current and evolving physical and chemical conditions in ... · hudson river physical and chemical...

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American Fisheries Society Symposium 51:39–61, 2006© 2006 by the American Fisheries Society

Current and Evolving Physical and ChemicalConditions in the Hudson River Estuary

KARIM A. ABOOD, THOMAS L. ENGLERT, SUSAN G. METZGER, CHARLES V.BECKERS, JR., TIMOTHY J. GRONINGER, AND SUMANT MALLAVARAM

HDR-LMSOne Blue Hill Plaza, Pearl River, New York 10965 USA

[email protected]

Abstract.—The physical and chemical environment of the Hudson River Estuaryis characterized from data collected during the last 6 decades, with a focus onhydrology (primarily freshwater flow) and water quality (mainly salinity,temperature, and dissolved oxygen [DO]). The most remarkable change duringthis period is the substantial improvement in dissolved oxygen levels, particularlyin the vicinity of New York City and around Albany. A primary reason behindthis improvement is wastewater treatment and minimization of point sourcedischarges (raw sewage and industrial effluents). During the summer of 1973,water quality was poor (DO of zero), with no fish life present in the Albanyregion. It is a given that water quality is but one element affecting biota andecosystem functions; however, current levels no longer support the septic, pollutedlabel given to the River in times past. The recent DO improvement is one measureof the resiliency of the Hudson River and its recovery from abuse. Also, the last2 decades experienced greater than average freshwater flows. The data suggest aslight increase in water temperature in certain segments and an apparent, yet tobe confirmed, small increase in salinity intrusion for a given flow.

Introduction

The Hudson River watershed and its estu-ary is a moderate-sized, but regionally andnationally important, mid-Atlantic systemwhich has received substantial study be-cause of management issues with its fishes(Daniels et al. 2005). To provide a physicaland chemical context for the Hudson’s fishes,we present a description of the Estuary andits basin, with emphasis on freshwater flowpatterns and recent and evolving water qual-ity characteristics, primarily water tempera-ture, salinity, and dissolved oxygen (DO).

Freshwater flow observations are from theGreen Island U.S. Geological Survey sta-tion (the most downstream gauging stationabove tidewater). Semiempirical methods toquantify the contribution of the basin areabelow Green Island at Poughkeepsie andthe Battery at the southern tip of Manhat-tan were used to estimate downstream flows.

Historical and recent salinity, water tem-perature, and dissolved oxygen observationsare derived from previous publications, aswell as from the multi-utility Long RiverSurveys (Klauda et al. 1988) conducted be-

40 ABOOD ET AL.

Figure 1. Map of the Hudson River drainage basin.

tween 1975 and 2000. These are followedby historical salinity, temperature, and dis-solved oxygen trends at Poughkeepsie andin the vicinity of New York City. Factorsinfluencing these parameters and a sum-mary of flow and water quality changes overthe last 50 years are also discussed.

Hudson River Basin

Basin Hydrology

The Hudson River basin drains an area of34,540 km2 (Abood et al. 1989). Most ofthis area lies in the east-central part of NewYork State, with small portions in Vermont,Massachusetts, Connecticut and New Jer-sey (Figure 1). Several locations of interestto this study are shown in Figure 2, whichalso illustrates the distance in river kilome-ters (river km) above the mouth of the River(the Battery, at river km 0). In addition,Figure 2 locates the multi-utility Long Riverstudy areas (Klauda et al. 1988), conductedfrom 1975 through 2000, and power plants.

The Hudson River basin is bounded on thenorth by the St. Lawrence and LakeChamplain drainage basins; on the east bythe Connecticut and Housatonic River ba-sins, and the Connecticut coastal area; onthe west by the Delaware, Susquehanna,Oswego and Black River basins; and on thesouth by the basins of small streams tribu-tary to the Hudson River in New York Har-bor. Its watershed extends 206 km east-to-west and 383 km north-to-south. TheHudson’s source is Lake Tear-of-the-Cloudsin the Adirondack Mountains of northernNew York State and from there it flowsgenerally south for 507 km to its mouth atthe Battery where it discharges into NewYork Upper Bay.

Major tributaries entering the river’smainstem include the Mohawk River, Hoosic

River, Kinderhook Creek, Indian River,Sacandaga River, Esopus Creek andRondout Creek. For convenience, the HudsonRiver basin has been separated into threeprincipal drainage areas: the Upper Hudson,the Mohawk River, and the Lower Hudsonsub-basins. The division between the Up-per and Lower Hudson basin is at the Fed-eral Dam at Troy, some 248 km above theBattery. The Federal Dam represents thehead of tidewater. Researchers have consid-ered the Battery (as well as the Narrows 8km south of the Battery) as its mouth.

The Upper Hudson River flows mainlysouth–southeast to the confluence with theSacandaga River, where it turns to the east.At Hudson Falls, it turns again to the south.The river’s total length to Green Island isabout 240 km and it drains approximately

41HUDSON RIVER PHYSICAL AND CHEMICAL CONDITIONS

Figure 2. Hudson River multi-utility, Long River study areas, 1975-2000. Power generating stations areshown in the callout boxes.

11,984 square km. From its source to TroyDam, the Hudson River drops about 552m, resulting in an average bottom slope ofabout 4.4 m per km.

The Mohawk River has its source in thehills near the boundary between Lewis andOneida Counties, New York. It flows in asoutherly direction to Rome, and then fol-

42 ABOOD ET AL.

lows a general east–southeast course to itsjunction with the Hudson River at Cohoes,New York. The total length of the MohawkRiver is about 250 km and it drains 8,967square km. The Mohawk River falls irregu-larly from its source at elevation about 550m above mean sea level to elevation 4.4 mwhere it joins the Hudson River at Cohoes.

The Lower Hudson River, the primary wa-ter body of interest in this paper, commencesat the junction of the Mohawk and UpperHudson Rivers at Troy and discharges intoUpper New York Bay. All of this section ofthe river is tidal and a substantial portionof it is subject to ocean-derived salt intru-sion. The Lower Hudson River is about 248km in length and it drains an area of 13,667km2. Its average slope, represented by thehalf tide level, is about 0.6 m in 241 km.The slope is greatest from Troy to Catskilland is smallest between Catskill andTarrytown. From Tarrytown to New YorkCity, the half tide elevation drops about 0.2m.

Basin Climate

The climate of the Hudson River basin maybe considered as moist continental (Abood1974). The Upper Hudson basin has com-paratively long, cold and snowy winters andshort, mild summers. In the Lower Hudsonbasin, where the climate is much milderdue to the modifying influence exerted bythe Hudson Valley, summers are usuallylonger and winters milder. The Mohawk Riverbasin has variable weather conditions withcharacteristics of both areas.

The average annual air temperature withinthe basin ranges from 10°C in the southernportion to 4.4°C in the Adirondack Moun-tains (Abood 1974). The corresponding av-erage July temperature varies from about24–18°C, and the average January tempera-ture is about –1°C and –9°C, respectively.

The maximum and minimum temperaturesrecorded in the basin are 41°C and –41°C,respectively.

The mean annual precipitation varies from86 cm in the center of the basin to morethan 127 cm in the Adirondacks, with mostof the basin receiving an average of about102 cm. Precipitation is distributed ratherevenly throughout the year, with a slightrise during the summer. The average an-nual snowfall for the basin ranges fromabout 76 cm in New York City to over 330cm in the Adirondack Mountains.

River Channel Geometry

The Lower Hudson River is a relatively deepand straight channel (Figure 3). The cross-sectional area of the channel ranges fromabout 23,336 m2 in Haverstraw Bay to lessthan 4,645 m2 in the most upstream reachof the estuary. Surface width in the LowerHudson varies significantly and somewhaterratically along the longitudinal axis of theestuary because of the presence of severalembayments, primarily Haverstraw andNewburgh Bays. The widest section of theriver is located at Haverstraw Bay, wherethe width reaches almost 5 km, and thenarrowest section, where the width is only175 m, is located some 10 km south of Troy.

From the Battery to the head of HaverstrawBay, the mean depth (mean tidal elevation),defined as the cross-sectional area dividedby the surface width, generally decreasesfrom about 10 m to 5 m. Upstream ofHaverstraw Bay, it abruptly increases, reach-ing a maximum of approximately 27 m inthe vicinity of West Point, where individualvalues approach some 60 m.

Tidal Characteristics

The Lower Hudson River is a tidal estuarybetween the Federal Dam at Troy and New


Figure 3. Hudson River Channel physical characteristics.

44 ABOOD ET AL.

York Harbor. It is also classified (Abood1974) as a dampened, reflected tidal waveregimen, with the dampening occurring asthe energy of the tidal wave is dissipatedby channel friction as the oceanic tidal waveprogresses upstream. Reflection includes thepropagation of secondary waves as a resultof channel obstructions. Complete reflectionoccurs at the Federal Dam at Troy, andadditional wave reflections result from sig-nificant changes in channel width. As widthincreases, wave amplitudes tend to decrease,whereas a decrease in channel width causesan increase in wave amplitudes.

Tidal behavior at any section is the com-posite effect of ocean tide, channel friction,and wave reflection. The primary ocean tidesare also variable, with maximum amplitudeoccurring during spring tide and minimumamplitude during neap tide. Variations infreshwater discharge and barometric condi-tions also contribute to changes in ampli-tude.

The half tide level indicates the averageslope in the river, and the total fall fromTroy to the sea is about 0.6 m. As the tidemoves upstream, its range diminishes fromabout 1.3 m at the Battery to a minimumof about 0.8 m near Storm King (river km90). The tidal range then increases to asecond maximum of 1.25 m near Catskilland then remains fairly constant (1.2 m)for the remaining distance to Troy.

The spring tide is characterized by a higherrange of high and low water elevations,about 1.6 m at the Battery, 0.95 m at WestPoint, and about 1.5 m at Troy. The varia-tion of mean sectional tidal velocity alongthe river was first observed in the 1929USC&GS study (USC&GS 1934). Both ebband flood strengths were averaged acrossthe river cross section, and mean absolutevelocity over a tidal cycle was obtained byaveraging the section-averaged ebb and flood

strengths. The Hudson River ebb strength(maximum ebb current) is generally greaterthan the flood strength in the entire estu-ary, primarily because the freshwater flowand the ebb flow proceed in the same direc-tion. The mean sectional tidal velocity inthe Hudson River ranges from just above60 cm/s near the mouth, 30 cm/s from theTappan Zee Bridge to Kingston, to just be-low 60 cm/s near Hudson, and zero at theTroy Federal Dam (Abood 1974).

The mean tidal flow decreases from a maxi-mum of 12,035 m3/s at the Battery to zeroat the Federal Dam at Troy (river km 247).The values of the mean tidal flow are af-fected by the freshwater flow, particularlyin the upstream reaches of the estuary.

Freshwater flow influences the tidal charac-teristics by increasing the surface ebb ve-locity and decreasing the surface flood ve-locity while prolonging the surface ebb andabbreviating the surface flood cycle. Theinfluence of freshwater flow on these char-acteristics decreases with depth.

Basin Sediments

A major source of sediments in the HudsonRiver is its watershed, particularly from ba-sin runoff erosion. The Hudson River’s sedi-ment load is approximately 800,000 tons/year. Secondary sources include wastewaterdischarges, bank erosion, dredged materialand nonpoint sources. Suspended sedimentconcentrations in New York Harbor rangefrom 3 to 276 mg/l and average about 30mg/l. The unit weight of dry sediments inthe lower reaches of the Hudson River rangesfrom about 400–800 kgm/m3 with wet den-sity of two to three times dry densities coun-terparts (Abood 1974). Abood (1979) esti-mated a theoretical sedimentation rate ofabout 3 cm/year to over 2 m/year in themost downstream reaches of the HudsonRiver and New York Harbor.


Figure 4. Hudson River historical monthly average flows (Green Island) 1946-2003.

Figure 5. Hudson River annual flow—(Green Island) 1951–2001.

Freshwater Flow

The major portion (about 75% under nor-mal summer, and 65% under mean an-nual conditions) of freshwater flow entersthe estuary at its head at Troy. The re-maining portion of freshwater flow con-sists of contributions by tributaries dis-

charging largely into the upper reach of theestuary. Measurement of the freshwater flowin the estuary is not possible because of tidaloscillation. Lower Hudson flow histograms inthe tidal portion of the river are usually con-structed by measurements of Hudson Riverflows at Green Island (the most downstreamUSGS gauging station above tidewater [see

46 ABOOD ET AL.

Hudson averaged 3,500 cfs, or about 100m3/s , during the 6-month period from Juneto November. This drought receded in thelate 1960s; during recent years flows havebeen slightly above normal (Figure 5). Dailyflows at Green Island during the most re-cent 7 years demonstrate high short-termvariability (Figure 6).

The travel time between the head (GreenIsland) and Indian Point (located at riverkm 69) may be estimated by the use of thefollowing relationships (Abood 1977):

in which:

tLH = travel time between Green Island andIndian Point in days;

QLH = estimated Lower Hudson freshwaterflow in cubic feet per second.

Figure 6. Hudson River daily flow—(Green Island).

(1)

Figure 1]), and by empirically developedflow and travel time relationships betweenthe flows at Green Island over a period of57 years (1946–2003).

Freshwater flow varies over the year, withmaximum flows occurring primarily duringthe spring months (March, April and May),and minimum flows usually beginning inJune and continuing until November. Theoverall monthly average flow range at GreenIsland from 1946 through 2003 (Figure 4)extends from a minimum of 83 m3/s (2,912cfs) during August to a maximum of 1,776m3/s (62,733 cfs) during February, with anannual average of 404 m3/s (14,252 cfs).

During the 1960s, the entire northeasternregion of the United States experienced asevere drought characterized by extremelylow flows. The most severe drought in theHudson River estuary was observed in 1964,when the freshwater flow in the lower


This equation was derived from Manning’sequation (negligible surface width with flowstages, Rh ≈ H, n ≠ f (Q) assumed) and isbased on the results of a detailed HudsonRiver flood routing study.

The contribution of the drainage area be-low Green Island (QGI) to the Lower Hudsonfreshwater flow (QLH) has been empiricallydetermined (Abood et al. 1989) with theuse of USGS estimates of ungauged andgauged tributaries from 1947 through 1965.Because of significant differences amongcalendar months and downstream location,24 equations were developed (Table 1). Al-though the correlation coefficients in Table1 are high (80–98%), flows generated usingthese relationships should represent only es-timates of Lower Hudson River flows andshould be considered accordingly. For con-venience, these relationships may be ap-proximated by:

Equation (4) was used to convert Green

Month a b R2 a b R2

Jan –413 1.477 0.96 -353 1.648 0.94Feb –1179 1.600 0.92 1105 1.600 0.90Mar 4718 1.319 0.92 5302 1.503 0.88Apr 1115 1.311 0.93 1626 1.432 0.88May –154 1.356 0.98 85 1.483 0.96Jun 711 1.231 0.91 1228 1.310 0.86Jul –12 1.228 0.94 52 1.317 0.95Aug –951 1.358 0.96 -1062 1.452 0.93Sept –192 1.237 0.87 -319 1.336 1.80Oct –1968 1.569 0.96 -2544 1.748 0.94Nov –964 1.516 0.95 -1678 1.741 0.92Dec –958 1.421 0.90 -1008 1.621 0.94

(QLH = a + b QGI) Poughkeepsie Battery

Table 1. Relationship between Green Island freshwater flow and lower Hudson River flow atPoughkeepsie and the Battery.

a Estimated by the USGS. Travel time not taken into account. Isolated points outside indicated range not used. Feb. valuesrounded.

(2)

Island flows to Lower Hudson River coun-terparts at Poughkeepsie and the Battery(Figure 7). The long term (1946 through2003) mean annual flow in the Hudson Riverhas increased by 4–5% since 1988; the mostrecent data set (prior to this paper) reportedby Abood et al. (1989).

Salinity Intrusion

The lower reach of the Hudson River issubject to salinity intrusion. During droughtperiods the salinity front (100 mg/l) canextend some 130 km above the ocean en-trance. Due to tidal motion in the ocean,turbulent eddies mix the lighter ocean-pro-duced saltwater upwards. This action di-lutes the landward flowing saltwater andincreases the density of the seaward flowingfreshwater. As a result, the seaward flow inthe upper layer increases as more ocean-derived water intrudes upstream in the lowerlayer.

Within the salt-intruded reach, the Hudson

48 ABOOD ET AL.

The length of the salt-intruded reaches,identified as the 0.1 ppt isosal, may be es-timated from the following relationship, de-veloped by the Abood (1977):

where:

L = location of 0.1 ppt isosal in miles abovethe Battery;

QLH = Lower Hudson flow in 1,000 cfs

Figure 7. Hudson River historical monthly flows at Poughkeepsie and the Battery, 1946-2003.

(3)

(4)

is partially stratified. Generally, the Hudsonis subject to a net upstream movement in itslower layer and a net downstream movementin its upper layer. This circulation is inducedby density differences caused by salinity dis-tribution. Upper layer flow values ranging from10 to 40 times the corresponding freshwaterflow have been reported (Abood 1977).

A set of generalized Hudson River mean sa-linity (averaged over tidal cycle and cross-sectional area) profiles, expressed as meansalinity versus Lower Hudson freshwater flowat fixed locations, is given in Figure 8 (Abood1974).


Figure 8. Salinity vs flow rate at mile points.

Figure 9. Hudson River salinity at the Battery vs Lower Hudson flow at Poughkeepsie.

These relationships were developed using20 sets of salinity observations made over a40-year period (1929 through 1969; Abood

1977). These data were synthesized to takeinto account the nature, characteristics, andextent of the individual surveys and to in-

50 ABOOD ET AL.

Figure 10. Salinity versus distance above the Battery.


Figure 11. Natural log of flow versus temperature.

corporate differences in key factors influenc-ing salinity intrusion in the Hudson River,including:

1. Sampling time within a tidal cycle. Abood(1977) has shown a sinusoidal relationshipbetween salinity and flow with a maximumand a minimum occurring at high waterslack water and low water slack, respec-tively.

2. Sampling point location with a cross-section. The Hudson is partially stratified(maximum salinity values near the bottomand minimum values near the surface).There is also a minor increase in salinityfrom the west shore towards the east.

3. Tidal range variation. Salinity is greaterat neap tide than at spring tide.

4. Meteorological conditions. Key param-eters include wind speed and direction, and,more importantly, an increase in mean sealevel (either natural or due to hurricanes).An increase in sea level causes a rise inwater elevation accompanied by an increasein the landward directed flow of seawater.

5. Freshwater flow conditions. As reflectedin equations 1 and 2, and in Figure 8, themost important and controlling factor in-fluencing salinity intrusion is the influenceof freshwater in the Hudson River. Thisincludes not only Upper Hudson and

52 ABOOD ET AL.

Mohawk River flows at Green Island, butalso the Lower Hudson tributaries and vari-able lag time from its head to its mouth(see Freshwater section, above).

6. Field instruments. The use of salinom-eters, conductivity meters, chemical titra-tions, or density can influence field read-ings.

A meaningful correlation among differentsalinity surveys must take these six factorsinto account (Abood 1977).

More recent observations made by NYCDEPfrom 1997 through 2003 and by Bokuniewicz(1996) from 1994 and 1995 indicate that al-though the overall trends produced by above-cited relationships (Figure 8) are still appro-priate, recent observations (1997–2003) showan increase in salinity for a given LowerHudson River flow of roughly 15%. Figures9 and 10 compare several recent salinity/flow observations versus their earlier coun-terparts. These comparisons are crude innature since the more recent salinity datawere not subjected to detailed analysis in-corporating the above-listed six key factors.The apparent increase in Hudson River sa-linity intrusion must, therefore, be viewedwith caution. A more thorough investigationis needed to confirm this finding.

Water Temperature

Four categories of parameters influence themagnitude and distribution of water tem-peratures in the Hudson River (Abood etal. 1976). These include atmospheric condi-tions (radiation, evaporation, and conduc-tion), hydrodynamic conditions (channelgeometry, flow, and dispersion), boundaryconditions (temperatures of the ocean andfreshwater inputs), and anthropogenic in-puts (primarily power plants using the Riveras a cooling water source). All of these pa-

rameters affect temperature distribution inthe Lower Hudson; however, feed-forwardand feedback dependencies exist amongmost of them. An example is given in Fig-ure 11 showing a joint probability plot ofJune Poughkeepsie flows and water tem-peratures (1951–1992). The centroid in thisfigure represents average (mean) conditions,and the oval curves represent probability ofoccurrence. For example, 1966 appears torepresent mean June conditions, while 1981conditions occur only about 30% of the time.

Inputs from human activities include mu-nicipal discharges, industrial discharges, and,more significantly, those of power plants.Some man-made sources exercise either aheating or a cooling effect on the River. Forexample, a 1-MGD sewage effluent whosetemperature is about 18°C, constitutes acooling factor in the Hudson during the sum-mer months, because River ambient tem-peratures are higher. The greatest percent-age of the artificial heat input results fromthe use of River water for condenser coolingpurposes for the electric power industry.There are presently six power plants with atotal electric generation of approximately6,000 Mwe using the Hudson River as acooling water source; these plants use a cool-ing water flow of approximately 290 m3/s,or 4,600,000 gallons/min, and constitute aheat input of approximately 8 × 1011 Btu/d. Data from the multi-utility Long RiverSurveys were used to construct a riverwidetemperature profile indicative of averageAugust conditions covering the period from1975 through 2000 (Figure 12), as well asApril, May, and June.

A long-term daily water temperature dataset was used to construct a 50-year trend atPoughkeepsie. The mid-Hudson reach(Poughkeepsie) appears to have experiencedan increasing trend of average temperaturesfrom 1951 through 2002 (Figure 13). How-ever, a significant variability existed during


Figure 12. Temperature by region for April, May, June, and August (1975–2000).

this period. The trend (r2 of only about 7%)may be expressed as follows:

Y=0.0106X–8.556 (5)

in which:

Y=average daily temperature at

Poughkeepsie in °C; X = calendar year.

The annual and 5-year running mean tem-peratures at Poughkeepsie from 1908through 2002 (Figure 14) show a relativelysteep decline over the 15-year period fromabout 1910 through 1935 of 2°C, followedby a more gradual warming trend of about

54 ABOOD ET AL.

Figure 13. Average temperature (Poughkeepsie) 1951–2001.

Figure 14. Average water temperature (Poughkeepsie) 1908–2002.

the same amount over the subsequent 67years (1935–2002). Thus, in general, thewarming trend evident in recent data anddocumented in equation 5 has apparentlyreturned temperatures to levels previously

observed in the early 1900s.

A long-term seasonal profile showing weeklyaverage Poughkeepsie and a two-standarddeviation band covering the period from 1951


Figure 15. Weekly average water temperature (Poughkeepsie) 1951–2001.

Figure 15. Weekly average water temperature (Poughkeepsie) 1951–2001.

Figure 16. Change in monthly average temperatures (Poughkeepsie) 1951–2001.

56 ABOOD ET AL.

Figure 17. 1929 Temperature profile in the Hudson River.

through 2002 is given in Figure 15. Changesin monthly average temperatures and de-viations by month observed at thePoughkeepsie Waterworks facility from 1951through 2002 are depicted in Figure 16. In-creases up to about 0.03°C per year duringspring and summer months, as well as de-creases of up to about 0.02°C per year wereexperienced during this 50-year period.

We also compared present day temperaturesto earlier (premajor power plant) counter-parts. The earliest riverwide temperaturesurvey conducted along the length of theHudson River in late August through earlySeptember 1929 as part of a current surveyby the U.S. Department of the Interior(USC&GS 1934). Temperatures for the 1929survey were about 18°C at the Battery, in-creased to about 23°C in the Nyack area (asignificant increase of about 5.5°C, over an

approximate 32 km river segment in twoweeks), due primarily to the influence ofcooler seawater on the Hudson River (Fig-ure 17). The next trend illustrated by thisprofile is a gradual decrease of about 1°Cbetween Nyack and Kingston, due mainlyto climatic patterns as well as to the exist-ing hydrodynamic conditions (lower veloc-ity and the presence of shallows). In thelast segment, a rapid decrease in tempera-tures (about 2°C) was recorded. This wasdue primarily to the hydrodynamic andchannel geometry differences. This segmentis deeper and has fewer shallow areas thanthe lower River reaches, resulting in highervelocity and less heat exchange.

A comparison between this 1929 trend (Fig-ure 17) and a similar one depicting Augusttemperatures from 1975 to 2000 (Figure 12)taken from the Long River Surveys, shows


the influence of the above-described param-eters. Both periods show a rapid increasefrom the Battery up to approximately theIndian Point vicinity, followed by a moregradual decrease near Hyde Park and a morepronounced decrease further upstream. Thesetrends seem to suggest that boundary (ocean,source, and tributary) temperatures andhydrodynamic and atmospheric parametersseem to exert more significant influences onlongitudinal trends during August thananthropogenic inputs.

Dissolved Oxygen

Two long-term sets of dissolved oxygen ob-servations considered in this paper were col-lected in conjunction with biological stud-ies of the fish populations of the HudsonRiver (see Figure 2) and by the New YorkCity Department of Environmental Protec-tion (DEP), covering the lower reaches ofthe Hudson River. Figure 18 depicts theaverage August dissolved oxygen observa-tions (DO in mg/l and % saturation) takenduring the Long River surveys from 1975through 2000 between the Battery and Al-bany. There has been a substantial increasein DO since the early 1980s (Figure 19,derived from the DEP data set), resultingfrom the significant upgrades to the Yon-kers and North River Sewage TreatmentPlants in the lower reach of the HudsonRiver. Both data sets show remarkable wa-ter quality improvements when comparedwith earlier observations.

A closer look at summertime DO levels isgiven in Figure 20. This figure composesAugust measurements made in 1967 and1978 against New York State standards andmodeling counterparts from various govern-mental agencies, utilities and private in-dustry were averaged by milepoint to pro-vide the 1970–1972 values (LMS 1976). The1978 results are averages by milepoint ofdata collected during the Hudson River Field

Week II (HRFW II), a one-week riverwidesampling effort by industries, utilities, con-sultants, governmental agencies, colleges,and others, and coordinated by the HudsonRiver Research Council (HRRC 1980).

The 1967 and 1978 values are the monthlyaverages for August of the respective years.The 1970–1972 flow is the mean of themonthly average flows for August from thethree years considered. The flows correspond-ing to these profiles range from 5,749–6,823cfs, or 163–193 m3/s; this tight range offlow makes this comparison reasonable.Freshwater flows affect the available dilu-tion and the movement downstream of oxy-gen-consuming discharges, as well as tem-perature and salinity, which in turn affectthe DO saturation level; that is, the maxi-mum amount of DO that the water canretain. Other parameters affecting DO lev-els include waste loads, temperature, andoverall climactic conditions.

Although detailed cause and effect compari-son among these profiles and the Long Riversurvey data (Figure 18) have not been made,a number of overall reach and Riverwideobservations based on these profiles can bemade below. The most obvious feature inFigure 20 is the sag in the region just down-stream of Albany (river km 193–241 [rivermile 120–150]). DO levels were below thestate standard in 1967 and approached 0.0mg/l in 1970–1972. In 1974, two waste treat-ment plants came on line in the Albanyarea, treating about 80% of the city’s do-mestic wastes. Dissolved oxygen levels ob-served in 1978 were above 6.5 mg/l andwere higher than the state standard (Figure20).

In the New York City area (river km 0–16),the 1967, 1970–1972, and 1978 values (Fig-ure 20) are almost identical and include thepresence of another sag below the State stan-dard. Construction delays of two major sew-

58 ABOOD ET AL.

Figure 18. DO profile in August by Hudson River region (1975–2000).

age treatment plants—the North River Plantin Manhattan and the Passaic Valley STPin New Jersey—are partially responsible forthe failure to achieve the projected 1977best practicable technology (BPT) level.However, substantial increases in DO wereobserved in the vicinity of New York Citysince the 1980s and continue to rise aftercompletion of the Yonkers and North RiverWater Pollution Control Plants (Figure 19).

In the mid-river region, the DO levels dur-ing the 1970s are irregular, but for all prac-tical purposes they are the same. There ap-pears to have been a slight increase overthe 1967 level, but profiles for this regionare well above the State standard.The overall water quality levels, as mea-sured by DO levels in the Hudson Riverhave improved substantially. In the areanorth of New York City extending as far


Figure 19. Dissolved oxygen percent saturation at 42nd Street, New York City.

north as the Federal Dam at Troy, waste-water treatment facilities, have resulted inwater quality levels that are more capableof supporting aquatic life throughout theyear. In the New York City area and aroundAlbany, the low DO conditions that existedof the summer months during the early 1960shave improved significantly.

Conclusions

The water chemistry and physics that makeup the biological environment of the HudsonRiver basin and estuary experience changeson many scales, both temporally and geo-graphically. However, the system also hasgone through notable environmental changesfrom a relatively pristine precolonial state,through the severe chemical and sewage con-tamination of its industrial era, to its par-tial recovery, due largely to the benefits ofthe Clean Water Act of 1972 (Waldman 1999;Limburg et al. 2006). This has resulted in a

cleaner habitat for fishes, with reduced con-taminant effects (Wirgin and Waldman, inpress). The return to New York Harbor ofmarine borers, particularly teredos, in re-cent years (Abood et al. 1995) is anotherindication of improved water quality; theseharmful invertebrates were largely absentin the harbor in the 1900s because of con-tamination. Although its changes throughtime in pollution levels may be the bestknown environmental factors that affectedfishes, we document changes in tempera-ture, freshwater runoff, and salinity thatlikely also may have been felt by fish popu-lations. Among these is the apparent de-mise of the river’s population of rainbowsmelt Osmerus mordax, probably due towarming (Waldman 2006).

References

Abood, K. A. 1974. Circulation in the Hudson Estu-ary. Annals of the New York Academy of Sciences

60 ABOOD ET AL.

Figure 20. August dissolved oxygen profiles (1967–1978).

250:39–111.Abood, K. A. 1977. Evaluation of circulation in par-

tially stratified estuaries as typified by the HudsonRiver. Doctoral dissertation., Rutgers University,New Brunswick, New Jersey.

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