Prepared in Cooperation with the UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WATER-RESOURCES INVESTIGATIONS REPORT 00-4124 U.S. DEPARTMENT OF THE INTERIOR DEPARTMENT OF ENVIRONMENTAL MANAGEMENT Breault, R.F., Barlow, L.K., Reisig, K.D. and Parker, G.W. U.S. GEOLOGICAL SURVEY NEW ENGLAND INTERSTATE WATER POLLUTION CONTROL COMMISSION Spatial distribution, temporal variability, and chemistry of the salt wedge in the Lower , Massachusetts 226,000 228,000 230,000 232,000 234,000 236,000

250,000 50,000 150,000 EXPLANATION INDEX MAPS MASSACHUSETTS 220,000 900,000 BATHYMETRY 910,000

230,000 0 50 MILES 800,000 0 to 1.9 meters 0 50 KILOMETERS 900,000 2 to 3.9 meters

210,000 4 to 5.9 meters 890,000

200,000 GRIDS SHOWN ARE METRIC 6 to 7.9 meters 230,000 235,000 880,000 10.5-C 8 to 9.9 meters

900,000 So 870,000 ld 10 to 11.9 meters 10.9-B ie r s

F

ii el Harvard University e Harvard University Eliot Bridge Eliot Bridge Herter East l LOWER CHARLES RIVER d 12 meters and over d CHARLES RIVER DRAINAGE BASIN STUDY AREA Park 1.5-B RR

oa 1.9-B o Commuter Rail Draw Bridge 1.0-C a Old 1.7-C 1.0-B 5.2-C d 2.0-B 0.0-C STATION AND IDENTIFIER 10.0-B 2.9-G 2.0-C 1.7-B 0.0-A 11.5-B 9.5-B Museum of Science 2.5-C 1.9-A 0.0-B 1.5-A 1.0-A1 8.9-B 2.5-B 2.0-A eeks Bridge 1.7-A 1.0-A Anderson Bridge 3.0-D 2.9-B 902,000 2.5-A 3.0-C New Charles River Dam Harvard Stadium Larz Anderson Bridge John WWeeks Bridge 3.0-B 2.9-A and Locks

3.2-A 3.0-A 3.5-C Watertown Dam Watertown Dam 3.5-B 3.7-C 3.5-A WWesternestern AAvenuevenue Bridge 3.7-B

3.7-A 3.9-C 3.9-B2 3.9-B 3.9-A Arsenal Street Bridge

4.0-B Beacon Street Bridge River Street Bridge 4.1-B 4.1-A2

6.9-B Massachusetts Institute 4.2-A of TTechnologyechnology 4.2-A1 Community Daly Field Boating Community Riverside Rowing Center urnpike Memorial Drive 4.5-B Massachusetts TTurnpike Boat Club 4.8-C Hatch 4.5-A Shell Massachusetts Institute of TTechnologyechnology 4.5-A2 Sailing Pavilion 4.9-C 5.1-C 5.2-C 5.1-B2 4.8-B 4.6-A 5.1-B 4.9-B University 4.7-A Esplanade 6.2-B Boathouse 5.2-B PondsPonds 5.5-B 4.9-A

5.9-B

Storrow Drive

Massachusetts Avenue

BostonBoston University University Bridge Bridge 900,000

A venue Massachusetts TTurnpikeurnpike

Fenway Park

Orthophoto map base by MassGIS, state plane coordinates, 2 km grid, NAD83 0 .5 1 1.5 2 MILES Figure 1. Water-quality sampling locations and bathymetry of the Lower Charles River, Massachusetts. 0 0.5 1 1.5 2 KILOMETERS

INTRODUCTION Salinity values between water-quality sampling sites were The Charles River is of great recreational and ecological estimated using GRID commands (a module of ARC/INFO) June 19, 1998 June 24, 1998 July 1, 1998 July 15, 1998 August 19, 1998 October 7, 1998 value to the Boston metropolitan region and the within the AML. Volumes of the salt wedge and salt mass 235,000 236,000 235,000 236,000 235,000 236,000 235,000 236,000 234,000 235,000 236,000 234,000235,000 236,000 Commonwealth of Massachusetts. It is also the focus of the were calculated with the AML for 1-m depth intervals. Total U.S. Environmental Protection Agency (USEPA) Region I, volume of the salt wedge and mass of salt in the Basin were 902,000 902,000 Clean Charles 2005 Task Force. The main goal of the Task calculated as the sum of the volumes and salt mass calculated Force is to make the Charles River “fishable and swimmable” for each 1 m of the water column. by the year 2005. Achieving “fishable and swimmable” SPATIAL DISTRIBUTION AND TEMPORAL conditions will require continued progress in addressing a VARIABILITY OF THE SALT WEDGE 902,000 902,000 902,000 902,000 range of environmental conditions now degrading water quality, including the infiltration of saltwater from Boston Because the density of the harbor water causes it to sink to Harbor into the freshwater Charles River. the bottom of the channel, its spatial distribution is dependent 901,000 901,000 To better understand the pattern of saltwater intrusion, the not only on the amount of harbor water infiltrating into and U.S. Geological Survey (USGS), in cooperation with the U.S. discharging from the Basin, but also upon the channel Environmental Protection Agency (USEPA), Massachusetts morphology of the Basin. The present-day channel morphology of the Basin is similar to that of the original tidal Department of Environmental Management (MADEM), and 0 2,000 FEET 0 2,000 FEET 0 4,000 FEET channel and associated mudflats surveyed in detail prior to 0 2,000 FEET 0 2,000 FEET 0 4,000 FEET 0 500 METERS 0 New England Interstate Water Pollution Control Commission 0 500 METERS 0 500 METERS 0 500 METERS 0 1,000 METERS 1,000 METERS (NEIWPCC), collected data on the spatial distribution, construction of the old dam (Pritchett and others, 1901); however, the bathymetry of the Basin surveyed in 1901 has 0 0 0 0 0 0 temporal variability, and chemistry of the saltwater that Longfellow Longfellow Longfellow Longfellow Harvard Longfellow Museum of Harvard Longfellow Museum of New Charles 2 Museum of New Charles 2 Museum of New Charles 2 Museum of New Charles 2 Museum of New Charles 2 New Charles 2 Bridge Bridge Science been slightly modified by sediment accumulation and man- Bridge Science River Dam Bridge Bridge Bridge Bridge Bridge Science River Dam River Dam entered the lower Charles River from June 1998 to July 1999. 4 4 Science River Dam 4 Science River Dam 4 Science River Dam 4 4 The purpose of this investigation is to extend and comple- made holes dug during the last 97 years. The denser harbor 6 6 6 6 6 6 water that infiltrates the Basin generally collects in these 8 8 8 8 8 8 ment a regional-scale study of Charles River water quality 10 10 10 10 10 10 1,400 1,200 1,000 800 600 400 200 0 1,400 1,200 1,000 800 600 400 200 0 1,400 1,200 1,000 800 600 400 200 0 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 DEPTH, IN METERS DEPTH, IN METERS 1,400 1,200 1,000 800 600 400 200 0

DEPTH, IN METERS 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 DEPTH, IN METERS holes (fig. 1). DEPTH, IN METERS DEPTH, IN METERS conducted in 1996 (T. Faber, U.S. Environmental Protection DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS Agency, written commun., 1997), and the ongoing water Other important morphological features of the Basin that monitoring activities of the Massachusetts Water Resources affect the distribution of the salt wedge are berms beneath the Authority (MWRA) and the Charles River Watershed Longfellow and Harvard Bridges (fig. 1). These berms are only about 4.6 m below the water surface at some spots, and November 17, 1998 December 29, 1998 February 10, 1999 March 17, 1999 April 28, 1999 Association (CRWA). The data collected by this investiga- 234,000235,000 236,000 234,000235,000 236,000 234,000 235,000 236,000 234,000 235,000 236,000 235,000 236,000 tion supports the Clean Charles 2005 Task Force by provid- they deter the movement of the salt wedge upstream, because ing detailed information concerning a major factor limiting the salt wedge must fill the downstream part of the Basin to a The average daily mean discharge from the Basin into “fishable and swimmable” conditions in the lower Charles level greater than the height of the berm before it can spill over the next 49 days (August 19 through 902,000 902,000 902,000 902,000 River. Finally, the study will be used to assist current plan- over the berm and move upstream. October 7) was the lowest average discharge measured in ning efforts of the Metropolitan District Commission (MDC) Several other factors control the distribution and temporal 1998 (5.7 m3/s) and, although there were fewer than 25 lock variability of the salt wedge, including freshwater discharge cycles per day, by October 7 the salt wedge had advanced to to restore the historic parklands of the lower Charles River. 902,000 The “Basin” is the local term for the reach of the Charles of the Charles River; number of lock cycles; pumping; and just downstream of Harvard Bridge, and reached its River that begins at the Watertown Dam in Watertown, Mass., opening and closing of the lock gates, culverts, sluice gates, maximum volume (2.6 x 106 m3) for 1998. In addition, and the fish ladder. These factors vary seasonally and with the much of the area occupied by the salt wedge below a depth and extends about 8 mi through suburban and urban areas to 901,000 901,000 901,000 901,000 Boston Harbor. Discharge to the harbor is controlled by the weather. For example, during the summer months boat traffic of 6 m had become anoxic. “new” Charles River Dam in Boston (fig. 1). The Basin was is heavy. The increased boat traffic allows more harbor water The average daily mean discharge between October 7, created by construction of the “old” Charles River Dam in to infiltrate into the Basin through opening of the locks than 1998 and November 17, 1998, more than doubled from 3.3 1908 to solve Boston’s sanitary problems. Prior to the in the winter, when the locks are seldom used. Also, m3/s to 8.5 m3/s; however, even the force of this increased 0 4,000 FEET building of the old Charles River Dam, the lower Charles freshwater discharge from upstream is generally lowest during flow was not sufficient to flush the salt wedge out of the 0 4,000 FEET 0 4,000 FEET 0 4,000 FEET 0 2,000 FEET River was a tidal estuary in which the water levels rose and the summer and highest in the winter and spring. The Basin because of the large amounts of harbor water that 0 1,000 METERS 0 1,000 METERS 0 1,000 METERS 0 1,000 METERS 0 500 METERS 6 3 fell twice daily with the tidal cycle. Low tide would expose increased discharge in the winter and spring increases the entered the Basin (0.009 x 10 m /day) during lock cycles. 0 0 0 0 0 Harvard Longfellow Museum of New Charles Harvard Museum of New Charles Harvard Longfellow Museum of New Charles Harvard Longfellow Museum of New Charles Longfellow Museum of 2 Bridge River Dam 2 Longfellow 2 Bridge 2 Bridge River Dam 2 New Charles untreated sewage that was discharged directly into the river. resistance of freshwater to harbor-water movement upstream Consequently, the net salt mass in the Basin during this Bridge Science Bridge Science River Dam Bridge Science River Dam Bridge Science Bridge Science River Dam 4 4 Bridge 4 4 4 6 6 Exposed sewage created noxious odors and served as a into the Basin. Finally, lock gates, culverts, and sluice gates period increased from 12 x 10 kg to 15 x 10 kg, which was 6 6 6 6 6 also are open more frequently during times of increased 8 8 8 8 8 breeding ground for mosquitoes that caused sporadic the maximum salt mass measured in the Basin for 1998. It 10 10 10 10 10

5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 DEPTH, IN METERS 5,000 4,500 4,000 3,500 3,000 25,00 2,000 1,500 1,000 500 0 DEPTH, IN METERS 1,4001,200 1,000 800 600 400 200 0 DEPTH, IN METERS 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0

discharge to prevent flooding. DEPTH, IN METERS epidemics of malaria and yellow fever (Jobin, 1998). is important to note that the spatial extent of the salt wedge DEPTH, IN METERS Damming of the river interrupted the normal tidal cycle and In addition to seasonal effects, storms can occur at any time and the salt mass in the Basin in 1998 were probably DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS flooded the estuary by creating a freshwater pool (the Basin) during the year, resulting in rapidly increasing freshwater considerably less than average because of the flushing of that had a constant water elevation of about 0.8 meters (m) discharge due to the large amount of impervious surface in the the salt wedge by runoff from the June storm. above mean sea level. Flooding of the estuary initially watershed. In order to prevent flooding during storms, the Between November 17, 1998, and March 17, 1999, lock May 6, 1999 June 4, 1999 June 18, 1999 June 23, 1999 dam operator will use pumps or open the lock gates, or both, 235,000 236,000 235,000 236,000 234,000 235,000 236,000 234,000 235,000 236,000 improved sanitary conditions and the Basin became a source cycles averaged fewer than two per day, resulting in very salinity of the salt wedge depend upon the natural forces to move water from the Basin into Boston Harbor. Pumping at of enjoyment for the local population and the focus of a large little harbor water infiltrating into the Basin. During the driving the flow of water through the Basin and anthro- waterfront park in Boston and Cambridge (Jobin, 1998). depth and opening the lock gates at low tide, in combination same period, the daily mean discharge of the Charles River pogenic influences on that flow. Natural factors include river 902,000 902,000 with the increased discharge from upstream, can flush the salt Although the infiltration of saltwater from the harbor into generally increased. As a result, the upstream advance of the discharge, precipitation, and stormwater discharges to the wedge completely out of the Basin. Tables 1 and 2 list the the Basin was anticipated when the old Charles River Dam salt wedge ceased, and the salt mass in the wedge declined. Basin. Anthropogenic influences include the operation of the summary statistics for some of these factors recorded during was built, neither the magnitude nor the consequences of the The decline in salt mass was probably due to entrainment dam at the mouth of the Basin (including lock cycles, the study period. 902,000 902,000 infiltration was considered. By 1975, the Metropolitan and export of saltwater caused by increased freshwater flow, pumping, and opening and closing of the lock gates, District Commission (MDC) determined that harbor water Just prior to the first water-quality survey of this study, the in conjunction with reduced salt input from Boston Harbor culverts, sluice gates, and the fish ladder). covered about 80 percent of the river bottom within the Basin Charles River drainage basin received more than 20 cm of during this period. Although the daily mean discharge A simple conceptual model that explains the importance 901,000 901,000 and composed about 50 percent of its depth. The MDC also rain as the result of an intense, slow-moving storm. This generally decreased from March 17 to May 6, 1999, the salt of these factors on saltwater intrusion into the Basin was concluded that fish kills and odors in the spring of 1975 were storm, which occurred June 12 to 15, 1998, resulted in a peak mass in the Basin continued to decline, again likely due to 3 developed and tested by using multiple regression analysis. likely the result of the sulfide-rich saltwater mixing with the flow of more than 62.0 m /s at USGS gaging station the low number of lock cycles (about 5 lock cycles per day). The dependent variable in this model was the change in salt overlying freshwater (Metropolitan District Commission, 01104500 on the Charles River at Waltham, Mass., and May 26, 1999, was the last day of measurable rain in the mass in the Basin between water-quality surveys at time 1 0 2,000 FEET 1975). equalled the 13-year flood for this station. A flow of this Boston area for 31 days and was the beginning of a period (t ) and time 2 (t ). The independent variables were the 0 2,000 FEET 0 4,000 FEET 0 4,000 FEET 1 2 0 500 METERS Saltwater from Boston Harbor that enters the Basin is magnitude at this station has an estimated recurrence interval of drought (R.S. Socolow, U.S. Geological Survey, oral number of lock cycles, the volume of water pumped from 0 500 METERS 0 1,000 METERS 0 1,000 METERS known as the “salt wedge” because of the shape it assumes as of 13 years (Parker and others, 1998), which means that the commun., 1999). The daily mean discharge of the Charles the Basin (in cubic meters), a variable (in square meters per 0 0 0 0 Harvard Harvard Longfellow Museum of New Charles it moves upstream. Freshwater discharge from upstream flow would be expected to be equaled or exceeded once in 3 3 2 Longfellow Museum of 2 Longfellow Museum of 2 Longfellow Museum of New Charles 2 River fell from 15 m /s on May 26 to less than 1.6 m /s by hour) related to the area and the time that the fish ladder, Bridge New Charles New Charles Bridge Science River Dam Bridge Bridge Science River Dam pushes against the intruding harbor water until the density 13 years. In addition to water discharged into the Basin from 4 Science River Dam 4 Bridge Science River Dam 4 Bridge 4 June 26. Freshwater flow in the river offered little resistance lock gates, lock culverts, and sluice gates were open; and the 6 6 6 6 differences cause stratification to occur; the freshwater then upstream, it is likely that discharge from numerous storm to the advance of the salt wedge upstream. By June 4, most 8 8 8 8 average daily mean discharge (in cubic meters per second) 10 10 10 10

overrides the denser harbor water (Fischer and others, 1979). drains and combined sewer overflows during intense storm 1,400 1,200 1,000 800 600 400 200 0 DEPTH, IN METERS 1,400 1,200 1,000 800 600 400 200 0 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 DEPTH, IN METERS 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 DEPTH, IN METERS of the Basin downstream of the Longfellow Bridge at between t and t , which was a surrogate for the force of the DEPTH, IN METERS events, doubles the volume of water entering the Basin (M. 1 2 The depth from the river surface to the top of the salt wedge depths greater than 5 m was occupied by the salt wedge, freshwater discharge (table 2). The force of upstream DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS is directly related to the force of the upstream flow and Roberts, Charles River Watershed Association, oral commun., and the salt mass in the Basin increased by 0.48 x 106 kg. freshwater is represented by the velocity head (E) 1998). During the June storm, the dam operator at the new inversely related to the difference in density between the Water-quality measurements recorded on June 4, 1999, component of the specific energy equation for a channel dam pumped about 19.0 x 106 m3 of water out of the Basin freshwater in the Basin and the harbor water. As the between the Museum of Science and Longfellow Bridge with a small slope and is directly related to the upstream June 29, 1999 July 6, 1999 July 19, 1999 and opened the locks to allow water to flow out of the Basin 234,000 235,000 236,000 234,000 235,000 236,000 232,000 233,000 234,000 235,000 236,000 freshwater flows over the harbor water, shear stress develops showed that most of the water column at depths greater than discharge (Q) by (Chow, 1958) into Boston Harbor, in order to prevent flooding. The REFERENCES CITED at the freshwater-saltwater interface, which causes the upper 6 m had become anoxic. It is likely that the water in the area surface of the salt wedge to form a slope that reflects the combined effect of the high discharge from upstream, the E = Q2 / 2gA2 , (1) between the new dam and the Museum of Science does not 902,000 pumping of water at depth out of the Basin, and the opening Cole, G.A., 1975, Textbook of limnology: Saint Louis, The 902,000 dynamic equilibrium between freshwater discharge and become anoxic because this area is frequently replenished where of the locks appears to have completely flushed the salt wedge C.V. Mosby Company, 283 p. saltwater intrusion. with oxygenated harbor water. In addition, the water also Q is discharge, in cubic meters per second and, 902,000 out of the Basin by June 15. The “new” Charles River Dam was built in the late 1970s became anoxic at depths greater than 6 m upstream of the Chow, V.T., 1958, Open-channel hydraulics: New York, as a replacement for the old dam. Features that were meant to During the four-day period following this storm, the salt Longfellow Bridge due to thermal stratification and g is acceleration due to gravity, in meters per McGraw-Hill Book Company, 680 p. significantly reduce saltwater intrusion of harbor water into wedge formed again in the Basin. By June 19, the wedge was hypolimnetic oxygen demand during this period. Decay of second the Basin were incorporated in the design of the new dam, detected at a depth of 6 m at the new dam locks, and at a Environmental Systems Research Institute, Inc., 1997, organic matter rapidly consumes dissolved oxygen under 901,000 A is cross-sectional area of the lock at the old dam, ARC/INFO (version 7.1.1): Redlands, Calif. 901,000 901,000 and reductions of harbor-water intrusion by as much as 80 depth of 8 m below the commuter rail draw bridge (fig. 1). conditions of thermal stratification; however, photosynthetic percent were predicted (Metropolitan District Commission, The salt wedge advanced despite relatively high average daily in square meters. dissolved oxygen production by phytoplankton in the Fischer, H.B., List, E.J., Koh, R.C.Y., Imberger, J., Brooks, 1975). Special features of the new dam included small locks mean discharge (estimated at 60 m3/s; table 1) and continual Two variables, number of lock cycles (LC) and average 6 epilimnion likely prevents the entire water column from N.H., 1979, Mixing in inland and coastal waters: New with tight seals and two sluice gates. One of the sluice gates pumping at the new dam (at an average rate of 5.5 x 10 becoming anoxic. mean daily discharge (Q), accounted for 63 percent of the 3 ∆ York, Academic Press, 483 p. 900,000 0 8,000 FEET is 6 m below the water surface and was designed to drain the m /day; table 2). During the next five days (June 20–24), daily variance of the change in salt mass ( SM) in the Basin and 0 4,000 FEET 0 4,000 FEET 3 By June 18, 1999, the salt wedge filled the Basin salt wedge at low tide. Six large pumps control the water mean discharge averaged 49 m /s and pumping at the new were significant at the 90th percentile (p = 0.1). The other 0 2,000 METERS downstream of Longfellow Bridge to the extent that it Hydrolab, 1996, Data Sond 4 and minisond water quality 0 1,000 METERS 0 1,000 METERS level in the Basin during flooding, and could be used to pump dam averaged 3.3 x 106 m3/day. Nevertheless, the salt wedge independent variables accounted for only a small amount of multiprobe user’s manual revision B: Austin, Tex., spilled over the berm beneath the bridge and into the first 0 0 0 Harvard Museum of Science New Charles Harvard Museum of Science Bridge Harvard Museum of Science the salt wedge out of the Basin. Each of these pumps is continued to move upstream and began to fill the area just the variance and were not significant (p > 0.1). The number Hydrolab Corporation, variously paginated. 2 2 New Charles 2 New Charles deep hole located upstream of the bridge (fig. 2). By June Bridge Longfellow Bridge River Dam Bridge Longfellow Bridge River Dam Bridge Longfellow Bridge River Dam capable of pumping about 40 cubic meters per second upstream of the old dam near the Museum of Science. The of lock cycles was directly related to the change in salt mass 4 4 4 6 6 6 3 18, the thermal stratification of the Basin was apparently (m /sec) of water from the Basin into Boston Harbor. lock at the old dam, which is always left open, is an important in the Basin; as the number of lock cycles increases, salt Ingmanson, D.E., and Wallace, W.J, 1989, Oceanography, 8 8 8 disrupted, probably due to wind, and much of the Basin an introduction: Belmont, Calif., Wadsworth Publishing 10 10 10 Although the new dam was designed to reduce the control point for the movement of the salt wedge upstream mass in the Basin increases. In contrast, the average daily 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 0 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 DEPTH, IN METERS upstream of Longfellow Bridge was aerobic, except for the DEPTH, IN METERS infiltration of harbor water into the Basin, its present-day because the salt wedge must pass through the lock, and most mean discharge was inversely related to change in salt mass Company, 511 p. DEPTH, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS DISTANCE FROM "NEW" CHARLES RIVER DAM, IN METERS deep hole just upstream of the bridge that recently had filled operation results in large volumes of harbor water entering of the freshwater moving downstream also must pass through in the Basin; as the average daily mean discharge or the with harbor water. The salt wedge area between Longfellow Jobin, W., 1998, Sustainable management for dams and into the Basin, particularly during times of high-recreational the narrow lock at the old dam, thereby increasing the force force due to the upstream freshwater increases, the mass of Bridge and the Museum of Science remained anoxic as it waters: Boca Raton, Fla., Lewis Publishers, 265 p. boating use. When the gates of the locks are opened to allow opposing the movement of the salt wedge upstream compared salt in the Basin decreases. was not mixed by wind forcing. By June 23, the salt wedge boats into or out of the Basin, freshwater flows out of the to other areas of the river. Therefore, the density of the salt The empirical model obtained by use of multiple linear filled much of the Basin between the Longfellow and Metropolitan District Commission, 1975, Artificial EXPLANATION Basin over the same volume of harbor water infiltrating the wedge must increase to a point at which it can overcome this regression is as follows: Destratification of the Charles River Basin: Boston, Basin. It has been estimated that about half of the volume of force before moving through the locks and into the main part Harvard Bridges below a depth of 5 m. From June 18 to 6 ∆SM = [(4.2 x 106) x log LC] - Mass., Metropolitan District Commission, variously water in the lock enters the Basin during each lock cycle (a of the Basin. June 23, the salt mass in the Basin increased by 3.3 x 10 SALINITY, IN PARTS PER THOUSAND kg. No oxygen data were collected during the June 23 6 6 paginated. lock cycle is composed of two lock openings—one for a boat A second storm of lesser intensity occurred on June 25. [(2.5 x 10 ) x log Q] -6.4 x 10 , (2) water-quality survey. entering the basin and one for a boat exiting the basin). There During this storm, the lock gates on the new dam were opened where Parker, G.W., Ries, K.G., III, and Socolow, R.S., 1998, < 0.5 5.5 to 10.49 15.5 to 20.4 RIVER CHANNEL 6 3 By June 29, the salt wedge filled in much of the Basin The flood of June 1998 in Massachusetts and Rhode are three locks at the new dam: two small locks (61.0 m x for more than 12 hours during low tide, and 13 x 10 m of ∆SM is the change in salt mass between t and t upstream of the Longfellow Bridge as far as the Harvard 1 2, Island: U.S. Geological Survey Fact Sheet 110–98, 4 p. BATHYMETRIC 7.6 m) and one large lock (91.4 m x 12.2 m). Assuming that water was pumped from the Basin at depth to prevent in kilograms, 0.5 to 5.49 10.5 to 15.49 20.5 to 25.49 each lock is used for one-third of the lock cycles and given Bridge at depths greater than 4 m, the elevation necessary to PROFILE flooding. By July 1, 1998, the combined effects of increased Prichett, H.S., Mansfield, S.M., and Dana, R.H., 1901, that the observed average height of water is 3.4 m in the small overtop the berm under the Harvard Bridge. Pockets of LC is the number of lock cycles between t1 and t2, discharge, pumping at depth, and lock openings (during low Report of committee on Charles River Dam: State locks and 4.6 m in the large lock, about 1,380 cubic meters saline water were measured in deep holes as far upstream as and tide) almost completely flushed the salt wedge out of the Printers, Boston, Mass., Wright & Potter Printing Co., (m3) of harbor water enters into the Basin per lock cycle. Basin for a second time. the Boston University Bridge. From June 23 to June 29, the salt mass in the basin increased by more than 7.1 x 106 kg Q is the average mean daily discharge between t1 579 p. The presence of the salt wedge can have profound effects More than 500,000 people attend the Fourth of July and t , in m3/s. on geochemical conditions within the Basin including: (about 1.2 x 106 kg/day), the largest net increase of salt per 2 Figure 2. Geographical distribution of salt wedge in the Lower Charles River on selected dates. celebration on the banks of the lower Charles River to watch The use of trade or product names in this report (1) increased salinity, (2) decreased oxygen concentrations, fireworks and listen to the Boston Pops Orchestra. Many of day observed during the study period. Probably the mass of Agreement between the conceptual model and the 2 is for identification purposes only and does not con- (3) increased production of hydrogen sulfide, (4) sequest- those attending come by boat from Boston Harbor; from July salt increased because the average daily mean discharge calculated salt mass in the Basin (r =0.92), using the salt ration and immobilization of trace metals, and (5) increased from the Basin between June 23 to June 29 was the mass calculated from measurements taken on June 19, 1998, stitute endorsement by the U.S. Geological Survey. 1 through 15, 1998, 5,441 boats entered and exited the Basin. 3 internal loading of nutrients, such as phosphorus. All of these About 728 lock cycles, lasting an average of 10 minutes per minimum for the study period (1.7 m /s) and nearly set a as the initial mass of salt in the Basin and the change in salt changes affect organisms that live within or pass through the cycle, were needed to convey this number of boats into and historical low based on a 67-year record at a streamgaging mass calculated using equation 2, indicates that lock oper- Basin, especially benthic organisms and fish. out of the Basin (an average of 52 lock cycles per day). station upstream of the Basin in Waltham, Mass. ations are the major pathway of saltwater intrusion into the Between June 29 and July 6, there were 425 lock cycles STUDY METHODS Hence, the locks were open 120 hrs during the 15 days Basin, compared to other possible pathways, such as fish between water-quality surveys (on July 1 and 15, 1998). In (an average of 61 lock cycles per day at 10 minutes for each ladders, culverts, and leaking sluice gates (fig 3; table 3). Water-quality surveys were conducted weekly, bi-monthly, Table 1. Estimated average mean daily discharge at the "new" Table 2. Summary of "new" Charles River Dam operations between water-quality Table 3. Volume of the salt-wedge (salinity greater than 0.5 ppt) and salt mass in the Lower Charles River addition, the average daily mean discharge of the Charles lock cycle) at the new dam. In other words, the locks were Additionally, this analysis indicates that current dam Charles River Dam between water-quality sampling surveys surveys, June 1998 through July 1999 and monthly to determine the seasonal variability in the River during this period had decreased to about 23 m3/s. The open 72 hours during the 7 days between the June 29 and operations (opening of culvert, lock gates, and sluice gates [All values in cubic meters per second. --, no change calculated] [All values are to be multiplied by 106. Changes and mean changes are increases or decreases from previous water-quality survey (date). Daily mean change in volume expressed as extent and chemistry of the salt wedge in the Basin (fig. 1). July 6 water-quality surveys. The results from July 6 at low tide, and pumping from the Basin) are not effective in [m3, cubic meter; m2/hr, square meters per hour] salt wedge on July 15 had moved upstream from the old dam, Daily mean Discharge harbor water: Harbor water is estimated as 30 ppt salinity. Volume of harbor water infiltration basin; Daily mean volume of harbor water infiltration basin; Salt mass Water-quality profiling locations were determined using 6 indicated that the salt wedge had increased in size and preventing the formation of the salt wedge within the Basin. Daily mean Mean Mean Area/Hours open (m2/hr) infiltration basin; Daily mean salt mass infiltration basin: Due to boat lockages (assuming each lock is used 33 percent of the time). kg, kilogram; kg/d, kilogram per day; m3, increasing the salt mass in the Basin by 1.2 x 10 kilograms between water-quality Volume channel morphology as a guide; these locations were discharge number of volume cubic meter; m3/d, cubic meter per day; ppt, parts per thousand; -, is a net loss from the saltwedge; --, change not measured] 6 salinity since June 29. Although the daily mean discharge sampling surveys Lock of water (kg) in 14 days, or 0.08 x 10 kg/day (table 3). on date of Date lock of water primarily along the deepest parts of the Basin because the increased by about 50 percent—from 1.7 m3/s on June 29 to Date Change cycles pumped Fish Lock Lock Sluice Although the boat traffic is greatest during the Fourth of water- cycles per pumped Daily Daily Daily density of the saltwater causes it to sink to the bottom and 3 25.0 Mini- Maxi- (x106 m3) ladder gate culverts gates Daily Volume July celebration, traffic remains heavy for the rest of the 2.6 m /s on July 6—the salt wedge advanced upstream to 70 Average quality day (x106 m3) Daily mean mean Salt mean 112.5 OBSERVED SALT MASS mum mum survey change Change mean of harbor reside in the deepest part of the channel. the John Weeks Bridge at Harvard University. Additionally, 106 IN CUBIC METERS PER SECOND (x 10 Change mean volume mass salt

summer. For example, lock cycles averaged 39 per day during ) Salt in the change water

6 June 19, 1998 42 21 22 5.5 24 0 0 28,000 in the Channel morphology was mapped from water depths most of the Basin at depths greater than 5 m from the PREDICTED SALT MASS 60 Volume in the change of harbor in l- mass the period July 15 to August 19; 6,777 boats entered the Basin 112.5 Date 3 volume mass salt in the in l- 20.0 DAM LOCK CYCLES PER DAY June 19, 1998 59 61 60 -- 61 June 24 146 29 16 3.3 34 84 8.9 38,700 (m ) volume in the water in l- trating in l- interpreted from video recordings of echo sounder output and Boston University Bridge to the Museum of Science was DAILY MEAN DISCHARGE, (kg) mass salt trating in 35 days, requiring 1,381 lock cycles. In addition, the June 24 42 56 49 -11 42 July 1 213 30 13 1.9 46 440 0 50,200 (m3) volume expressed trating basin trating a portable, high-precision global positioning system (GPS). DAILY MEAN DISCHARGE as harbor (kg) mass basin anoxic. 50 July 1 28 39 34 -15 37 July 15 728 52 7.3 .5 11 120 33 76,000 (m3/d) basin (kg) basin average daily mean discharge of the Charles River during this water (kg/day) (m3) Data was processed using the triangular irregular network 3 6 July 15 14 35 23 -10 14 August 19 1,381 39 1.2 .03 0 54 0 132,000 (m3/d) (kg/d) period dropped to less than 7.0 m /s and only about 1.2 x 10 During the next week, more than 500 lock cycles 15.0 m3/d (TIN) data model (Environmental Science Research Institute, August 19 4.5 12 7.0 -16 9.6 October 7 1,192 24 1.4 .03 0 18 220 195,000 m3 of water was pumped from the Basin into Boston Harbor. occurred and the daily mean discharge decreased by more 40 June 19, 1998 0.02 ------0.05 -- -- 0.05 0.028 1.7 0.87 Inc., 1997) (fig. 1). 3 3 October 7 2.8 9.7 5.7 -1.3 3.3 November 17 271 7 3.1 .08 0 96 70 344,000 The combined effects of the large number of lock cycles, than half from 2.6 m /s on July 6, 1999, to 1.2 m /s on July June 24 .08 0.06 0.01 0.001 .21 0.16 0.03 .20 .040 6.1 1.2 Field measurements of water depth, specific conductance, November 17 3.9 24 11 5.0 8.5 December 29 62 1 2. .05 0 0 0 237,000 reduced upstream flow, and reduced pumping resulted in the 19, 1999, the lowest daily mean discharge estimated during 10.0 30 February 10, 1999 24 1 8. .19 0 1,400 1,500 533,000 July 1 .05 -.03 -.004 -.001 .03 -.18 -.03 .29 .042 9.0 1.3 salinity, pH, temperature, dissolved oxygen, and turbidity, 42 December 29 5.0 10 7.2 -3.5 5.4 advance of the salt wedge to the vicinity of the Harvard the study period. These low flows and the high number of February 10, 1999 7.3 36 22 15 30 March 17 34 1 .0 .0 0 3,400 1,700 888,000 July 15 .28 .23 .016 .003 1.2 1.2 .08 .99 .071 31 2.2 were made with a Hydrolab instrument at 1-m intervals from Bridge by August 19. lock cycles resulted in the salt wedge moving further 20 March 17 17 28 22 -.41 20 April 28 176 4 .0 .0 200 740 1, 400 290,000 August 19 .94 .66 .019 .005 6.0 4.8 .14 1.9 .054 58 1.7 October 7 2.6 1.6 .033 .004 12 6.5 .13 1.6 .033 50 1.0 the water surface to the river bottom at each water-quality August 19, 1998, marked the first time anoxic conditions upstream to a point near Harvard Stadium, affecting about 5.0 April 28 7.6 24 16 -6.0 9.8 May 6 90 11 .0 .0 61 0 180 27,100 profiling location. To minimize data collection error, the 6 3 May 6 6.4 9.3 8.0 -8.0 8.6 June 4 654 23 1.9 .07 210 0 648.1 89,800 November 17 2.2 -.38 -.009 .002 15 2.8 .07 .37 .009 11 .28 were measured in the Basin during the sampling period. 10 x 10 m of the Basin (about 92 percent of the total 10 Hydrolab was calibrated at the beginning of the sampling day June 4 4.9 16 9.1 1.2 4.9 June 18 447 32 .0 .0 110 0 218.5 48,200 December 29 1.8 -.36 -.008 -.004 9.3 -5.9 -.14 .08 .002 2.6 .06 volume of the Basin). This intrusion represents the 5 6 ) Concentrations of dissolved oxygen (DO) were less than 1.0 MASS OF SALT, IN KILOGRAMS (x 10 June 23 208 42 .0 .0 37 140 0 0 February 10, 1999 1.2 -.66 -.015 -.003 5 -4.4 -.10 .03 .0001 1.0 .02 and its calibration was checked at the end of the day to 0 0 0 June 18 2.6 4.8 3.4 -5.7 2.6 milligrams per liter (mg/L) at stations 3.5-A, 3.5-B, and 3.7-A maximum extent of the salt wedge and the largest mass of June 23 2.0 2.5 2.3 -1.1 2.0 June 29 252 42 .0 .0 46 0 0 7,760 March 17 5.9 4.8 .14 -.001 4.1 -.84 -.02 .05 .0013 1.4 .04 determine if the Hydrolab readings had drifted outside the 6 (fig. 1) at a depth of about 6 m. Anoxic conditions develop in salt observed (over 22 x 10 kg) in the Basin during the June 29 1.5 1.8 1.7 -.62 1.7 July 6 425 61 .0 .0 40 0 13.4 399,000 April 28 .49 -5.5 -.13 -.001 2.7 -1.5 -.03 .24 .0057 7.4 .18 instrument specifications (Hydrolab, 1996). Analysis of post- study period. Most of the Basin at depths greater than 5 m July 6 1.3 3.0 2.5 .83 2.6 July 19 501 39 .0 .0 4.5 0 0 15,800 May 6 .46 -.03 -.003 -.002 2.2 -.44 -.05 .12 .015 3.8 .47 the salt wedge due to oxygen uptake by heterotrophic bacteria -5.0 calibration data indicate that the Hydrolab operated within July 19 1.1 2.2 1.7 -.78 1.2 June 4 .57 .11 .004 .001 2.7 .48 .02 .90 .031 28 .95 in the sediment and the slow transfer of oxygen from the remained anoxic downstream of the Boston University 1-June-98 1-Sept.-98 1-Dec.-98 1-March-99 1-June-99 1-Sept.-99 acceptable limits for all of the water-quality surveys. overlying freshwater to the salt wedge. Anoxia in the Basin is Bridge, from July 6–19, 1999. June 18 1.6 1.0 .073 .007 5.5 2.8 .20 .61 .044 19 1.3 Salinity measurements that were greater than 0.5 parts per June 23 2.2 .62 .12 .021 8.7 3.3 .66 .28 .057 8.7 1.7 important not only because aerobic organisms cannot survive A CONCEPTUAL MODEL OF SALT June 29 3.5 1.3 .22 .039 16 7.1 1.2 .34 .057 11 1.8 thousand (ppt) at each of the water-quality sampling sites in oxygen-deficient environments, but because naturally Figure 3. Comparison between measured and were entered into an Arc Macro Language (AML) program MASS IN THE BASIN calculated salt mass in the basin between water- July 6 5.5 1.9 .28 .013 19 2.7 .39 .58 .083 18 2.6 occurring sulfate in the salt wedge is quickly converted to July 19 10 4.5 .35 .008 22 2.9 .23 .68 .053 21 1.6 (Environmental Science Research Institute, Inc., 1997). The salt wedge in the lower Charles River is not a quality surveys from estimated daily mean discharge hydrogen sulfide (H2S) under anoxic conditions. Hydrogen Salinity is the sum of the concentrations of major ions in sulfide is extremely toxic to aerobic organisms because it stagnant mass of saltwater residing only in the deepest parts for the Charles River at the new Charles River Dam seawater and includes chloride, sodium, sulfate, magnesium, inactivates the enzyme cytochrome oxidase, interfering with of the channel. Rather, it varies in area and salinity and the number of lock cycles (one cycle is one calcium, potassium, bicarbonate, bromide, boric acid, energy metabolism at the cellular level (Cole, 1975). throughout the year. Temporal changes in the area and opening and closing of the locks) per day at the new strontium, and fluoride (Ingmanson and Wallace, 1989). Charles River Dam.

SPATIAL DISTRIBUTION, TEMPORAL VARIABILITY, AND CHEMISTRY OF THE SALT WEDGE IN THE LOWER CHARLES RIVER, MASSACHUSETTS, JUNE 1998 TO JULY 1999 by R.F. Breault, L.K. Barlow, K.D. Reisig, and G.W. Parker 2000