Right: Wetland Development near Tappan Zee Bridge, Hudson River Estuary
Summary
The Hudson River has a severely altered hydrologic regime stretching from the Adirondack Mountains to the southern tip of Manhattan, where it opens into the Atlantic Ocean via the New York Bight. Extending from the head of the tide in Troy, New York, 250 km to its mouth at the Atlantic Ocean, the Hudson River estuary is the southernmost segment of the Hudson River. On average, the estuary experiences brackish water conditions as far north as Newburgh (100 km north of the Battery). Compared to other large urban estuaries, the Hudson River estuary has a large ratio of basin area (13,370 square miles) to estuary (154 miles). Such a large basin area deposits sediment, TSS, and other contaminants to the Hudson River estuary. Throughout its course, the Hudson River has been engineered to provide water for canals, prevent flooding, develop drinking water lakes and reservoirs, generate power, and for recreational purposes. There are over 2,000 dams modifying freshwater hydrology in the Hudson River watershed. Other anthropogenic modifications to the main stem Hudson River include: dredged removal of sediment, shoreline hardening, and construction of bulkheads. Hydraulic stress on the channel thalweg, shoaling (sediment deposition) in the estuarine turbidity maxima (ETM) and other regions of the estuary, as well as subaqueous erosion, are all bi-products of these modifications observed in the estuary mainstem. At average peak flow, the Hudson River discharges nearly 2,000 m^3/sec, occurring during the spring freshet in March or April. During the summer months, freshwater discharge decreases to an average of 100-200 m^3/sec. Low flow events produce longer freshwater residence times in the Hudson River and have been shown to cause hypereutrophic conditions, especially in the saline estuary. Although approximately 80% of freshwater entering the Hudson River estuary flows from above the Federal Dam, the tides provide most of the energy and fluid transport within the estuary. Velocities due to the tides are 5 to 10 times as great as estuarine circulation and about 100 times as great as river flow.
Historically, hydrologic investigations conducted in the estuary were driven primarily by the need to improve navigation and to understand the circulation of sewage and industrial waste in surface waters. Annual discharge has been measured at various points in the Hudson River from 1918 to the present. However, a 76-year period is not sufficient to fully analyze and distinguish between human impacts and nature’s influence on estuarine processes. The USGS has put real-time discharge gaging stations at several mainstem sites and on four major Hudson River estuary tributaries. Further research is needed to determine discharge rates and hydrologic influence on the estuary from all of its tributaries. Few studies have investigated tidal wetlands as a link between the uplands and the estuary, or their hydrological characteristics. There are 2,895 ha of tidal wetland in the estuary. Wetlands and marsh ecosystems have been modified, and in certain cases filled to accommodate human development in the estuary. The presence of anthropogenic features such as railroads has caused changes in wetland hydrology and shaped their ecological structure and function.
Literature Review
The following literature review provides a synthesis of the main physical features of the Hudson River estuary. Three principal aspects of the Hudson River watershed are considered, which include: (1) the size and shape of the Hudson River channel; (2) freshwater discharge rates and tidal motions in the estuary; and (3) estuarine processes such as vertical mixing, stratification, and circulation.
I. Length, Width, Depth and Gradient
The Hudson is a 315-mile river flowing southeast from its headwaters at Lake Tear of the Clouds in the Adirondack Mountains to the Battery in New York City where it discharges into New York Bay. The Hudson River watershed collects freshwater from an area of 13,370 square miles, covering five states. The watershed can be geographically divided into four sub-watersheds: (1) upper Hudson River, extending from its source at Lake Tear of the Clouds to the federal dam at Troy; (2) the Mohawk river; (3) the upper Hudson River estuary, from the federal dam at Troy, NY to the Bronx-Westchester County boundary; and (4) lower Hudson River estuary in the New York-New Jersey metropolitan region from the Bronx-Westchester County line to the Verrazano-Narrows bridge (Brosnan et al. 2006). The Hudson River estuary is tidal and extends 240 km (154 miles) south from Albany to New York City, NY, where it empties into the sea.
The division between the upper and lower Hudson River watershed is the confluence with the Sacandega River of the Mohawk River with the Hudson River at Green Island and the location of the Federal Dam. The lower Hudson River is part of the Hudson-Raritan system, which is a varied and complex coastal plain estuary. The system is dominated by a drowned river valley (the Hudson), with a network of tidal straits (Arthur Kill, Kill Van Kull, and Harlem and East Rivers), open and enclosed bays (Raritan, Jamaica, and New York bays), tidal wetlands, marshes, swamps and mud flats. The lower tidal estuary connects with the Atlantic Ocean via New York Bay just south of the Battery. There are 198 tributaries to the Hudson River estuary ranging from first to ninth order with variable contributions to estuary hydrology (NYS DEC 1998).
From Lake Tear of the Clouds to Green Island, the upper Hudson River's length is about 150 miles and drains an area of some 4,627 square miles (Abood et al. 1992). The Hudson River drops 1,810 feet from its source to Troy Dam, an average bottom slope of about 12 ft/mi. The Mohawk River is 155 miles long and drains some 3,462 square miles. From its source at 1,800 feet above mean sea level, the Mohawk River falls irregularly to an elevation of 14.3 feet, at which point it joins the Hudson. Finally, the lower Hudson River is approximately 154 miles long and drains some 5,277 square miles with an average slope (represented by the halftide or mean tide level) of 2 feet per 150 miles (Abood et al. 1992). Thus, the main stem Hudson River has a steep gradient from its source to Troy and flows along a flat gradient from Troy to NY Harbor.
The average width of the Hudson River is 1.5 km. However, exceptionally wide portions along the main stem include: (1) Tappan Zee, width of 4 km, and (2) Haverstraw Bay, width of 4.6 km. The average depth in the Hudson River is approximately 10 m. Brosnan and O’Shea (1996) estimate that water depths range from 11 m to 21 m. However, channel irregularities like the one near the southern boundary of the Hudson Highlands, where the channel suddenly deepens to 30 meters, are important regions in the estuary producing greater tidal mixing. The morphology of the estuary floor is smooth on the shoal, changing abruptly to rough toward the channel thalweg (Klingbeil and Sommerfield 2005). The roughness elements of the estuary floor are flow transverse, wave-like erosional marks with heights of 0.25-0.5 m and wavelengths of 4-9 meters at the observed height range. These erosional bedforms being distinct from accretionary sandwaves and dunes, which generally exhibit wavelengths of 4-9 m at the observed height range. Studies of the erosion characteristics of the estuary suggest that the system is close to equilibrium with hydrodynamic conditions, so an increase in near-bottom velocities would likely result in enhanced erosion (Geyer 2005). This has been observed where shoreline modification resulted in a decrease in cross-sectional area of 10-20% and resulted in a deepening of the channel to restore cross-sectional area and and a new equilibrium condition (Klingbeil and Sommerfield 2005).
II. Freshwater Discharge, Tidal Flow, Flushing Rate, and Water Residence
South of the Federal Dam at Troy, the Hudson River is tidal. Freshwater outputs produce a net southward motion in the tidal river, but tidal velocities are usually much higher than the net southward motion of river flow. Thus, in all but the most extreme outflow conditions, the estuary flows in both directions, following the influence of the tides (Geyer and Chant 2006). Although the tidal river flows both north and south, the net southerly river flow persists to create the freshwater estuary. This freshwater source is a dominant contributor to the physical regime of the estuary and harbor, as it controls the salinity regime, the vertical stratification and the exchange of properties between the estuary, ocean, and atmosphere (Geyer and Chant 2006). Freshwater pulses also enhance the erosive sheer stress of ebb tidal currents (Klingbeil and Sommerfield 2005).
About 80% of freshwater entering the Hudson River system does so above the Troy Dam. Most of the remaining freshwater enters the rivers from tributaries in the lower Hudson River. At peak flow, the Hudson River discharges nearly 2,000 m^3/sec. This occurrence, known as the freshet, occurs in late March or early April. During the summer months, discharge typically decreases to 100-200 m^3/sec. Abood et. al (1992) estimate that the longterm average freshwater flow in the Hudson River at the Battery in New York City is 20,936 cubic feet per second (cfs). The 7-day, 10-year low flow (7Q10) for the Hudson-Raritan System has been estimated at 82 m^3/sec, and the 100-year flood flow is 5,760 m^3/sec.
The tidal cycle on the Hudson River is 24 hours and 50 minutes, during which two low tides and two high tides are experienced. Maximum flood occurs within an hour of high tide, and the flood continues for the first two hours of the falling tide. Tidal pulse is semi-diurnal throughout (Brosnan and O’Shea 1996). Although frictional effects slow down the tides by about 20% (Geyer and Chant 2006), average tidal currents move at a pace of 0.7 m/sec. Tidal flows are strong and have been found to range from between 50 to 120 cm/sec (Heyes et al. 2004). The mean tidal range in the lower estuary is 1.5 m (Klingbeil and Sommerfield 2004), however the tidal amplitude or the height of the intertidal zone varies from about 0.75-1.8 m, lowest in the Hudson Highlands and highest in the southern and northern ends of the estuary (Kiviat et al. 2006). Montalto and Steenhuis (2004) report that average tidal variation of the Hudson River near Piermont Marsh is estimated to be 1.1 m, which interestingly is the same range reported about 130 km further upriver at Tivoli North Bay. It was reported by Gross (1974) that deep channel dredging in the Hudson River between the town of Hudson and the city of Albany almost doubled the tidal range in that portion of the river (Montalto and Steenhius 2004). Currents are stronger in the middle of the channel and near the surface, averaging closer to 1 m/sec (1000 cm/sec) or 2 knots. The tidal currents are considerably stronger than the velocity due to freshwater outflow of the tidal river, which is on the order of 0.01 m/sec during the dry summer months, and reaches 0.2-0.5 m/sec during the spring freshet. Thus, the tides provide most of the energy and fluid transport within the river below the dam at Troy. In fact, the velocities due to the tides are 5 to 10 times as great as the estuarine circulation (see next section for description) and about 100 times as great as the river flow.
The lower estuary has a partially mixed regime, with vigorous, tide-induced mixing between fresh and salt waters. The sea water is progressively diluted by river water as it extends up the estuary, and even during low flow conditions the water is nearly fresh at Peekskill, 70 km to the north of the harbor. Four salinity zones are classified within the Hudson River estuary based on salt concentrations: (1) polyhaline (18-30%), (2) mesohaline (5-18%), (3) oligohaline (0.5-5%), and (4) tidal fresh (>0.5%). The estuarine turbidity maximum (ETM), an area where maximum sediment resuspension occurs, is in the polyhaline portion of the lower estuary between 10 km and 35 km north of the battery (Heyes et al. 2004).
The flushing rate, or the ratio of water volume to mean annual freshwater flow, is 0.35 years (126 days). The Hudson River estuary flushes at a rate faster than other East Coast estuaries, e.g., Chesapeake Bay (Limburg et al. 1986). Howarth et al. (2006) estimate that water residence times in surface waters that comprise the photic zone of the mesohaline estuary range from 0.1 to 4 days; considered “rapid” when compared to other large estuaries. However, flushing rate and water residence times are affected by the tidal cycle and freshwater discharge into the Hudson. An important link in the system is that water residence times strongly regulate primary productivity in the Hudson River estuary. When residence times are greater than 2 days, production is extremely high in the saline estuary. If the residence times are less than 2 days, production is low to moderate. Freshwater discharges in the 1990s were low, and as a result the lower estuary tended to be hypereutrophic (Howarth et al. 2006). When freshwater discharge is high during spring-tide periods of the month and tidal mixing is greater, then water residence times are low (i.e. less than 1 day). At lower rates of freshwater discharge, water residence times vary greatly, with at least some variation related to the tidal cycle; neap tides favor longer water residence times.
III. Stratification (Density, Salinity, Baroclinic), Estuarine Circulation, and Tidal Mixing
Ocean water contains about 3% salt by weight (or 30 ppt, referred to by oceanographers as practical salinity units or psu), which renders salt water about 3% more dense than freshwater. This density contrast causes the freshwater to flow over the salt water and the formation a “salt wedge” on the bottom layer. The Hudson exhibits a more extreme range of stratification between spring and neap tides than any estuary where this phenomenon has been observed (Geyer and Chant 2006). There is a strong horizontal salinity gradient along the estuary that causes a horizontal density gradient (due to the difference between fresh and salt water), which in turn induces a depth varying, or “baroclinic” pressure gradient in the estuary. The baroclinic pressure gradient drives the deep water landward, and a compensating tilt of the water surface drives the surface water seaward (Geyer and Chant 2006). This vertical motion is called estuarine circulation.
The tides dominate the density driven estuarine circulation. The stronger the density gradient between the fresh and salt water, the stronger the estuarine circulation. Estuarine circulation always increases the salinity of deep water and decreases the salinity of surface water due to horizontal advection. Although, channel irregularity in the Hudson Highlands, where depth goes from 10-12 m to 30 m, creates turbulence and promotes vertical mixing.
A byproduct of the interaction of estuarine circulation and the salinity gradient is stratification. Stratification is generally considered the most important variable for classification of estuaries. Networks of varying freshwater inflows, tides and tidal flows produce the vertical density stratification in the Hudson River estuary and Upper New York Bay, with seasonal and tidal variations in stratification. Average summer salinities in the lower Hudson River are typically between 10-15 mg/g (%) in surface waters, and 15-25 mg/g in bottom waters. Temperature stratification is normally less than 2 degrees C. However, the magnitude of temperature differences at any one time between the upper and lower reaches of the channel can approach 11 degress Celcius. Heyes et al. (2004) found that water temperatures in the ETM ranged from 4 through 24 degrees Celcius, with peak temperatures in August. Because the difference between surface and bottom salinities in the estuary is generally less than 10 mg/g, the estuary is classified as moderately stratified and partially mixed (Brosnan and O’Shea 1996). Tidal mixing affects the stratification directly, by producing vertical exchange between upper and lower layers, and indirectly, by influencing the strength of estuarine circulation, which provides the source of stratification. Vertical mixing, due mainly to the tidal currents, partially counteracts the stratifying tendency of estuarine circulation. As tidal currents increase, there is greater vertical mixing and less stratification for a given amount of estuarine circulation.
Cited References
Abood, K. A., G. A. Apicella, and A. W. Wells. 1992. General evaluation of Hudson River freshwater flow trends. Pages 3–28 in L. C. Smith, edito,. Estuarine Research in the 1980s. State University of New York Press, Albany, New York, USA.
Ashizawa, D., and J. C. Cole. 1994. Long-term temperature trends of the Hudson River: A study of historical data. Estuaries, 17(1): 166-71.
Brosnan, T. M., and M. L. O’Shea. 1996. Long-term Improvements in water quality due to sewage abatement in the Lower Hudson River. Estuaries, 19(4): 890-900.
Brosnan, T. M., A. Stoddard, and L. J. Hetling. 2006. Hudson River sewage inputs and impacts: Past and present. In J.S. Levinton and J.R. Waldman (eds), The Hudson River Estuary. Cambridge University Press.
Reid, G.K. and R.W. Wood. 1976. Ecology of Inland Waters and Estuaries. 2nd ed. van Nostrand, New York: pp. 93-107.
Dyer, K. R. 1997. Estuaries, A Physical Introduction. 2nd Edition. John Wiley & Sons, Chichester 195 pp.
Geyer, W. R., and R. Chant. 2006. The physical oceanography processes in the Hudson River Estuary. In J.S. Levinton and J.R. Waldman (eds), The Hudson River Estuary. Cambridge University Press.
Gross, M.G. 1974. Sediment and waste deposition in New York Harbor. Annals of the New York Academy of Sciences, 250: 112-128.
Heyes, A., C. Miller, and R. P. Mason. 2004. Mercury and methylmercury in Hudson River sediment: Impact of tidal resuspension on partitioning and methylation. Marine Chemistry, (90): 75-89.
Howarth, R. W., R. Marino, D. P. Swaney, and E. W. Boyer. 2006. Wastewater and watershed influences on primary productivity and oxygen dynamics in the Lower Hudson River Estuary. In J.S. Levinton and J.R. Waldman (eds), The Hudson River Estuary. Cambridge University Press.
Kiviat, Erik, S.G. Findlay, and W.C. Neider. 2006. Tidal wetlands of the Hudson River. In J.S Levinton and J.R. Waldman (eds), The Hudson River Ecosystem, Cambridge University Press.
Klingbeil, A.B. and C.K. Sommerfield. 2005. Last Holocene evolution and human disturbance of a channel segment in the Hudson River estuary. Marine Geology 218: 135-153.
Limburg, K.E., M.A. Moran, and W.H. Mcdowell. 1986. The Hudson River Ecosystem. Springer-Verlag, New York. 331 pp.
Mccrone, A.W. 1966. The Hudson River estuary: Hydrology, sediments and pollution. Geographical Review 56(2): 176-189.
Montalto, F.A. and T.W. Steenhuis. 2004. The link between hydrology and restoration of tidal marshes in the New York/New Jersey Estuary. Wetlands 24(2): 414-25.
Montalto, F.A., T.S. Steenhuis, and J.-Y. Parlange. 2005. The hydrology of Piermont Marsh, a reference for tidal marsh restoration in the Hudson River estuary, New York. Journal of Hydrology: 1-21.
NYS DEC. 1998. HIstorical Fisheries Database. New York State Department of Environmental Conservation, Biological Survey Unit, Albany, NY.
Stanne, S.P., R.G. Panetta, and B.E. Forist. 1996. The Hudson: An Illustrated Guide to a Living River. Rutgers University Press, New Jersey.