshwater discharge from upstream pushes against the intruding harbor wateruntil the density differences cause stratification to occur; the freshwater thenoverrides the denser harbor water (Fischer and others, 1979). The depth from theriver surface to the top of the salt wedge is directly related to the force ofthe upstream flow and inversely related to the difference in density between thefreshwater in the Basin and the harbor water. As the freshwater flows over theharbor water, shear stress develops at the freshwater-saltwater interface, whichcauses the upper surface of the salt wedge to form a slope that reflects thedynamic equilibrium between freshwater discharge and saltwater intrusion.

The "new" Charles River Dam was built in the late1970s as a replacement for the old dam. Features that were meant tosignificantly reduce saltwater intrusion of harbor water into the Basin wereincorporated in the design of the new dam, and reductions of harbor-waterintrusion by as much as 80 percent were predicted (Metropolitan DistrictCommission, 1975). Special features of the new dam included small locks withtight seals and two sluice gates. One of the sluice gates is 6 m below the watersurface and was designed to drain the salt wedge at low tide. Six large pumpscontrol the water level in the Basin during flooding, and could be used to pumpthe salt wedge out of the Basin. Each of these pumps is capable of pumping about40 cubic meters per second (m3/sec) of water from the Basin intoBoston Harbor.

Although the new dam was designed to reduce the infiltrationof harbor water into the Basin, its present-day operation results in largevolumes of harbor water entering into the Basin, particularly during times ofhigh-recreational boating use. When the gates of the locks are opened to allowboats into or out of the Basin, freshwater flows out of the Basin over the samevolume of harbor water infiltrating the Basin. It has been estimated that abouthalf of the volume of water in the lock enters the Basin during each lock cycle(a lock cycle is composed of two lock openings �one for a boat entering thebasin and one for a boat exiting the basin). There are three locks at the newdam: two small locks (61.0 m x 7.6 m) and one large lock (91.4 m x 12.2 m).Assuming that each lock is used for one-third of the lock cycles and given thatthe observed average height of water is 3.4 m in the small locks and 4.6 m inthe large lock, about 136.7 cubic meters (m3) of harbor water entersinto the Basin per lock cycle.

The presence of the salt wedge can have profound effects ongeochemical conditions within the Basin including: (1) increased salinity, (2)decreased oxygen concentrations, (3) increased production of hydrogen sulfide,(4) sequestration and immobilization of trace metals, and (5) increased internalloading of nutrients, such as phosphorus. All of these changes affect organismsthat live within or pass through the Basin, especially benthic organisms andfish.

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