Sediment transport and trapping in the Hudson River estuary

Estuaries Vol. 24, No. 5, p. 670-679 October 2001

Sediment Transport and Trapping in the Hudson River Estuary

W. ROCKWELL GEYER*, JONATHAN D. WOODaUFF, and PETER TaAYKOVSKI

Woods Hole Oceanog'raphic h~stitution, Woods Hole, Massachusetts 02543

ABSTRACT: The Hudson Rh,er estuary has a pronounced turbidity maximum zone, in which rapid~ short-term depo- sition of sediment occurs during and following the spr ing freshet. Water-column mea.surements of currents mid stLs- peuded sediment were p e r f o r m e d during the spring of 1999 to determine the rate and mechanisms of sediment t ranspor t and trapping in file estuary. The net convergence of sediment in the lower estuary was approximately 300,000 tous~ cotLsi.stent with an estimate based on sediment cores. The major input of sediment f rom the watershed occurred duriug the spring freshet, as expected. Unexpected~ however~ was that an even larger quantity of sediment was t ransported landward into die ~stuary during the 3-too observation period. The landward movement wa.s largely accomplished by tidal pumping (i.e., die correlation between concentration and velocity at tidal frequencies) during spring tides, when the concentrations were 5 to 10 dines higher than during neap tides. The landward flux is not consistent with die long~ term sediment budg'et~ which requires a seaward flux at the mouth to account for the exce.ss input f rom the watershed relative to net accumulation. The anomalous, laaldward t ransport in 1999 occurred in part because file freshet was relatively weak, aud the f reshet occnrred during neap ddes when sediment resuspeusion was miuimM. An extreme freshet occnrred during 1998, which may have provided a repc~sitory of sediment jus t seaward of the mouth fliat re-entered the estnary in 1999. The amplitnde of the spr ing freshet and its timing with respect to file spring-neap cycle canse large iuteranunal variations in estuarine sediment fltLx. These variations can resnlt in the remobilizatiou of previonsly deposi ted sedimen b the ma.ss of which may exceed file annual inputs f rom the watershed.

In t roduc t ion

Meade (1969) no ted that estuaries slowly fill up w-ith fluvial sed iment until they are shallow enough to reach an equi l ibr ium condi t ion in which the sed iment supply is m a t ched by the expor t to the sea. He also no ted that dur ing mrerage condit ions, r iverine sed iment may be comple te ly t rapped w-ith- in an estuary due to the landward bo t t om flow of the estuarine circulation, which also carries sedi- m e n t into the estum-y f rom the sea. Thus, the ex- por t of s ed imen t required for the long- term mass balance may be accompl i shed only dur ing ex t reme events when the saline wedge is pushed to the m o u t h of the estuary and the nea r -bo t tom flow be- comes seaward t h r o u g h o u t the estuary. O t h e r es- tuar ine observat ions (e.g., Migniot 1971; Grabe- m a n n et al. 1997) indicate that increased river flow shifts the zone of sed iment t rapping seaward. Cas- taing and Allen (1981) provide an example of the expor t of s ed imen t to the sea dur ing high flow.

T h e H u d s o n River valley was carved by glaciers dur ing the Pleistocene (Neum an et al. 1969), and its estuary wa~s originally a fiord with depths of S0- 200 m. This glacial t rough is now mostly filled with estuarine sed iment (Weiss 1974), and the estuary is 10-20-m deep (Fig. 1). The estuarine s t ructure is partially mixed with m o d e r a t e vertical stratifica- tion and tidal currents of 100 cm s -1 (Abood 1974; Geyer et al. 9000). T h e r e is an estuarine turbidit)r

* Corresponding autEor; tele: 508/'289-2868; fax: 508/'457- 2194; e-mail: [email protected].

m a x i m u m (ETM) zone in the lower estuary 10-25 km nor th of T h e Batter}, (Fig. 2) where suspended sed iment concent ra t ions can reach m o r e than 1,000 mg 1-1 (Geyer et al. 1998). Sedimentologica l observat ions by Olsen et al. (1978) showed depo- sition rates of up to SO cm yr -1 in certain parts of the ETM on timescales of 5-10 yr. Woodru f f et al. (in press) found deposits of light brown m u d in the ETM as m u c h as 40 cm thick. T h e p resence of Be-7 at the base of these deposits revealed that they had been deposi ted no m o r e than 6-mo pr ior to sampl ing and most likely had been deposi ted over a t imescale of several months . Woodru f f et al. (in press) also showed that there are large seasonal variations in the spatial distr ibutions and thickness of these m u d deposits. Dur ing the spring fi-eshet in April 1998, the m u d deposits are absent fi-om the ETM zone at km 12-25, but there are fresh m u d deposits closer to the m o u t h at km 1-5. Two mon ths after the spring fi-eshet, the lower estuary deposits have vanished, and thick deposits are ob- served within the ETM zone.

T h e long- term sed iment accumula t ion rate in the H u d s o n estuary is only 1-3 m m yr -I (Olsen et al. 1978); the rapid, shor t - term deposi t ion appears to be roughly ba lanced by erosion on longer t ime scales. T h e local, long- term rate of sea-level rise is approx imate ly 2-g m m yr 1, based on salt marsh accret ion at P i e rmon t Marsh, 5 km nor th of the study site (Peteet and Wong 9000). It appea r s that the sTstem is in a p p r o x i m a t e equi l ibr ium with sea- level rise at this time. Averaged over the 100 km

�9 2001 Estuarine Research Federation 670

7 4 ~ 1 5 ' 7 4 ~ 7 3 ~ '

4 1 ~ '

4 1 ~ ,

4 0 ~ '

4 0 " 3 0 '

Fig. 1. Map of the study area, showing the northern, ETM, and southern sections. The positions of the sections are 1.5, 14, and 35 km north of the Battery. The 10-m isobath is indicated. The inset sections show the locations of the bottom tripods.

l e n g t h o f t he es tuary , a p p r o x i m a t e l y 3 • 105 m s yr -* o r r o u g h l y 1.5 • 105 tons ~- ~ o f s e d i m e n t a r e r e q u i r e d to m a i n t a i n the e s t u a r y at a c o n s t a n t d e p t h . I n c l u d i n g the e n t i r e r iver a r e a b e l o w the d a m at T roy ( the l a n d w a r d l imi t o f t ida l inf lu- e n c e ) , this n u m b e r i n c r e a s e s to 3 • 10 r' t ons yr ~.

W o o d r u f f e t al. ( in press ) e s t i m a t e t ha t o n av- e r a g e the w a t e r s h e d s o f t h e H u d s o n River a n d its t r i b u t a r i e s p r e s e n t l y supply 4 x 105 tons yr 1 o f s e d i m e n t to t he es tuary , b a s e d o n 18 yr o f s e d i m e n t d i s c h a r g e d a t a b e t w e e n 1976 a n d 1994. T h e p re - s en t s u p p l y f r o m the w a t e r s h e d p r o v i d e s 30% m o r e s e d i m e n t t h a n is r e q u i r e d to m a i n t a i n e q u i l i b r i u m c o n d i t i o n s b a s e d o n sea-level r ise. Th i s sugges t s t ha t in t he p r e s e n t r e g i m e , t he e s t u a r y s h o u l d ex- p o r t r o u g h l y 100,000 t ons o f s e d i m e n t p e r y e a r o n a l o n g - t e r m ave rage basis to m a i n t a i n m o r p h o l o g - ical e q u i l i b r i u m wi th sea level r ise.

Based o n p r e v i o u s o b s e r v a t i o n s o f t he p r e d o m - i n a n c e o f f i n e - g r a i n e d s e d i m e n t s in the ETM de- pos i t s (O l son e t al. 1978; W o o d r u f f e t al. in p re s s ) , this s tudy f o c u s e d on ly on s u s p e n d e d s e d i m e n t

Sediment Transport and Trapping 671

M i d - R i v e r Sec t i on , 5 / 3 / 9 3 1 6 2 1 - 1 9 0 8

sa l in i t y c o n t o u r i n t e r va l 2 psu

2 0 , , , ,

5 10 15 2 0 k m f r o m B a t t e r y

Fig. 2. Salinity contours and suspended sediment concentra- tions from a survey of the Hudson River estuary during mod- erate discharge and spring-tide conditions in May 1993. The high suspended sediment concentrations within the ETM zone are evident.

t r a n s p o r t . T h e r e a r e b e d f o r m s in t h e lower H u d - son River e s t u a r y i n d i c a t i n g b e d l o a d t r a n s p o r t ( F l o o d a n d B o k u n i e w i c z 1 9 8 6 ) , a l t h o u g h t h e coa r se s e d i m e n t t r a n s p o r t a p p e a r s to b e a s soc i a t ed wi th t i le r e w o r k i n g o f o l d e r d e p o s i t s r a t h e r t h a n the i n p u t o f new s e d i m e n t .

W h e r e a s t he s e d i m e n t mass b a l a n c e o f t he es- t u a r y sugges ts t ha t t h e r e s h o u l d b e s ign i f i can t sed- i m e n t e x p o r t f l 'om the es tuary , p r e v i o u s obse rva - t ions do n o t p r o v i d e an a d e q u a t e i n d i c a t i o n o f t he m e c h a n i s m s o f s e d i m e n t t r a n s p o r t w i th in t he sys- t e m to d e t e r m i n e h o w t h a t e x p o r t m a y t ake p lace . Th i s issue m o t i v a t e d a s tudy o f s e d i m e n t f lux in t he l ower H u d s o n River e s t u a r y to d e t e r m i n e the m e c h a n i s m s o f s e d i m e n t f lux a n d p o s s i b l e e x p o r t . T h e s e n e w o b s e r v a t i o n s in u w n p r o v i d e a p e r s p e c - tive on t h e l o n g - t e r m e v o l u t i o n o f t he e s t u a r i n e s e d i m e n t o l o g i c a l r e g i m e .

M e t h o d s

M o o r i n g s a n d t r i p o d s w e r e d e p l o y e d f r o m M a r c h to J u n e 1999 to d o c u m e n t t he m e c h a n i s m s o f s e d i m e n t t r a n s p o r t a n d t r a p p i n g in t he H u d s o n River e s t u a r y d u r i n g a n d fo l lowing the s p r i n g f reshe t . C ros s -e s tua ry ar rays were d e p l o y e d l and - w a r d a n d seaward o f t he ETM zone , a n d a t r i p o d a n d m o o r i n g we re d e p l o y e d wi th in t he ETM. T h e c ross -e s tua ry a r rays i n c l u d e d b o t t o m t r i p o d s at two c ross -e s tua ry l o c a t i o n s a n d n e a r b y tau t - l ine m o o r - ings. T h e t r i p o d s in t he t ha lweg h a d u p w a r d - l o o k - ing, RDI w o r k h o r s e acous t ic d o p p l e r c u r r e n t p ro - f i lers (ADCP) . T h e f r e q u e n c y o f t he A D C P at t he s o u t h e r n n ' ansec t was 600 kHz, a n d the ETM a n d n o r t h e r n t r ansec t s h a d 1,200 k H z A D C R In add i - t i on to w a t e r - c o l u m n ve loc i ty p rof i l es , t h e A D C P p r o v i d e d acous t i c b a c k s c a t t e r m e a s u r e m e n t s t ha t we re u s e d to e s t i m a t e s u s p e n d e d s e d i m e n t c o n c e n - t r a t i o n ( T h o r n e 1993). C u r r e n t s we re m e a s u r e d at o t h e r l o c a t i o n s wi th $4 a n d M a r s h - M c B i r n e y elec- t r o m a g n e t i c c u r r e n t m e t e r s ( see Fig. 1). O p t i c a l

672 w . R . Geyer et al.

backscatterance sensors (OBS) were deployed on all of the bot tom tripods at 50, 100, and 200 cm above the bottom. Additional OBS were deployed at 100, 200, and 300 cm above bot tom on the moorings. Salinity and temperature were mea~sured w-ith Seabird Seacats on all of the tripods and moorings. Surface salinity and backscatterance were measured with a Seacat and OBS at the sur- face moor ing at the ETM.

Ins t ruments were deployed on March 9-10, 1999, recovered for cleaning and redeployed on April 14-15, and finally recovered on June 17, 1999. Fouling of salinity sensors due to clogging w-ith sediment occurred at several of the bot tom tripod locations. Some of the OBS sensors showed evidence of fouling starting in the middle of May,

Lab calibration of all of the OBS sensors was per- formed prior to the deployment using Hudson Riv- er mud, In situ calibrations were pe r fo rmed dur- ing shipboard surveys using an OBS on an Ocean Sensors CTD and a 1.8-1 Nisken bottle. The Nisken bottle was tripped by a bot tom-actuated trigger at 1 m above the bottom, Samples taken during pe- riods of low near-bot tom concentra t ion indicated that there was no contr ibut ion of resuspension by the ins t rument in the water samples, The water samples were filtered in the lab with an 8-1J~m glass- fiber filter, dried, and weighed to determine total suspended solids, A total of 50 samples with con- centrations ranging from 10 to 1,000 mg 1-1 pro- vided calibration for the OBS on the CTD. The uncertainty of the OBS estimates in comparison with the bottle samples was a round 20%, Compar- ison of the moored instruments with the profiler indicated that these measurements were also con- sistent with an uncertainty of 20-S0%, some of the uncertainty owing to slight differences in time and space of the samples.

The OBS time series were used to calibrate the ADCP backscattered intensity for sediment concen- tration. A non-linear regression wa~s used to relate the lowest bins of the ADCP data (located at S m above bot tom) to the highest tripod OBS. Error bounds on this regression are approximately 80% to 40%. At the no r the rn and southern tripods the highest OBS was at the same elevation as the lowest ADCP bin, while at the ETM tripod the highest OBS was located 2 m above bottom. At this site a vertical extrapolation based on an exponential fit to the g OBS sensors was used to estimate concen- tration at the lowest ADCP bin. The vertical de- pendence of the calibration coefficients at the ETM site was determined by using an OBS located on a surface moor ing nearby. The vertical decay of acoustic energy spreads geometrically as range squared, and there is an additional exponential de- cay due to at tenuat ion both from water and sus-

pended sediment (Thorne 1993). Applying this geometric decay to the ADCP data resulted in con- centrat ion estimates that were too low at the sur- Face compared to the OBS that had been calibrat- ed with in-situ bottle samples. In order to generate AC, DP concentra t ion estimates that were consistent with the OBS data, an exponen t of 1.4 rather than - 2 was used for the range-dependen t decay. There are two possible reasons for the discrepancy between the theoretical decay factor and the one actually used. The ADCP backscattering data is re- turned as a number that is propor t ional to deci- bels. The manufac ture r suggests a coefficient to scale this n u m b e r into actual decibels, but this co- efficient may be incorrect. This would result in a backscattered intensity that is not actual acoustic intensity, but intensity raised to some undeter- mined power. A second possible factor is that there is consistent grain size variation with height. A low- er than expected decay, based on OBS measure- ments, is consistent with finer sediments higher in the water column, because the OBS is more sen- sitive to fine particles than the acoustics. Because there were no surface moorings at the no r the rn or southern ADCP tripods this same decay factor was applied to these data.

The time series data at the no r the rn and south- ern transects were used to estimate the cros~sec- tionally integrated sediment flux for the deploy- ment period of March 10 to J u n e 15, 1999. Direct estimates of both concentra t ion and velocity were obtained in the bot tom bounda ry layer (0.5-8 m above bottom) at the tripod locations. "velocity and c o n c e n t r a t i o n (based on acous t ic backsca t te r ) through the whole water column were obtained at the eastern tripod at each site. In order to obtain cross-sectional average estimates based on the tri- pod measurements , both lateral and vertical ex- trapolation were per formed. Vertical extrapolation of the suspended sediment data at the western tri- pods was pe r fo rmed by assuming an exponential reduct ion in concentra t ion beyond 3-m, as indi- cated by the ADCP data as well as the shipboard profiles. The shipboard ADCP profiles indicate mean e-folding scales of 10-m for both sites, al- though there was considerable variability among individual profiles. A range of 5-m to 15-m was used to bracket the uncertainty in the estimate of the vertical structure.

The velocity above the boundary layer at the western tripods was assumed to have the same ver- tical structure as the eastern tripods with a shift in the mean flow based on the difference in record- length means between the bot tom currents at the eastern and western sites. The assumption of the similarity in vertical structure was based on prior shipboard observations (Nepf and Geyer 1906),

t \~,~, �9 i / E 5 4 I I a::" ~1o

', - -

Ebb

" " : J I i,& l

:(7 " I . . . . I "

F lood

0 0 .5 1 0 0 .5 1 km km

Fig. 3. Cross-sections of suspended sediment at the northern and southern sections dtuing maximum ebb and maximm~n flood. The northern section data were obtained on year day 105, and the southern section data were obtained on year day 104 (spring tide conditions).

which i nd i ca t e tha t the ver t ical shears are relatively u n i f o r m in the c ross -channe l d i rec t ion , a l t h o u g h the tidally averaged velocity may vary significantly. For b o t h the n o r t h e r n a n d s o u t h e r n transects , a 5 cm s ~ seaward offset was app l i ed to the c u r r e n t s above the b o u n d a r y layer o n the wes t e rn s ta t ion locat ions , based o n the obse rved la teral va r ia t ions in tidally averaged, n e a r - b o t t o m velocity.

T h e la teral i n t e g r a t i o n was g u i d e d by the cross- sec t ional d i s t r i bu t ions of s u s p e n d e d s e d i m e n t (Fig. 3). T h e b o u n d a r y layer es t imates (over the lowest 2 m) were e x l r a p o l a t e d across the e n t i r e w id th of the estuary, a n d the u p p e r water c o l u m n es t imates were app l i ed to the r e m a i n d e r of the cross-section- al a rea with equa l weights app l i ed to the da ta on b o t h sides of the estuary. This a s s u m p t i o n may n o t have b e e n valid for all flow cond i t ions , b u t t he re were n o a d d i t i o n a l t ime series data with which to c o n s t r a i n the la teral d i s t r i bu t i on of s e d i m e n t flux.

Resu l t s

FLOW AND SUSPENDED SEDIMENT CONCENTRATIONS

River d i scharge of the H u d s o n in the sp r ing of 1999 p e a k e d in early Apr i l (year day 95) with a m a g n i t u d e of 1,700 m S s -1 (Fig. 4). T h e f reshe t peak has r a n g e d fl 'oui 1 ,000-4 ,000 m s s 1 in lhe last 54 yr; the 1999 value is in the 5th pe rcen t i l e , i.e., it was a relatively small freshet . T h e m a x i m u n i net , seaward velocity in the es tuary d u e to the r iver flow was app rox in i a t e ly 20 cm s 1, based o n the cross-sect ional area of 104 me. Tidal c u r r e n t s pro- vide the d o m i n a n t flow in t im estuary, even d u r i n g pe r iods of h igh flow. T h e a m p l i t u d e of the near - surface t idal c u r r e n t s m e a s u r e d at the s o u t h e r n sec t ion var ied f rom 65 to 120 cni s 1, a n d near- b o t t o m c u r r e n t s r a n g e d f ron i 50 to 85 cm s 1 (Fig. 4). T h e d o m i n a n t var ia t ions were at fo r tn igh t ly a n d m o n t h l y t imescales. T h e m o n t h l y m a x i m a in

Sediment Transport and Trapping 673

2000 , , J J t IRiver Discharge 1 5 0 0

~ lOOO

500

0 120 ~ ~ a l Velocity Amplitude

~176 0 , ,

i i J i i i J

"~ 20 O Southern Non-T ida l Velocity

-20

-40

30 I i i I Near-Bottom Sa,i.ity I Southern

g 2 0 ~

Q" 10 TM

0

70 80 90 100 110 120 130 140 Year Day, 1999

Fig. 4. Time series of river flow (lst panel), tidal velocity amplitude (from the ADCP at the southern section; End panel), non-tidal velocity at the northern and southern sections (3rd panel), near-bottom salinity (4th panel), and stratification (as salinity difference between near-surface and near-bottom at the ETM location, 5th panel). Shaded zones correspond to monthly maximum spring-tide conditions (near-bottom tidal velocity ex- ceeding 70 cm s 1). The river discharge data were obtained from the U.S. Geological Survey gauges at Waterford and Co- hoes.

velocity were f o u n d to be the mos t i m p o r t a n t with respec t to s e d i m e n t r e s u s p e n s i o n a n d t ranspor t ; these t ime in te rva ls are s h a d e d in Fig. 4 a n d sub- s e q u e n t f igures to p rov ide a t e m p o r a l r e f e r e n c e for the la rge sp r ing tides.

T h e non - t i da l velocity' showed large var ia t ions d u r i n g the d e p l o y m e n t (Fig. 4) d u e b o t h to the i n f l u e n c e of s p r i n g - n e a p t idal variabil i ty a n d vari- a t ions in r iver d ischarge . T h e non - t i da l velocity was gene ra l ly l a n d w a r d at the b o t t o m a n d seaward at the surface i n d i c a t i n g the p r e d o m i n a n c e of the es- m a r i n e c i rcu la t ion . T h e n e a r - b o t t o m flow at the n o r t h e r n t ransec t b e c a m e seaward for sus ta ined pe r i ods d u r i n g h i g h r iver flow. Th i s also ind ica tes the seaward shift of the es tuary d u r i n g h i g h dis- cha rge cond i t ions . T h e r e was only a b r i e f p e r i o d (year day 801) w h e n the non - t i da l flow at the south- e r n s ta t ion was seaward; this o c c u r r e d d u r i n g ris- i ng d i scharge a n d sp r ing tides. T h e flow at this sta-

674 w . R . G e y e r e t a l .

< - 5 % . iL/ i Lj i i = = = = =

E 5 ~ j=-

15- SOUTH

; ;0 ;s 2'0 & a.0 aa km from Batter,/

Fig. 5. Salinity section during high flow, nearly spring-tide conditions on year day 103 (April 14, 1999).

t ion r e m a i n e d l a n d w a r d d u r i n g the p e a k f r e s h e t (year day 9 2 - 1 0 5 ) , d u e in p a r t to the re la t ive ly weak t ida l f lows d u r i n g t ha t in te rva l . S t r o n g ver t i - cal m i x i n g d u r i n g s p r i n g t ides t e n d to r e d u c e t h e ve r t i ca l shea r s (e.g., y e a r day 107 a n d 135) , w h i c h can l e a d to s eaward flows t h r o u g h o u t t h e e s t u a r y if t h e r e is s t r o n g e n o u g h r iver outf low.

N e a r - b o t t o m sal ini ty was i n f l u e n c e d b o t h by t h e s p r i n g - n e a p cycle a n d r iver d i s cha rge . T h e lowes t n e a r - b o t t o m sa l in i t ies o c c u r r e d fo l l owing h i g h riv- e r f low even t s (Fig. 4, 4 th p a n e l ) , p a r t i c u l a r l y at t he n o r t h e r n sec t ion , i n d i c a t i n g a s eaward shi f t in t he e s t u a r i n e s t ruc tu re . Th i s s eaward shif t in s t ruc- tu re was s t r o n g e r d u r i n g s p r i n g t ides. Ver t ica l strat- i f i ca t ion (Fig. 4, b o t t o m p a n e l ) v a r i e d f r o m 0 to 17 psu ( d i f f e r e n c e f ron t su r f ace to b o t t o m sal ini ty) a t t he ETM site, t he m a x i m u m va lues o c c u r r i n g dur - i ng n e a p t ides a n d the m i n i m a d u r i n g spr ings .

T h e a l o n g - e s t u a r y sal in i ty d i s t r i b u t i o n d u r i n g the f r e s h e t (Fig. 5) shows t h a t the e s t u a r y is p u s h e d seaward re la t ive to low flow c o n d i t i o n s . T h e sal ini ty g r a d i e n t s a r e weak t h r o u g h m o s t o f t he es tuary, b u t s t r o n g h o r i z o n t a l sa l in i ty g r a d i e n t s a re f o u n d neat" the m o u t h . Th i s is in c o n t r a s t to low f low c o n d i t i o n s , d u r i n g w h i c h the h o r i z o n t a l sa l in i ty g r a d i e n t is re la t ive ly u n i f o r m t h r o u g h o u t t he es tuary .

S u s p e n d e d s e d i m e n t c o n c e n t r a t i o n s s h o w e d d r a m a t i c t e m p o r a l a n d spa t ia l var iab i l i ty in the es- t u a r y (Fig. 6). C o n c e n t r a t i o n s g r e a t e r t h a n 5,000 m g 1 1 we re r e c o r d e d by the OBS in the n e a r - b o t - t o m wa te r s a t t h e ETM d u r i n g t h e first s p r i n g t ide , a n d c o n c e n t r a t i o n s g r e a t e r t h a n 4.000 m g 1 ~ w e r e o b s e r v e d t h e r e d u r i n g t h e m o n t h l y m a x i m u m s p r i n g t ides . A t t he n o r t h e r n a n d s o u t h e r n sec- t ions , c o n c e n t r a t i o n s we re n e a r l y an o r d e r o f mag- n i t u d e lower wi th p e a k c o n c e n t r a t i o n s r e a c h i n g 2 0 0 - 5 0 0 m g 1 ~. At all o f t h e s ta l ions t h e r e w e r e l a rge f l u c t u a t i o n s b o t h at t ida l a n d s p r i n g - n e a p t ime scales. C o n c e n t r a t i o n s d e c r e a s e d to less t h a n 100 m g 1 ~ d u r i n g s lack w a t e r a t the n o r t h e r n a n d ETM sites a n d to less t h a n 30 m g 1 1 at t h e sou th -

Near-Bottom Suspended Sediment Concentration 1000 t i = i t = i

Northern

s0

70 80 90 100 110 120 130 140 Year Day, 1999

Fig. 6. Suspended sediment time series at 1-m above bottom at the northern, ETM, and southern sections, all on the west side (where concentrations are highest). The shading indicates monthly mmxilnum spring tides, as in Fig. 3. The high-frequency variability is due to tidal resuspension.

e r n site. Mos t o f t he s u s p e n d e d s e d i m e n t a t all o f t h e si tes was a s soc i a t ed wi th t ida l r e s u s p e n s i o n .

T h e r e was a cleat" s p r i n g - n e a p a n d m o n t h l y m o d - u l a t i o n o f s u s p e n d e d s e d i m e n t , wi th p e a k c o n c e n - t r a t i o n s d u r i n g n e a p t ides a p p r o x i m a t e l y an o r d e r o f m a g n i t u d e sma l l e r t h a n t h e p e a k s d u r i n g s p r i n g t ides at all o f t he loca t ions . T h e s p r i n g - n e a p vari- a t i o n s in c o n c e n t r a t i o n s h o w e d d i f f e r e n t r e s p o n s e s at t he d i f f e r e n t s t a t i o n s - - i n p a r t i c u l a r t h e sou th - e r n t i m e ser ies was d o m i n a t e d by t h e m o n t h l y vat'- i a t ions , w h e r e a s the o t h e r s ta t ions s h o w e d sp r ing- n e a p v a r i a t i o n s m o r e c o n s i s t e n t wi th t he v a r i a t i o n s in t ida l a m p l i t u d e . Th i s d i f t ~ r e n c e may r e su l t f ron t a h i g h e r t h r e s h o l d fo r r e s u s p e n s i o n at t h e sou th - e r n s ta t ion , a l t h o u g h n o s e d i m e n t o l o g i c a l evi- d e n c e was o b t a i n e d .

T h e s u s p e n d e d s e d i m e n t c o n c e n t r a t i o n s v a r i e d s ign i f i can t ly in t he ve r t i ca l a n d l a t e r a l d i r e c t i o n s as e x e m p l i f i e d by s h i p b o a r d o b s e r v a t i o n s a t t h e n o r t h e r n a n d s o u t h e r n t r ansec t s (Fig. 3). A t b o t h t ransec ts , c o n c e n t r a t i o n s w e r e g e n e r a l l y h i g h e r on the wes t s ide. T h e n o r t h e r n t r a n s e c t is a t t he up- e s t u a r y e n d o f a s e d i m e n t t r a p p i n g z o n e t ha t ex- t e n d s a l o n g the west s ide o f t h e e s tua ry ( W o o d r u f f e t al. in p ress ) . T h e u n c o n s o l i d a t e d s e d i m e n t s a l o n g this t r a p p i n g z o n e a r e easily r e s u s p e n d e d by the t ida l c u r r e n t s a n d l e a d to h i g h e r c o n c e n t r a - t ions. T h e l a t e r a l va r i a t i ons in c o n c e n t r a t i o n at t h e s o u t h e r n t r a n s e c t were n o t as c o n s i s t e n t as t h e n o r t h e r n site, a l t h o u g h the c o n c e n t r a t i o n s w e r e g e n e r a l l y h i g h e r o n t h e west side. S o m e o f t h e h i g h e s t c o n c e n t r a t i o n s o n t h e west s ide fo r t he s o u t h e r n site o c c u r r e d d u r i n g the s p r i n g t ides a r o u n d y e a r day 135 (Fig. 6). T h e s e c o n c e n t r a t i o n s

1 i i i i i i 1 Northern

100'

0' ~ ~ - -

~ - t 0 O

-2OO

- 3 0 0 300 I I I I I I I I Southern

100

O - -

- 1 0 0 -

7'0 80 90 100 110 120 130 I;0

Year Day, 1999

Fig. 7. Sediment fltLx estimates at the northern and south- ern sections. The solid lines correspond to the west side and the dashes to the east side. Positive values indicate sediment fllLx into the estuary. Tidal fluctuations have been filtered out of these time series. Shading is the same as Fig. 3.

o f a r o u n d 500 m g 1 : c o m p a r e to on ly 100 :lag 1 1 at t h e e a s t e r n t r i p o d at t h e s a m e t ime . Th i s m a y r e f l e c t l a t e r a l d i f f e r e n c e s in loca l e rodib i l i ty , b u t g iven t h e mass o f s e d i m e n t t r a n s p o r t e d at t h e s o u t h e r n t r ansec t , it m o r e l ike ly r e f l ec t s l a t e r a l dif- f e r e n c e s in t i le s e d i m e n t supp ly f r o m the h a r b o r .

SEDIMENT FL::XES

T h e e s t i m a t e d s e d i m e n t f lux at t i le n o r t h e r n t r a n s e c t (Fig. 7, u p p e r p a n e l ) was s eaward d u r i n g m o s t o f t he f l ' e she t p e r i o d , c o n s i s t e n t wi th the sea- w a r d f low in t h e b o t t o m b o u n d a r y layer at th is site. T r a n s p o r t o n the wes t s ide was l a rgm, p a r t i c u l a r l y d u r i n g the f reshe t . Th i s was d u e b o t h to i n c r e a s e d c o n c e n t r a t i o n s a n d i n c r e a s e d seaward ve loc i t i e s on t h e sha l lower west s ide. F o l l o w i n g t h e f reshe t , sed- i m e n t f lux was g e n e r a l l y n o r t h w a r d d u e to t h e n o r t h w a r d - d i r e c t e d f low in t he b o t t o m b o u n d a r y layer. At t h e s o u t h e r n t ransec t , s e d i m e n t f lux was s ign i f i can t ly h i g h e r d u r i n g s p r i n g - t i d e i n t e rva l s w h e n r e s u s p e n s i o n was h ighes t . T r a n s p o r t was gen- e ra l ly l a n d w a r d on t h e west s ide a n d s eaward o n t h e eas t s ide, b u t t he h i g h e r c o n c e n t r a t i o n s o n t h e wes t s ide c a u s e d these f luxes to d o m i n a t e .

T h e s e d i m e n t f lux was d i v i d e d i n to two c o m p o - nen t s : an advec t ive f lux a n d a t ida l p u m p i n g f lux. T h e advec t ive f lux is d u e to t ida l ly a v e r a g e d veloc- it.}" a n d c o n c e n t r a t i o n

Qo=fff;(y,z)C(y,z)dyaz (1)

w h e r e u is t he a l o n g - c h a n n e l velocity, C is t i le con- c e n t r a t i o n , a n d the o v e r b a r s i n d i c a t e t ida l ave rages

Sediment Transport and Trapping 675

i I i i i i i i

50 tl tl #l I ~ s t [

}," ,~, [!~',* , , l l~ ' . ,z ,J ' . """ v'4~-'̂ ~ , ' .R.- ^ ,.,%- _ _ _ , , _ ~ t ~ ~ 1 - _ r - i . a ~ - - ~ - a

- 1 5 0 I I I I I I I I

II Southern. W e s t 250 - It

II 200- It

150- i l l 'i t

= 1 0 0 " I~ ! 11 I ~=~ ,rig t, I i l l t ; I l l 1

.50- I ~ �9 I I I �9 I I

- 5 0 . rl .

- t011.

70 8~ 90 100 1;0 '1~)0 130 140 Year Day, 1999

Fig. 8. Sediment fltLx at the western side of the northern and southern sections divided into non-tidal advection (solid lines) and tidal pumping (dashed lines). Shading is the same as Fig. 3.

( t he t ides a r e r e m o v e d u s i n g a 35-h f i l te r ) . T h e t ida l p u m p i n g f lux is d u e to the c o r r e l a t i o n o f t ida l f l u c t u a t i o n s o f ve loc i ty a n d c o n c e n t r a t i o n s

Qt=ffu'(>z)C'(>z)~yaz (2)

w h e r e t i le p r i m e s i n d i c a t e t e m p o r a l dev i a t i ons f r o m the t ida l averages .

T h e c o n t r i b u t i o n s o f t h e m e a n advec t ive a n d t id- al p m n p i n g s e d i m e n t f lux fo r t he w e s t e r n s t a t ions (wh ich d o m i n a t e d the overa l l s e d i m e n t f lux) a r e shown in Fig. 8. At t h e n o r t h e r n t ransec t , t h e ad- vect ive f lux was g e n e r a l l y seaward , w i th t he l a rge s t c o n t r i b u t i o n c o m i n g d u r i n g t h e f r e s h e t . T h e p m n p i n g t e r m was g e n e r a l l y n o r t h w a r d i n d i c a t i n g h i g h e r c o n c e n t r a t i o n s d u r i n g f l o o d t h a n e b b t ides . Th i s f lux m i g h t be e x p l a i n e d by h i g h e r c o n c e n t r a - t ions o f s e d i m e n t to t i le s o u t h o f th is t r a n s e c t con- s i s t en t wi th t he p o s i t i o n o f t he ETM.

A t the s o u t h e r n t ransec t , fi le advec t ive s e d i m e n t f lux was va r i ab le , b u t g e n e r a l l y n o r t h w a r d d u r i n g n e a p t ides a n d s o u t h w a r d d u r i n g s p r i n g t ides. Th i s r e f l ec t s t he va r i ab i l i ty o f t h e e s t u a r i n e c i r c u l a t i o n wi th t he s t r o n g e s t b o t t o m in f low o c c u r r i n g d u r i n g n e a p t ides. T h e t ida l p u m p i n g t e r m s h o w e d a m u c h l a r g e r s p r i n g - n e a p cycle, w i th s t r o n g pu l s e s o f in f low d u r i n g l a rge s p r i n g t ides . Th is p a t t e r n s u g g e s t s i n t e n s e r e s u s p e n s i o n d u r i n g f l o o d i n g s p r i n g t ides to f i le s o u t h o f this s t a t ion wi th in t h e h a r b o r p r o d u c i n g t i le l a rge n o r t h w a r d f lux at th is t r ansec t . T h e sp r ing - t i de p u m p i n g s ignal m a y b e e x p l a i n e d by a l a rge s o u r c e o f easi ly e r o d i b l e sed- i m e n t in the harboL; w h i c h w o u l d have b e e n de- p o s i t e d d u r i n g an e a r l i e r f r e s h e t event , p e r h a p s in

676 w . R . G e y e r e t a l .

3 0 0 , 0 0 0 I I I I I I I

2 0 0 , 0 0 0 " , , ~ " a J ' ' e * ' : , " �9

100,000 , Southern

-100,000 70 80 90 100 110 120 130 140

Fig. 9. Integrated sediment flux at the northern and south- ern sections for the period of reliable suspended sediment data. P~sitive values indicate northward (landward) flux. The shaded area on either side of the solid lines indicates the uncertainty of the estimate associated with different assumptions about the vertical distribution of sediment on the west side (see text). At the northern section, the net sediment flux was southward (sea- ward) during the freshet (year day 80-100), but the northward transport later in the record exceeded the initial southward mass filL':. At the southern section, the net flux was northward throughout the record, with mmximum sediment inflmr occur- ring during the large spring tides.

1998. T h e r e was on ly m o d e s t e v i d e n c e o f p u m p i n g on the eas t s ide at t i le Bat tery , a n d c o n c e n t r a t i o n s t h e r e w e r e s i g n i f i c a n t l y l o w e r ; a p p a r e n t l y t h i s s o u r c e o f s e d i m e n t was c o n f i n e d to t h e west s ide o f t i le u p p e r h a r b o r .

T i l e s e d i m e n t f luxes were i n t e g r a t e d over t he l e n g t h o f t he d e p l o y m e n t to d e t e r m i n e the to ta l mass o f s e d i m e n t t r a n s p o r t e d p a s t e a c h s ec t i on (Fig. 9). T h e u n c e r t a i n t y a s soc i a t ed wi th t he ver t i - cal d i s t r i b u t i o n o f s e d i m e n t is r e f l e c t e d in t he s h a d e d r e g i o n in e a c h p lo t . T h e u p p e r l imi t as- s u m e s an e - f o l d i n g scale o f 15 m, a n d the lower l imi t an e - f o l d i n g scale o f 5 m, c o r r e s p o n d i n g wi th t he 95~, c o n f i d e n c e l imi ts o f t he o b s e r v e d ve r t i ca l d i s t r i bu t i ons . T h e e r r o r s a s s o c i a t e d wi th l a t e r a l vat'- labi l i ty a r e n o t r e ad i l y e s t i m a t e d , b e c a u s e t h e r e were t oo few r e a l i z a t i o n s o f t h e l a t e r a l s u s p e n d e d s e d i m e n t s t r u c t u r e to o b t a i n m e a n i n g f u l statistics. As a c o n s e q u e n c e , t i le va lues o f n e t s e d i m e n t f lux s h o u l d b e t r e a t e d wi th c a u t i o n , a c k n o w l e d g i n g tha t t he t ime v a r i a t i o n o f t r ansve r se s u s p e n d e d sedi- m e n t s t r u c t u r e a u g m e n t s t h e u n c e r t a i n t y o f the n e t f lux es t ima tes .

Even in l i g h t o f t he u n c e r t a i n t y o f t h e m e a s u r e - men t s , t he n e t f lux e s t ima te s shown in Fig. 9 cleat'- ly dev ia t e f r o m the e x p e c t a t i o n o f a n e t s o u t h w a r d s e d i m e n t f lux t h r o u g h the es tuary . T h e n o r t h e r n t r a n s e c t h a d a n e t s eaward f lux d u r i n g the f r e s h e t (day 8 0 - 1 0 5 ) as e x p e c t e d , b u t t he s o u t h e r n t ran- sect e x h i b i t e d a n e t l a n d w a r d f lux even d u r i n g this p e r i o d . A f t e r t h e f l 'eshet , t he n e t f lux was l a n d w a r d at b o t h t ransec ts . T h e i n t e g r a t e d t r a n s p o r t a t t he n o r t h e r n t r a n s e c t over t h e p e r i o d c a r r i e d a mass o f 65,000 _+ 30,000 tons to t h e n o r t h w a r d . At t h e s o u t h e r n t r ansec t , t h e i n t e g r a t e d t r a n s p o r t t o t a l e d 280,000 _+ 70,000 to l ls n o r t h w a r d . Even wi th t he

l a rge u n c e r t a i n t y o f t h e s e f lux ca l cu la t ions , t he f lux at b o t h s t a t ions was c lear ly l a n d w a r d , a n d t h e r e was a s ign i f i can t n e t c o n v e r g e n c e i n to t he e s t u a r y o f r o u g h l y 200,000 tons ove r t he in te rva l . T h e f lnxes at t i le s o u t h e r n t r a n s e c t w e r e pa r t i cu - lar ly e n h a n c e d d u r i n g the two l a rge s p r i n g t ides fo l lowing the f reshe t . T h e s e two in t e rva l s p r o v i d e d r o u g h l y ha l f o f t he n e t n o r t h w a r d flux.

D i s c u s s i o n

SEDIMENT INPUT FROM THE WATERSHED

W o o d r l f f f e t al. ( in press) c a l c u l a t e d b a s e d on h i s to r i ca l s e d i m e n t l o a d d a t a a n d r iver d i s c h a r g e o b s e r v a t i o n s t ha t 90,000 t ons o f s e d i m e n t we re de- l ive red f r o m the H u d s o n River w a t e r s h e d b e t w e e n J a n u a r y a n d J u n e 1999. D u r i n g the d e p l o y m e n t pe- r i o d b e t w e e n M a r c h a n d J u n e , a p p r o x i m a t e l y 5 0 , 0 0 0 t o n s o f s e d i m e n t w e r e d e l i v e r e d . T h e m o o r e d f lux c a l c u l a t i o n s at t i le n o r t h e r n t r a n s e c t i n d i c a t e a n e t s eaward f lux o f a p p r o x i m a t e l y tha t m a g n i n l d e d u r i n g t h e f r e s h e t p e r i o d (un t i l yea r day 90; Fig. 9). F o l l o w i n g t h e f reshe t , t he d i r e c t i o n o f t r a n s p o r t r e v e r s e d p r o d u c i n g a n e t l a n d w a r d f lux o f n l o r e t h a n 50,000 tons over t he o b s e r v a t i o n p e r i o d . I t a p p e a r s t ha t in 1999, t h e r e was a n e t t r a p p i n g o f a t l eas t 100,000 tons o f s e d i m e n t in t he u p p e r e s t ua ry a n d r iver d u r i n g the o b s e r v a t i o n pe- r iod . A l t h o u g h smal l re la t ive to t h e 200,000 tons t ha t was t r a p p e d in t h e e s t u a r y ( b a s e d o n the a b o v e f lux c a l c u l a t i o n s ) , t he H u d s o n River was par- t i cu la r ly ine f fec t ive at t r a n s p o r t i n g s e d i m e n t sea- wa rd d u r i n g the s p r i n g o f 1999.

T h e s p r i n g f r e s h e t o f 1999 was re la t ive ly smal l c o m p a r e d to t he l o n g - t e r m average , f a l l ing wi th in t he lowest 5 th p e r c e n t i l e wi th r e s p e c t to p e a k f low b a s e d on t h e last 54 yea r s o f data . 1998 was in t he h i g h e s t 10th p e r c e n t i l e , wi th a m a j o r f l o o d occur- r i n g in J a n u a r y 1998. T h e h i g h p e a k f low in 1998 w o u l d have b e e n m u c h m o r e ef fec t ive at t rans- p o r t i n g s e d i m e n t seaward t h a n the m o d e s t dis- c h a r g e o f 1999 d u e b o t h to t he d i r e c t i n f l u e n c e o f t he s t r o n g s eaward a d v e c t i o n a n d the i n d i r e c t ef- fec t o f p u s h i n g the sa l in i ty i n t r u s i o n seaward . As i n d i c a t e d in b o t h t he t i m e ser ies d a t a (Fig. 4) a n d the sa l in i ty s ec t ion (Fig. 5), t he sa l in i ty i n t r u s i o n was d i s p l a c e d s eaward d u r i n g p e r i o d s o f h i g h r iver outf low. As t h e sal t was p u s h e d seaward , t he n e t n e a r - b o t t o m f low w o u l d b e c o m e seaward t h r o u g h m o s t o r p e r h a p s t i le e n t i r e lower e s t u a r y d u e to t he h i g h e r flows a n d l o n g e r p e r s i s t e n c e o f t he 1998 f reshe t . Th i s seaward , n e a r - b o t t o m f low c o u l d h a v e p r o d u c e d a n e t s e a w a r d s e d i m e n t f l u x t h r o u g h o u t t h e f r e s h e t in 1998. A l a rge e x p o r t o f s e d i m e n t d u r i n g the f r e s h e t o f 1998 m a y h e l p ex- p l a i n t he l a rge i n p u t o f s e d i m e n t f r o m t h e h a r b o r in 1999. A n e s t i m a t e d 500,000 t ons o f s e d i m e n t

e n t e r e d f r o m the wa te r shed be tween J a n u a r y a nd J u n e in 1998 ( W o o d r u f f et al. in press) , a signifi- can t f rac t ion o f wh ich may have set t led in the hat'- bor. S o m e o f this newly depos i t ed s e d i m e n t was then r emob i l i z ed in 1999 p rov id ing the source fo r the l andward t r anspo r t o b s e r v e d at the s o u t h e r n t ransec t d u r i n g the spr ing o f 1999.

THE COMBINED EFFECT OF TIDES AND DISCHARGE

O n e fac to r c o n t r i b u t i n g to the weak s e d i m e n t e x p o r t d u r i n g the 1999 f r e she t was the t int ing o f the f reshe t relative to the sp r ing-neap cycle. T h e peak o f the f r e she t o c c u r r e d d u r i n g n e a p tides, wh ich effectively m i n i m i z e d the seaward e x p o r t o f s e d i m e n t due to two effects. T h e r e is a lmost an orders-of-magni tude var ia t ion in s u s p e n d e d sedi- m e n t c o n c e n t r a t i o n be tween n e a p and spr ing tides, so the seaward advec t ion associated with the f reshe t wou ld have the least i m p a c t on s e d i m e n t t r anspor t d u r i n g n e a p condi t ions . T h e g rea t e r a m o u n t o f m i x i n g d u r i n g spr ing t ides tends to re- d u c e the m a g n i t u d e o f the es tuar ine c i rcula t ion (Geyer et al. 2000), wh ich leads to a s t ronge r sea- ward c o m p o n e n t o f the n e a r - b o t t o m flow if the f reshe t occurs d u r i n g spr ing tides. Even with the relatively small r iver ou t f low obse rved in 1999, sig- n i f icant e x p o r t o f s e d i m e n t w o u l d have o c c u r r e d if the p e a k o f the f reshe t h a d o c c u r r e d d u r i n g spr ing tides.

SEDIMENT TRAPPING IN THE LOWER ESTUARY

T h e sed imen to log ica l studies by M~oodruff et al. (in press) ind ica te a seasonal var ia t ion o f s e d i m e n t t r app ing in the lower H u d s o n River estuary, shown schemat ica l ly in Fig. 10. R e c e n t s e d i m e n t deposi- t ion was ident i f ied by the p r e s e n c e o f Be-7 (half- life 53 d) in the surficial sed iment . D u r i n g the f reshe t p e r i o d o f 1999, a p p r o x i m a t e l y 100,000 tons o f s e d i m e n t were f o u n d in the seaward p o r t i o n o f the es tuary in the r eg ion o f m a x i m u m hor i zon ta l salinity g r a d i e n t d u r i n g that h igh-f low p e r i o d (cf., Fig. 5), a n d no new s e d i m e n t was f o u n d in the ETM z o n e excep t a local ized depos i t n e a r the bank. In J u n e 1999, a m u c h la rger deposi t , to ta l ing 300,000 tons, was f o u n d in the ETM zone, a n d the seaward r e a c h was now devo id o f sed iment . This seasonal shift in depos i t ion is cons is ten t with the o b s e r v e d var ia t ions in s e d i m e n t f lux with m o r e sea- ward t r anspo r t d u r i n g f reshe t cond i t i ons and land- ward t r anspo r t in the s u b s e q u e n t low flow. This seasonal pa t t e rn o f s e d i m e n t depos i t ion has also b e e n o b s e r v e d in the G i r o n d e es tuary by Mign io t (1971), and similar m o v e m e n t o f the ETM was ob- served in the T a m a r and Weser estuaries by Gra- b e m a n n et al. (1997). T h e c h a n g e s in pos i t ion o f s e d i m e n t t r a p p i n g pr incipal ly ref lect the c h a n g e in pos i t ion o f n e a r - b o t t o m c o n v e r g e n c e due to vari-

30 I

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Sediment Transport and Trapping 677

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g o

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10 15 20 25 30 35 ETM NORTH

5 10 15 20 2 5 30 35 SOUTH ETM Distance from Battery {kin) NORTH

Fig. 10. Short-term bed sediment accumulation observed in April 1999 (upper panel) and June 1999 (lower panel), based on Be-7 data fl'om Woodruff et al. (in press). The numbers indicate inventories and integrated fltLxes in thousands of t(ms. The integrated fltL,:es for the upper panel represent the intel'val of year days 70-105 and for the lower panel year days 105-165.

at ions in pos i t ion o f the salinig' i n t rus ion as a func- t ion o f d ischarge .

T h e new s e d i m e n t tha t was d e p o s i t e d in file H u d s o n River d id n o t necessari ly c o m e f r o m out- side the e s t u a r y - - i t cou ld have b e e n e r o d e d within the es tuary a n d e x p o s e d to Be-7 in the water col- u m n before b e i n g re-depos i ted . A s ignif icant frac- t ion o f the o b s e r v e d depos i t ion appea r s to have e n t e r e d f r o m the harbor . A p p r o x i m a t e l y 280,000 tons were t r a n s p o r t e d in to the es tuary f r o m the h a r b o r be tween year day 70 a nd 140, a n d a n o t h e r 100,000 tons were p robab ly t r a n s p o r t e d over file nex t 24 d p r e c e d i n g the co r ing study, a l t h o u g h the fou l ing o f the OBS sensors p r e c l u d e d the accura te es t imate o f this la t ter flux. Virtually all o f the newly a c c u m u l a t e d s e d i m e n t wi thin the es tuary cou ld be the resul t o f the i n p u t f r o m the harbor . This find- ing is cons i s ten t with the analysis o f F e n s et al. (1999) o f Be-7 and Th-234 ratios in the H u d s o n River estuary, wh ich suggests tha t the s e d i m e n t ac- c u m u l a t i n g in the ETM of the H u d s o n o r ig ina ted f r o m the Harbor . This a p p r o x i m a t e mass ba lance also provides s o m e s u p p o r t fo r the quant i ta t ive as- sessments o f s e d i m e n t f lux p r e s e n t e d here .

SHORT-TERM DEPOSITION VERSUS LONG-TERM ACCUMULATION

T h e rate o f s e d i m e n t depos i t ion der ived b o t h by the f lux m e a s u r e m e n t s and by the W o o d r u f f et al. (in press) es t imates o f new s e d i m e n t a t i o n great ly exceed the es t imates by Olsen et al. (1978) o f long- t e rm s e d i m e n t a c c u m u l a t i o n based on Pb-210 an d Cs-137 o f 1 -2 m m yr ~. If the o b s e r v e d s e d i m e n t

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Fig. 11. Historical changes in bed elevation at two sites with- m the ETM, between 1865 and 1998. The changes in deptl~ in the last 68 years do not indicate any significant infilling, except for tl~e oh'edged channel on tl~e west side at km 14. The deposits indicated in Fig. 10 and observed by Woodruff et al. (in press) were generally outside the regions of historical ohedging.

convergence were distr ibuted uni formly over the surface area of the estuary, it would a m o u n t to ap- proximate ly 8 cm of net deposi t ion (based on a bulk density of 400 kg m -s dr), weight and an a rea of the lower estuary of 3 • 10 v m2). T h e results of Woodru f f et al. (in press) indicate that the depo- sition is not un i fo rm, but ra ther it is focused in discrete zones within the ETM with local accumu- lations of as m u c h as $5 cm over the Apr i l - June interval.

Addi t ional suppor t for Olsen 's modes t estimate of long- term sed imen t accumula t ion comes fi-om historical ba thymet ry data (Fig. 11). Over the last 150 yr, the depth profile of the estuary has been relatively constant with mino r changes a~ssociated with shorel ine modif icat ion. Dredging was a signif- icant factor in the mass balance dur ing the 1960s and 1970s, when approx imate ly g00,000 tons were dredged annual ly f rom a channel within the ETM (Gross 1974). Dredging in this par t of the river has been discont inued as of 1994 and was no t signifi- cant dur ing the late 1800s and early 1900s. It ap- pears that dredging causes an intensif ication of sed iment t rapping due to artificially deepen ing the channel , a l though as the channel re turns to its nat- ural cross-sectional shape, it mainta ins an equilib- r ium mass balance on decadal or longer t ime scales based both on Olsen et al. 's (1978) radio- nuclide data and the long- term bathymetr ic data.

T h e large accumula t ion of s ed imen t in the es- tuary observed in 1999 is not consistent with this l o n g - t e r m e q u i l i b r i u m . Even on d e c a d a l t ime scales, the rapid accumula t ion of s ed imen t would result in significant changes in the channel mor- phology that would al ter the hydrodynamic re- gime. Al though not observed in this stud?,, there must be erosion events at roughly decadal t ime scales that expor t sed iment to the ocean and main-

tain the long- term equil ibrium. T h e mos t plausible mechan i sm for these expor t events is large freshet events, part icularly those that occur during spring t ides. Given a r a n d o m o c c u r r e n c e of f r e s h e t events, abou t 95% of them should occur within the several days of the for tn ight with spr ing tides. It is likely that at least once per decade, a large freshet event will occur dur ing spring tides, providing a large expor t of sed iment f rom the estuary. T h e large event in 1998 is one potent ia l e x a m p l e - - i t certainly had the ampl i tude and the t iming with respect to the s p r i n g - n e a p cycle to accompl ish a significant a m o u n t of erosion. T h e magn i tude of sed iment expor t associated with this event is un- known, and it is difficult to assess the potent ia l magn i tude of erosion, particularly if the sed iment has u n d e r g o n e consolidation. T h e overall mass balance of the system requires that the episodic expor t of s ed imen t ma tch the shor te r - te rm accu- mulat ion, and the most likely m e c h a n i s m for such expor t events appears to be high discharge com- bined with high tidal range.

C o n c l u s i o n s

This set of observat ions in the H u d s o n River es- tuary suppor ts Meade ' s (1969) observat ion that unde r no rma l flow condit ions estuaries tend to im- por t s ed imen t f rom the seaward direction. T h e ob- servations also indicate that the direct ion of sedi- m e n t tim-{ is control led by the river flow while its magn i tude is control led by the spr ing-neap varia- tion in tidal ampli tude. Sed imen t expor t front es- tuaries is expected to occur when spring tides co- incide with s t rong river outflow. During the 1999 observations, the peak river flow occur red dur ing neap tides, and there was virtually no expor t of sed iment fi-om the estuary. T h e t rapping of sedi- m e n t within the estuary observed in 1999 is incon- sistent with the long- term morpologica l equilibri- um of the estuary, which is evident f rom historical bathymetr ic data ex tend ing over the last 150 ?,ears. Major sed iment expor t events must occur episod- ically, in order to mainta in the estuary at a roughly un i fo rm depth. T h e events most likely to accom- plish the major episodes of sed iment expor t are large f reshet events that occur dur ing spring tides.

ACKNOWLEDGMENTS

This research was supported by the Hudson River Foundation Grant 004/98A and fl~e Office of Naval Research Harbor Pro- cesses Program award N00014-99-0039. This is Wc_~ods Hole OceanogTaphic Institution contrikaltion 10285.

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Received for consideq'atio~, Az~g~st 25, 2000 Accepted for pz~blication, April 23, 2001