Wind-forced salt intrusion into a tributary estuary

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. C8, PAGES 13,357-13,371, AUGUST 15, 1990

Wind-Forced Salt Intrusion Into a Tributary Estuary

LAWRENCE P. SANFORD AND WILLIAM C. BOICOURT

University of Maryland System Center for Environmental and Estuarine Studies Horn Point Environmental Laboratories, Cambridge, Maryland

Moored measurements and hydrographic surveys were carried out during the summers of 1986 and 1987 to examine interaction between the mainstem of the Chesapeake Bay and the Choptank River, an eastern shore tributary estuary. The data show that an important mode of interaction is through wind-forced intrusion of saline, hypoxic water from below the pycnocline of the Bay into the lower river. Intrusions are driven by lateral tilting of the pycnocline in the Bay, when high salinity water is upwelled on the eastern side of the Bay in response to a southward pulse of wind stress. The resulting internal surges propagate up the relict Choptank entrance channel at a speed of about 20 cm/s and spill onto the broad sill inside the mouth of the river. Intrusion-favorable pycnocline tilts in the Bay do not always result in lower layer intrusion into the Choptank, but may be blocked or choked in the entrance channel on occasion. The data suggest that wind-forced intrusion of salt leads to increased gravitational circulation in the Choptank during the summer months, providing a mechanism through which high frequency energy may be directly translated into lower frequency motion.

1. INTRODUCTION

Tributary estuaries, or estuaries that empty into larger estuaries, can be quite sensitive to changes in their seaward boundary conditions [Carter and Pritchard, 1988]. Circulation patterns in the tributary estuaries of the northern Chesa- peakc Bay are controlled largely by the salinity of the adjacent Bay, which is itself controlled by freshwater inflow from the Susquehanna River [$chubel and Pritchard, 1987]. Bottom water intrusions over the entrance sill to Puget Sound are modulated by deep water salinity variation in the adjacent Strait of Juan de Fuca, which appears to be driven by coastal storms [Cannon et al., 1990]. Sea level fluctuations and near-bottom currents in the lower Potomac River

are related more to wind-driven motion in the adjacent Chesapeake Bay than to local wind driving [Wang and Elliott, i978]. An important aspect of all of these interactions is that salinity at the seaward boundary of a tributary estuary is frequently more variable than salinity at the seaward boundary of a primary estuary.

In this paper, we consider interactions that occur during the summertime across the seaward boundary of the Chop- tank River, an Eastern Shore tributary of the Chesapeake Bay (Figure 1). In particular, we focus on processes that are responsible for the intrusion of saline waters from below the pyenoeline of the mainstem Bay into the lower River. In fjords, intrusions o f relatively dense water over the entrance sill have been attributed with increased flushing and preven- tion of anoxie conditions in deep interior waters [e.g., Gade and Edwards, 1980; Cannon et al., 1990]. In the mid-Chesa- peake Bay, however, subpyenoeline waters are generally anoxie during the summertime [O•'•cer et al., 1984; Malone et al., 1988], such that intrusion of these waters into the productive tributaries that border the mid-Bay has been

Copyright 1990 by the American Geophysical Union.

Paper number 90JC00717. 0148-0227/90/90JC-00717505.00

implicated as a probable cause of deteriorating water quality in the tributaries [Seliger et al., 1985]. Concern over this possibility, fueled by an apparent observation of anoxie intrusion well into the Choptank River in the summer of 1984 [Seliger et al., 1985; Magnien, 1987], motivated the observational programs that are reported on here.

Lower layer intrusion from the Bay into the Choptank is complicated by the bathymetry of the lower River and adjacent Bay (Figures 1 and 2). The lower Choptank stretches from the Cambridge bridge downstream to the mouth of the river [e.g., Ward and Twilley, 1986]. From a surface perspective, the mouth is defined by a line drawn between Blackwalnut Point at the southern tip of Tilghman Island and Cook Point. However, a closer examination of the bathymetry of the Bay near the Choptank shows a relict entrance channel stretching about 15 km southwest from the mouth of the Choptank, separated from the Bay by Sharps Island Bar. Sharps Island Bar has an average depth of about 3 m; in fact, Sharps Island has become a subtidal feature only within the recent past, through very rapid shoreline erosion [Hanks, 1975]. The pycnoeline in the Chesapeake Bay is usually at or below 9 m depth in the summertime [Carter and Pritchard, 1988], so that lower layer water from the Bay can only enter the Choptank by travelling up this entrance channel. This water is forced through a narrow contraction 8 km outside of the mouth of the river, and must pass over a secondary sill with a minimum depth of 11 m at 3 km outside of the mouth of the river. When it reaches the

head of the channel just inside the mouth of the river, it is further impeded by the primary Choptank sill where the depth decreases from 15 m to 7 m. This broad, shallow sill extends over another 5 km of river bottom before a true

channel begins to reappear. At the outset of our study, little was known about

circulation in the Choptank during the summer. Previous observations of the hydrographic structure [Ward and Twilley, 1986] had shown the lower River to be fairly well mixed (surface to bottom salinity differences of < 2 psu) with a weak longitudinal salinity gradient (OS/0y < 0.1

13,357

13,358 SANFORD AND BOICOURT: WIND-FORCED SALT INTRUSION I•ro A TRIBUTARY ESTUARY

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SANFORD AND BOICOURT: WIND-FORCED SALT INTRUSION INTO A TRIBUTARY ESTUARY 13,359

1986 Moored Array 1987 Moored Array Chesapeake Bay Choptank River Chesapeake Bay Choptank River

o o

-5 -5

-20 -20

-25 -25

-•0 -•0 ,-. i-. ,.. i.. , .. i.. i .. i .. i.. i .. i.. i.. i . . i .. i .. -27 -24 -21 -18 -15 -12 -9 -8 -3 0 3 8 9 12 15 -27 -24 -21 -1• -15 -12 -9 -8 -3 0 3 8 9 12 15

a Distance from Mouth (kin) b Distance from Mouth (km) Fiõ. 2. (a) ;[986 moored array, along the transect of the mooring locations in Figure 1 and the axis of the Choptank River. The deep hole at km 14 is at Castle Haven in Figure 1 and the dotted line shows the minimum depth of Shaq•s Island Bar, which separates the relict Choptank entrance channel from the Chesapeake Bay. (b) 1987 moored array.

psu/km). Under similar circumstances, Wang and Elliott [1978] had shown subtidal variability in the lower Potomac River to be dominated by wind-forced fluctuations over time scales of 2-20 days. Thus, it was anticipated that subtidal velocities strong enough to produce advective intrusion of saline, hypoxic water from the mainstem of the Bay well into the Choptank River (a distance of about 20 km) in a matter of hours or days would be more likely associated with wind- forced motion than with the gravitational circulation. That being the case, intrusions would be modulated at wind-forced time scales. The observational plan was designed according- ly. The results reported herein verify this hypothesis, reveal the dynamics of the intrusions, and suggest that wind-forced intrusions may have important consequences for the gravitational circulation in the Choptank River.

2. METHODS

Moored observations were made at three stations in the

Choptank entrance channel and the lower estuary from late June through late September, 1986. The mooring program was modified and expanded slightly in 1987, with the addition of several sensors and a fourth mooring on the western shelf of the Bay. Measurements wcrc made from mid-June to mid-September in 1987. Mooring plans for both years arc shown in Figures 1 and 2. Most current measurements wcrc made with Endcco 174 and SSM ducted impcllor-vanc current meters, but Aandcraa Savonius rotor-vane meters wcrc used occasionally for the lower layer measurements in 1986 and an InterOcean S4 E/M meter was mounted on a

bottom tripod at site CS1 from July, 1986 on. DO was measured with Endcco Pulsed DO Sensors. All current

meters wcrc equipped with temperature and conductivity (T/C) sensors. Additional T/C measurements wcrc attempted at mid-depth in 1987 using Sea Data Conductivity, Temperature and Depth Recorders (CTDRs). Data return was good in 1986 and adequate in 1987. Instrumentation failure in 1987 caused the loss of much of the velocity information from the Endcco SSM's and the loss of all ofthc

conductivity information from the Sea Data CTDR's, but the

T/C sensors on the current meters and the DO sensors all

functioned well. Calibration procedures, data processing procedures, and data return arc described in more detail by Sanford and Boicourt [1990].

A hydrographic survey plan was designed for the Chop- tank entrance channel and the lower River from mooring CE1 to the Choptank River Bridge at Cambridge, with 26 stations in 1986 and 27 stations in 1987. Surveys wcrc attempted at approximately semiweekly intervals, using Hydrolab Surveyor II's to measure T, C, DO, pH, and oxidation-reduction potential. Survey coverage was reason- ably complete, with an average of 22 stations completed per survey in both years at an average interval of :5 days in 1986 and 4 days in 1987. Salinity and DO data as a function of depth and location for each survey wcrc griddcd and contoured using a commercially available contouring package, SURFER (Golden Software, Golden, Colorado). Average distributions of salinity and DO for each of the summers of 1986 and 1987 also wcrc calculated from the griddcd data sets, using the results of all surveys with sufficient spatial coverage.

Wind data wcrc obtained from the NO AA weather station

at Baltimore Washington International Airport (BWI), and tidal height data wcrc obtained from the NOS gauge in Cambridge; these locations arc shown in Figure 1. Wind data also wcrc obtained from the weather station at Patuxcnt NAS and from BG&E weather stations at Calvert Cliffs

Nuclear Power Plant. Comparisons of 34 hr low-pass filtered wind data from BWI, Patuxcnt NAS, and Calvert Cliffs showed no major, repeatable differences. The BWI wind data appeared to bc slightly more consistent with the observed velocity and salt time series. BWI wind data arc used exclusively for the analyses presented here.

3. RESULTS

3.1. Hydrographic Structure and the Relationship Between Salt and DO

Average axial distributions of salt and DO for 1986 and 1987 arc shown in Figure 3. These represent a vertical slice

13,360 SANFORD AND BOICOURT: WIND-FORCED SALT INTRUSION II•rro A TRIBUTARY ESTUARY

' ? 7 7 ? 7

7 ? 7 7 ?

' • 7 7 ? 7

SANFORD AND BOICOURT; WIND-FORCED SALT INTRUSION INTO A TRIBUTARY ESTUARY 13,361

up the axis of the river, calculated from those hydrographic survey stations closest to the centerline of the channel. There are several conclusions to be drawn from an examination of these distributions:

1. The influence of the entrance channel bathymetry is clear. The secondary sill at -3 km clearly blocks intrusion of the most saline ( > 18 psu in 1986, > 17 psu in 1987), most hypoxie (< 2 mg/1) lower layer water from the Bay. The primary sill at 3 km also acts as an effective block to the intrusion of subpyenoeline water from the mainstem of the Bay ( > 15.5 psu, < 5 mg/1), though there is a clear indica- tion of some intrusion over the edge of the sill (15 psu isohaline in 1986, 14.5 psu isohaline in 1987).

2. There is a high inverse spatial correlation between average salt and average DO in the lower layer of the Choptank entrance channel (> 5 m depth, < km 0). Advection of low DO by intrusion of lower layer water from the mainstem of the Bay thus appears to control spatial variability of DO in this region.

3. Well inside the mouth ofthe river ( > km 15) and near the surface (depths < 4 m) everywhere, there is no obvious spatial correlation between salt and DO, at least in the mean. This would argue that the distribution of DO in these locations is due mostlyto local production and consumption of DO, though occasional intrusion of high salt/low DO well into the River was observed.

4. There are two notable interannual differences in the

mean salinity distributions. First, the 1987 distribution is 0.5- 1 psu fresher than the 1986 distribution. Second, the mean position of the halocline in the mainstem ofthe Bay is higher in the water column in 1986 than in 1987. Defining this position as the mean depth of the 17.5 psu isohaline in 1986 and the 17 psu isohaline in 1987, the halocline was at about 7 m depth in 1986 and at about 12 m depth in 1987.

5. Interannual differences between the DO distributions

are less than the differences between the salt distributions.

Inside the mouth of the river, the DO distributions for 1986 and 1987 are virtually identical. Outside the mouth, the oxyeline position closely matches the halocline position, but the isohaline associated with a particular value of DO changes; in 1986, 17 psu corresponds to 4 mg/1, while 16 psu corresponds to 4 mg/1 in 1987.

Mean plan view distributions of near-bottom salinity and DO for 1986 and 1987 are shown in Figure 4. These represent the horizontal distribution of average near-bottom salt and DO calculated from the deepest points sampled during the hydrographic surveys. For the most part, these distributions confirm the impressions gained from the axial distributions in Figure 3, and also confirm the initial expectation that the Choptank entrance channel acts as a localized source of high salt and low DO for the lower river. The distributions of both salt and DO, but particularly that of DO, show that the head of the entrance channel at the mouth of the river

has the character of a point source of high salt/low DO for the near-bottom environment in the lower river. The

distributions tend to be elongated in the channel axis direction, consistent with a combination of upstream advection and radial dispersion. Apparent disagreement between absolute near-bottom salt and DO values in Figures 3 and 4 is an artifact of the different contouring schemes; the axial distributions are extrapolated to the bottom of the river, while the plan near-bottom distributions are contoured at the lowest depth actually sampled (usually 1-1.5 m above the bottom).

The high spatial correlation between the average salinity and DO distributions near the mouth of the river is even

more apparent in individual hydrographic surveys, and it translates directly into a high, inverse temporal correlation in simultaneous moored observations of salinity and DO. The combination of spatial and temporal correlation estab- lish advection as the dominant mode of DO variability near the mouth of the Choptank. In the remainder of this paper, we concentrate on exploration and explanation of the physical processes responsible for advection.

3.2. Kinematics of Lower Layer Intrusions

Figures 5a and 5b show progressive vector diagrams (PVDs) calculated from velocity measured at mooring CE2 during the first deployment in 1986. The surface layer record in Figure 5a is dominated by the semidiurnal tide, with a small net movement out of the river (at about 240 deg T) and an apparent cross-channel net motion that is not relevant to the present problem. The lower layer record in Figure 5b is characterized by periods of tidal oscillation with slow inward motion, punctuated by 1-2 day pulses that represent substantial transport into the river. Three pulses (7/10-12, 7/15-16, and 7/19-20) account for about two thirds of the total transport into the river during the first three weeks of July.

The ratio of the lower layer average velocity to the upper layer average velocity, (2.3 em/s/0.5 em/s = 4.6), approximately balances the ratio of the cross-sectional areas of the upper and lower layers (34000 m2/7600 m 2 = 4.5) if 9 m is chosen as the average divisor between upper and lower layers at CE2. The average inward excursion during the pulses shown in Figure 5b, multiplied by a cross-sectional area of about 7600 m 2 below 9 m, amounts to a total transport of about 1 x 10 a m 3 at an average rate over a 2 day period of 5.8 x 102 m3/s. This represents a volume of water extending about 15 km inward from the mouth of the river below the

5 m isobath (approximately to the deep hole at Castle Haven), neglecting mixing.

Figures 5c and 5d show the salinity records that corre- spond to the velocity records o f Figures 5a and 5b; again, the record from the surface layer (Figure 5c) shows very little activity, while the record from the lower layer (Figure 5d) shows marked pulses of high salinity. The timing of the high salinity pulses corresponds closely to timing of the intrusions in Figure 5b.

A more complete picture of the nature of lower layer intrusion into the Choptank in 1986 is presented in Figure 6, where subtidal (low-pass filtered; Lanezos taper with a 34 hr half-power point) records of BWI wind, Cambridge tidal height, and principal axis velocity and salt from the moorings near the mouth of the river are shown for the entire period of the 1986 observational program. Several additional characteristics of lower layer intrusions are apparent from this figure:

1. Inward velocity pulses in the lower layer at the mouth of the river (CE2) are nearly coincidental with high salinity pulses, with salinity lagging slightly behind velocity. In general, the initial increase in salinity is more rapid than the decrease, and the initial inward pulse of lower layer velocity is stronger than any subsequent outward motion; in fact, only half of the major intrusions show any appreciable outward velocity in the lower layer du•ing the period of decreasing salinity.

13,362 SANFORD AND BOICOURT: WIND-FoRcED SALT INTRUSION I•o A TRIBUTARY ESTUARY

1986 Average Near Bottom Salinity 1987 Average Near BottOm Salinity , ,

1986 Average Near Bottom Dissolved Oxygen 1987 Average Near Bottom Dissolved Oxygen

Fig. 4. Average plan view distributions of salinity and dissolved oxygen for the summers of 1986 and 1987, calculated by averaging the results of all hydrographic surveys with near-complete mapping coverage. The crosses show measurement locations in a complete survey. Salinity in psu, DO in mg/l.

2. The strength and character of the salt intrusions changes markedly between the mouth of the river (CE2) and the top of the sill (CS1). The increases in salt are much smaller on top of the sill, and the intrusions also appear to be spread out over a longer time.

3. Salt intrusions appear to occur in response to southward pulses of wind, though the magnitude and phasing of the response are not simply connected to the strength or duration of the southward wind. Intrusions are always preceded by a southward wind pulse, but a southward wind pulse is not always followed by an intrusion.

4. Although the increases in salt associated with intrusions are always associated with inward pulses of lower layer velocity, the converse is not true; that is, inward pulses of lower layer velocity are not always associated with increases in salt. The velocity pulses that result in salt intrusion are most often depth dependent, with large inward motion of

water in the lower layer relative to the upper layer at the mouth of the river.

Figure 7 is a presentation of data obtained during the 1987 observational program, in the same format and from the same locations as the 1986 data shown in figure 6; velocity data from the mouth of the river are not shown in figure 7 because the data collected were not continuous enough to yield meaningful subtidal records. The qualitative nature of lower layer salinity intrusion is much like that observed in 1986, except for a general impression of fewer and weaker intrusion events. This impression is quantified in Table 1, where characteristics of salt intrusions larger than 1 psu observed in 1986 and 1987 are compared. Intrusions in 1987 were slightly weaker and marginally longer in duration than in 1986, but these differences are not statistically significant. The only significant interannual difference is in the number and average separation of intrusion events; on average,


a

CE2 2.4 METERS

6/25 - 7/24/1986 MEAN 0.5 CM/S TONARD 240ø

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16

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7/12

7/10

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b

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Fig. 5. (a) Progressive vector diagram (PVD) generated from the near-surface velocity data obtained during the first 1986 deployment at mooring CE2, at the mouth of the Choptank River; crosses mark 00:00 each day; mean velocity is projected into the channel direction. (b) PVD of the lower layer velocity data from the same mooring and same interval as in Figure 5a. (c) Salinity measured at the same location, depth and time as in Figure 5a. (d) Salinity measured at the same location, depth and time as in Figure 5b.

13,364 SANFORD AND BOICOURT: WIND-FORCED SALT INTRUSION INTO A TRIBUTARY ESTUARY

0.0 lOm/s

BWI Wind

CAMB Tide (m)

0.5

CE2 V' o (cm/s)

CE2 Salt (psu)

CS• •ø F V' o

(cm/s) I 16

Salt (psu)

25 30 5 10 15 20 25 30 5 10 15 20 25 30 5 10

July, 1986 August September

Fig. 6. Subtidal (low pass filtered, 34 hr halfpower point with a Lanczos taper) 1986 data: (a) BWI wind vectors. (b) Cambridge tidal height. (c) Principal axis velocities from the mouth of the river, positive into the river (solid lines, lower layer; dashed lines, upper layer). (d) Salinity at the mouth of the river (solid line, lower layer; dashed line, upper layer). (e) Near-bottom principal axis velocity on top of the main sill, positive into the river (solid line, 34 hr low pass; dashed line, 10 day low pass). (f) Near-bottom salinity on the sill.

0.0 lOm/s

BWI W•nd

CAMB Tide (m)

CE2 Salt (psu)

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cs'l V' (cm/s)

Salt (psu)

16

14

20 25 30 5 10 15 20 25 30 5 :•0 :•5 20 25 30 5

June, 1987 July August

Fig. 7. Subtidal 1987 data: (a) BWI wind vectors. (b) Cambridge tidal height. (c) Salinity at the mouth of the river (solid line, lower layer; dashed line, upper layer). (d) Near-bottom principal axis velocity on top of the main sill, positive into the river (solid line, 34 hr low pass; dashed line, 10 day low pass). (e) Near-bottom salinity on the sill.

SANFORD AND BOICOURT: WIND-FORCED SALT I•rRUSION INTO A TRIBUTARY ESTUARY 13,365

Year

TABLE 1. Characteristics of Salt Intrusions at Mooring CE2, From the Lower Layer Records

Record Number of Peak to Peak

Length, intrusions Interval, days

days • I psu Ave Min Max

Duration, Magnitude, days psu

Ave Min Max Ave Min Max

3.1 2 5 2.7 1.3 4.7

3.7 2 5 2.5 1.3 4.2

1986 80 14 5.7 2 12

1987 78 10 7.8 3 12

Ave is average, Min is minimum, Max is maximum.

intrusions occurred more frequently in 1986 than they did in 1987, and there were a greater number of intrusions in 1986 than in 1987.

Hydrographic surveys carried out on July21 and 23, 1987, show the spatial structure of an intrusion of about average magnitude that occurred from July 21-25, 1987. Axial and plan views of salinity from these surveys are presented in Figure 8 for comparison to the subtidal time series of Figure 7. Figures 8a-Sb show the Choptank entrance channel and lower river just before the intrusion reaches the mouth of the river (first vertical line in Figure 7). The nose of the intrusion is apparent as it carries high salinity/low DO water over the top of the secondary sill at -3 kin. The lower river (• km 3) shows little vertical structure. Figures 8c-d show the salinity distribution at mid-intrufion (second vertical line in Figure 7). The appearance of the intrusion here is of a large pulse of lower layer water that is being forced over the edge of the primary sill, with its leading edge 7-8 km inside the river. Isohalines are further into the river, 5oth axially and laterally, and higher in the water column than prior to the intrusion. The intrusion is still in progress (note that the inward subtidal velocity at mooring CS1 is just at its peak). Thus these distributions represent about half of the full extent of high salinity intrusion into the fiver, in approximate agreement with the calculations of intrusive transport presented above.

3.3. Dynamics of Lower Layer Intrusions

Lower layer intrusions from the Chesapeake Bay into the Choptank River are apparently wind-driven, but the dynami- cal connection between the wind and the response of the River is as yet unclear. We have seen, for example, that intrusions are always preceded by a pulse of southward wind, but that pulses of southward wind are not always followed by intrusions. Again, salinity pulses are always accompanied by pulses of inward velocity, but inward velocities do not always produce a corresponding increase in salinity.

The complicated relationship between the wind and the response of the Choptank River is due to the fact that there are several discrete modes of •nd-driven response possible in any given system of interconnected, stratified basins [e.g., Wang and Elliott, 1978; Vieira, 1986; Goodrich et al., 1987; Chao, 1988; Cannon et al., 1990]. These include an essentially barotropic response to remotely imposed changes in sea level [Wang and Elliott, 1978], a depth dependent response to local longitudinal wind driving [Vieira, 1986; Goodrich et al., 1987; Chao, 1988], and an entirely baroclinic response to remotely imposed changes in the density structure [Cannon et al., 1990]. We have seen that intrusions in the Choptank River are accompanied by predominantly depth dependent

motion, which rules out the depth independent influence of remotely imposed changes in sea level. This type of response is clearly present in the Cambridge tidal height records and the velocity records, but it is not associated with salt intrusions. It thus remains to distinguish between a locally forced depth dependent response and a remotely forced, entirely baroclinic response.

Consider first the locally driven response. Vieira [1986] has described this response in some detail using data collected in the mainstem Chesapeake Bay in 1977. Longitu- dinal winds drive down-wind transport in the surface layer, which creates a setup or setdown of sea level at the head of the Bay with very little change in sea level at the mouth of the Bay. The resulting barotropic pressure gradient drives a counter flow against the wind that begins at depth and propagates upward through the water column. When the longitudinal wind changes direction, the surface layer flow actually precedes it slightly as it responds to the pressure gradient in the absence of direct wind driving. Since the return flow is driven by a surface pressure gradient, its set,up is characterized by longitudinal propagation at the long surface wave speed. In addition, there is no vertical motion of the pycnocline required.

A remotely driven baroc!inic response occurs when there is a sudden change in the density structure at one end of a basin, such as that produced by a rapid increase in salinity at the mouth of an estuary [e.g., Cannon et al., 1990]. The density change then propagates into the basin as an internal surge, which must move at roughly the long internal wave speed, and which must be accompanied by vertical motion of the pycnocline. Lower layer velocity under a wave of elevation is in the direction of propagation, and upper layer velocity is counter to the direction of propagation.

Analysis of the unfiltered time series of salinity from the lower layer at the mouth of the river (Figure 5d) indicates that a rapid upward motion of isohalines must be present during intrusion events in addition to the observed horizontal advection. A typical horizontal salinity gradient at the mouth of the river (about 0.3 psu/km -- 3 x 10 '3 psu/m), advected at the maximum velocity observed during the intrusions shown (about 30 cm/s), would yield a rate of salinity increase of about 1 x 104 psu/s. The observed rates of increase at the leading edges ofthe intrusions are about 4 psu in 12 min, or 5.5 x 10 '3 psu/s; these are more than an order of magnitude larger than can be accounted for by horizontal advection alone. The onlywayto account for the observed rate of salinity increase is to invoke a relatively rapid (about 0.5 cm/s) upward motion of the isohalines in the presence of vertical salinity gradients that are on the order of 1 psu/m. This rapid upward motion, which lags slightly behind the beginning of the intrusive velocity pulse, can only be pro-

13,366 SANFORD AND BOICOURT: WIND-FORCED SALT I•rraUSION I•rro A TRIBUTARY ESTUARY

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SANFORD AND BOICOURT'. WIND-FORCED SALT INTRUSION INTO A TRIBUTARY ESTUARY 13,367

duced if the intrusions take the form of an advancing internal long internal wave speed may be obtained from the average wave of elevation or the leading edge of an advancing density distribution corresponding to the 1986 salt distribu- internal bore. tion of Figure 3a, approximating the density structure as

Figure 9 provides further evidence that the lower layer surface and bottom mixed layers with a stratified interior and intrusion response in the Choptank River is a remotely using the formulae presented by Sanford and Grant [1987]. driven baroclinic response. In this figure, BWI wind records are compared to lower layer salt records from the outer entrance channel (CE1) and the mouth of the river (CE2) in 1986, and from the western shelf of the Bay (CBW) and the mouth o f the river (CE2) in 1987. The time series have been low pass filtered with a 3 hr half power point to remove confounding high frequencies, while leaving the essential rapid rise in salt that marks the leading edge of an intrusion.

This calculation yields an estimated speed of about 25 cm/s, as compared to an estimated surface wave speed of about 1,200 cm/s. Coupled with the previous conclusion about the rate of increase of salinity at the leading edge of an intrusion, this confirms that lower layer intrusions in the Chop- tank entrance channel take the form of advancing internal waves of elevation or internal bores. Propagation of intrusions over the primary sill inside the mouth of the river is slowed even more by decreased depth and decreased stratifi- cation. Reference to Figures 6 and 7 indicates that it takes about 17 hrs for an intrusion to travel over the 5 km between cm/s.

Figure 9 also provides evidence that the intrusion the pycnocline in the Bay. A lateral pycnocline tilt in the

Bay that results from upwelling on the eastern shore of the Bay and downwelling on the western shore is marked by a

'-' , •i1 ]] ] . sudden decrease in saltinthe 1987record from mooring .... CBW. Comparison of the 1987 salt record from the CE2 to the record from CBW shows that every intrusion that appears at the mouth of the Choptank is preceded by a downwelling response on the western shore, with an average lead of 17.5 hrs. If we assume that upwelling on the eastern shore occurs at the same time as downwelling on the western shore, then the data in Figure 9 indicate that the resultant internal surge takes 17.5 hrs to propagate along the entrance channel in 1987. The additional 4 hrs in 1987 relative to

1986 is explained by the fact that mooring CE 1 in 1986 was about 3.5 km inside the entrance channel from its connection

point with the Bay. Since 3.5 km in 4 hrs is equivalent to a propagation speed of 24 cm/s, the 4 hr difference simply represents the additional time lag for a surge to reach CE1 from the Bay in 1986.

Lateral tilting o f the pycnocline in the Bay is driven most •_5 20 25 30 5

AugusL, I_987 Sepf•embep

Fig. 9. Time series of wind and salinity during selected intervals of the 1986 and 1987 programs. A 3 hr low pass filter has been applied to remove the highest frequencies. (a) Lower layer salinity at the outer end of the entrance channel in 1986. (b) Lower layer salinity at the mouth of the river in 1986; pairs of vertical lines show propagation of intrusions up the entrance channel. (c) Lower layer salinity on the western side of the Bay in 1987. (d) Lower layer salinity at the mouth of the river in 1987; pairs of vertical lines show relationship between downwelling on the western shore and lower layer intrusion into the Choptank.

(Note that the lower layer current record from CE1 in 1986 actually was located in the lower portion of the pycnocline, such that the large tidal excursions of salt represent an internal tidal oscillation of the pycnocline and an intrusion is marked by cessation of tidal oscillations as the pycnocline rises above the current meter.) The average time lag in 1986 between the leading edge of an intrusion at CE1 and the leading edge of an intrusion at CE2 is 13.5 hrs. Over the 9.5 km that separate the two moorings, this amounts to a propagation speed of about 20 cm/s. An estimate of the

strongly by north-south reversals in wind. Longitudinal downwind motion in the surface layer is accompanied by lateral Ekman transport that results in downwelling on one side of the Bay and upwelling on the other side of the Bay. This behavior is consistent with the numerical results of Chao [1988], that predict a preferential response to longitudinal winds. Thus, the normal local wind-driven response of the mainstem Bay [e.g., Vieira, 1986; Goodrich et al., 1987] includes a lateral adjustment that can drive baroclinic intrusions into adjacent tributaries.

Asymmetrical aspects of the wind/intrusion relationship may be due to internal hydraulic controls in the Choptank entrance channel. Two classes of internal hydraulic control are implicated. In the first of these, control of maximal flow over a sill [e.g., Storereel and Farmer, 1953; Farmer and Armi, 1986; $tigebrandt, 1988], flow speeds that exceed local maximum internal wave speeds (supercritical flows) result in specific limitations on the geometry of the flow and the maximum transport possible. Locally enhanced mixing and a rapid transition back to subcritical flow occur at some point downstream of the control point as the flow goes through an internal hydraulic jump. The second likely type of control is flow blocking, which occurs when there is not

13,368 SANFORD AND BOICOURT: WIND-FORCED SALT INTRUSION INTO A TRIBUTARY ESTUARY

enough energy in an imposed flow to lift dense fluid from below the pycnocline over an intervening sill [e.g., Turner, 19731.

Axial salinity and DO distributions have indicated that the highest salinity/lowest DO water from below the pycnocline of the Bay can be effectively blocked by both the secondary sill at -3 km and the primary sill at the mouth of the river. Furthermore, maximum internal wave speeds in the entrance channel are on the order of 30 cm/s, and maximum tidal velocities are of about the same magnitude. Thus, especially over the tops of the sills where the internal wave speed decreases as the depth decreases, internally supercritical conditions are likely and we may expect to see an internal hydraulic control with an internal hydraulic jump downstream. Such a jump was observed over the secondary sill at -3 km in 300 KHz echo sounder images collected on August 19, 1986, on a flood tide transcot up the axis of the entrance channel (Figure 10).

The hydraulic controls in the Choptank entrance channel determine whether intrusions that are imposed as increases in pycnocline height at the outer end of the entrance channel will ever arrive in the lower Choptank, and if so, how much total transport will be associated with a given intrusion. Thus, any relationship between wind forcing and lower layer intrusion response must include the strength of the stratifica- tion and the height of the pycnocline relative to the sills in the entrance channel. This is the most probable explanation for a lower number of intrusions during the summer of 1987 relative to 1986, recalling that the mean position of the pycnocline was about 5 m deeper in 1987 than in 1986.

3.4. The Consequences of Intrusions

Gravitational circulation in estuaries is driven by the longitudinal density gradient that results from the difference in density between fresh water at the head and salt water at the mouth, divided by the length of the estuary [e.g., Prit- chard, 1956]. The source that maintains this longitudinal density gradient is usually thought of as fresh water inflow at the head of the estuary, but a forced influx of higher salinity water at the mouth should act equivalently, at least in the short term. In the case of the Choptank River, evidence to be presented here suggests that the forced influx of salt that occurs during wind-driven intrusions increases gravitational

circulation during the summertime by increasing the longitudinal density gradient.

Figure ! 1, derived from data compiled by Fisher [1988] on watershed inputs to the Choptank, shows total freshwater input by month for the Choptank basin from 1980-1987. Data for 1986 and 1987 are plotted separately for comparison to the 8 year average values. Springtime inflow peaked in February, 1986 and in January, 1987 (with a small secondary peak in March), as opposed to average peak inflow during March-April. The summers of 1986 and 1987 were both very dry, with much lower than average fresh water inflow during the three months preceding the field programs in both years. Slightly higher streamflow in the months preceding the field program in 1987 relative to 1986 may account for the slightly lower average salinities observed in 1987 (Figures 3 and 4).

TOTAL FRESH WATER INPUT

lOO

o 60

,.-. 4.0 o

n- 20

I I I I I I I I I I I I

/\ Field / \ Programs

i I \

I I I I I I I I I I I I

Oct Nov Dec Jan Feb Mar Apr May Jun Jly Aug Sep

1986 --, 1987 -- -, Average of 1980-1987 O

Fig. 11. Estimated total freshwater input into the Choptank River by month, calculated as in Fisher (1988). Data courtesy of T.R. Fisher.

Mean near bottom flow on top of the primary sill (CS1) in 1987 was near zero at the beginning of the observational period, and only began to move into the river in a quasi- steady fashion after the sequence of large intrusions in late July (Figure 7d). Salinity over the sill began to increase at the same time (Figure 7e). If this apparent increase in

Fig. 10. 300 KHz echo sounder image from August 19, 1986, on a transect up the Choptank entrance channel. Approximate location is at km -2 in Figure 2, descending from the secondary sill at km -3 into the basin at the mouth of the river; upstream is to the right. The lowest continuous mid-water scattering layer marks the pycnocline. Note the very thin lower layer on the left and the apparent internal hydraulic jump on the right.


gravitational circulation over the sill in July were a delayed response to the spring freshet in 1987, a lag of at least 5 months would be indicated. This is much too long, with an implied propagation of the peak inflow over the 70 km between the head of the river and the sill at a rate of about

0.5 cm/ s. Ward and Twilley [1986] observed a rapid response to a short term freshwater inflow event in the spring of 1983, with a lag of O(days) between upstream inflow and downstream response (L. Ward, personal communication). Thus, it appears that the delivery of salt by lower layer intrusions is the proximate cause of the developing inward flow in 1987.

Examination of Figure 6 reveals a paradox. Very low frequency near bottom inflow occurred over the sill during periods of active intrusion in 1986 (Figure 6e), but no steady inflow developed as it did in 1987, even though intrusions were more frequent and presumably caused a greater total influx of salt water (Table 1).

This paradox may be reconciled by a simple conceptual model of the intrusive process. After an intrusion is forced over the edge of the sill, it propagates forward as a time dependent internal surge, which may become an isolated parcel of water if it is cut off at the sill edge. This propagation is highly damped by friction and mixing over the shallow sill. At some point, forward propagation is arrested. A relatively steady gravity current then develops upstream,

has pushed the average position of the salt resevoir past mooring CS1, leading to time dependent inflow with no apparent steady component. In 1987, mooring CS1 was apparently on the upstream side of the salt resevoir, leading to less time dependence and a developing steady inflow. The 1986 and 1987 CS1 inflows thus appear to be different, but consistent aspects of a single process that leads to the ultimate conversion of a wind-forced influx of salt into

increased gravitational circulation in the Choptank. The gravitational circulation that results from the

intrusion process is essentially independent of concurrent fresh water inflow, but it cannot be independent of yearly averaged fresh water inflow. There must also be a large enough resevoir of low salinity water in the upper reaches of the river to maintain the longitudinal salinity gradient, which in turn depends on fresh water input during the preceding winter and spring. Thus, we do not argue here that the wind-forced intrusion process is solely responsible for the summertime estuarine circulation, but rather that it acts to increase that circulation by supplying an additional push to an otherwise sluggish system.

4. DISCUSSION

Increased flushing due to time dependent intrusions over possibly decreasing over time as the available salt is deplet- sills has been recognized for fjord estuaries for some time ed. Successive intrusions build up a "resevoir" of available [e.g., Gade and Edwards, 1980]. However, previous investi- salt, increasing the steadiness of the upstream gravity gations of lowfrequencymotion in partially mixed estuaries current; if the rate of salt addition is greater than the rate of have generally considered wind-driven and tidal variability as loss, the strength of the gravity current increases. Down- modulations that can be averaged out of the equations of stream (Bayward) ofthe average position of the salt resevoir, motion that govern the gravitational circulation, except for currents are more time dependent and have less of a steady their effects on mixing and dissipation. The present results component than currents upstream of the salt resevoir. demonstrate a situation in which a wind-driven influxof salt Figure 12 shows that the greater total influx of salt in 1986 "pumps" the gravitational circulation, such that it is not

1986 Average Salt 0 1 2

0 ! • i "l, i I• ! i i , , [ 0 -1 -

•k. "' ••••-• - -2 -2 h......... •

-4 • 14.75

_,b-..- -8

v 0 1 2 3 4 5 6 7 8 9 10 11 12 3

1987 Average Salt 0 1 2 3 4 5 6 7 8 9 10 11 12 13

o L! i ,.• i i i ! i t i o -1 7• \ \\ -1 ,, \ ,,

--4 '" • -• -.,. \\ -- --4

.,-"--' ,N',.\',, I::' / \',\', \ ',

-8 I I -8

-9 ••• •• '\•/ /t -9 -10 -10

-11 -11

-12 -12.

-13 • • -13 0 1 2 3 4 5 6 7 8 9 10 11 12 13

Fig. 12. Expanded view of the main sill section from the average axial distributions of salinity during 1986 and 1987 (Figure 3). The dots mark the location of the current meter at mooring CS1. The greater inward extent of salt intrusion in 1986 is evident; compare, for example, the 14.625 isohaline in 1986 to the 14.25 isohaline in 1987. The implication is that the source of salt for enhancing the mean gravitational circulation is upstream of the current meter in 1986, but downstream of the current meter in 1987. Isohaline patterns in the immediate vicinity of the sill edge are manifestations of the time dependent intrusion process itself, and do not represent a steady forcing.

13,370 SANFORD AND BOICOURT: WIND-FORCED SALT I•rrRUSION l•rro A TRIBUTARY ESTUARY

separable. This effect is similar to the "tidal pumping" discussed by, for example, $tigebrandt [1988]. In a broader sense, wind-driven and tidal motions may be expected to contribute significantly to low frequency exchanges between stratified basins whenever the additional kinetic energy contributes significantly to overcoming a potential energy barrier; for example, through raising dense water over an intervening sill. Consideration of the axial bathymetry of the Chesapeake Bay [Carter and Pritchard, 1988] reveals a number of deep basins separated by shallower sills; in addition, many of the tributary estuaries of the Chesapeake are separated from the mainstem by shallow sills across their mouths. Thus, the mechanism identified here may be representative of a broad class of tidal and wind-driven motions that contribute in an as yet undetermined wayto the net circulation of the Chesapeake system.

Wind-driven motion in estuaries, and in particular the separation o f local forcing from remote forcing, has been the topic of much research in recent years [e.g., Wang and Elliott, 1978; Vieira, 1986; Goodrich et al., 1987; Chao, 1988; Cannon et al., 1990]. The majority of these studies have considered the response of the estuary to some combination of surface wind stress and a surface pressure gradient. The present study, however, joins those few that have considered an entirely baroclinic wind-driven response [e.g., Cannon et al., 1990], apparently resulting from rapidly imposed changes in the salt structure at the mouth of the estuary. Cannon et al. [1990] suggested that longitudinal advection in the Strait of Juan de Fuca was responsible for initiation and cessation of intrusions into Puget Sound. We have shown here that lower layer intrusion into the Choptank River is driven by lateral tilting (upwelling-downwelling) of the pycnocline in the mainstem of the Chesapeake Bay. This lateral tilting is characterized by rapid vertical motion of the pycnocline on either side of the Bay, which produces a much more rapid increase in salinity on the upwelling side than would be possible from longitudinal advection in the mainstem Bay. The present results thus indicate the potential importance of time dependent internal dynamics in partially mixed estuaries. Our present understanding of time dependent internal phenomena in the estuarine environment is limited.

The height of the pycnocline also varies over both seasonal and interannual time scales, as demonstrated by the difference between the 1986 and 1987 average salinity distributions presented in Figure 3. Given the potential importance of internal hydraulic controls, changes in the height of the pycnocline over seasonal and interannual time scales maybe implicated as additional factors contributing to variability in circulation and mixing processes. These changes and the processes that control them are not well understood.

It is important to point out that seasonal and interannual variability in fresh water inflow will probably change the influence of wind-driven intrusions relative to the classical

gravitational circulation. The observations reported here were for two very dry summers in an estuary that has low fresh water inflow even in normal years. Like the tributaries of the northern Chesapeake Bay [Pritchard, 1968], the Chop- tank is a prime candidate for control by circulation processes in the adjacent mainstem. Higher fresh water inflow would likely increase the gravitational circulation in both the Bay and the Choptank. The occurrence of lower layer intrusion,

however, has been seen to depend primarily on the wind, the density structure in the Bay, and the internal response o f the Bay to changing wind forcing. Wind forcing should be independent of freshwater inflow, but it is not known how the internal dynamics of the Bay change from year to year in response to freshwater inflow variations. Thus, the low DO at shallow depths reported by Seliger et al. [1985] and Magnien [1987] to have occurred well within the Choptank may have been a unique, short-lived occurrence due to a single large intrusion, or it may have been locally produced because of a higher delivery of nutrients to the Choptank in 1984, or it may have been indicative of normal conditions in 1984 (though this seems unlikely). Ward and Twilley [1986] never observed low DO in the Choptank during the summer of 1983 except at the mouth of the river, though their monthly sampling interval could easily have missed intrusion events. 1983 had nearly as much freshwater runoff as 1984 [Fisher, 1988]. Two things seem clear: a better understanding of year to year variability in the internal dynamics of the Bay is needed, and data on the intrusion process from high freshwater flow years would be most helpful.

We have not considered in detail the fate of an intrusion

once it enters the Choptank River proper, though we have implied that the salt mostly stays within the system. The bases of this assumption are the lower layer current meter records from CE2 and CS1 for both 1986 and 1987, which show little if any return flow towards the Bay, and the averaged salinity distributions, which show no major reverse salt gradient at the mouth o f the river. Loss o f salt delivered in intrusions by vertical mixing or vertical advection and subsequent outward advection in the surface layer is a possibility that cannot be ruled out, but the apparent increase in gravitational circulation indicates that at least some of the salt remains in the river.

5. CONCLUSIONS

The observations reported here indicate that remotely wind-forced, baroclinic intrusion of saline, low DO waters from below the pycnocline of the Chesapeake Bay into the lower Choptank River estuary is an important mode of interaction between the River and the Bay during the summertime. Intrusions occur as large internal surges of lower layer water that travel up the relict Choptank entrance channel at a speed of about 20 cm/s and spill over the edge of the main entrance sill just inside the mouth of the river. They are driven by lateral internal tilting of the pycnocline in the Bay, when high salinity water is upwelled on the eastern side of the Bay after a southward pulse of wind stress. Intrusive pulses tend to be unidirectional; outward flow in the lower layer at the mouth of the river sometimes occurs, but it seldom produces as large a transport as the inward intrusions. Intrusions are modulated on meteorologi- cal time scales; a typical intrusive pulse lasts 3-4 days, and pushes lower layer water about 15 km into the Choptank below the 5 m isobath, neglecting mixing; the actual extent of individual intrusions will be quite variable.

Intrusion-favorable pycnocline tilts in the Bay do not always result in lower layer intrusion into the Choptank, but may be blocked in the entrance channel on occasion. The lower average height of the pycnocline in the Chesapeake Bay in the summer of 1987 relative to the summer of 1986 may have been the reason that fewer intrusions occurred in

SANFORD AND BOICOURT: WIND-FORCED SALT INTRUSION I•rro A TRIBUTARY ESTUARY 13,371

1987. In the summer of 1986, large intrusive pulses occurred at intervals of about 6 days; in 1987, fewer intrusions occurred, and the average interval between intrusions increased to about 8 days.

Advection of low DO by intrusion of lower layer water from the mainstem of the Bay dominates both the temporal and spatial variability of near-bottom DO near the mouth of the Choptank. The head of the entrance channel at the mouth of the river has the character of a point source of high salt/low DO for the near-bottom environment in the lower

river, with distributions of near-bottom salt and DO that resemble radial diffusion elongated in the axial direction by upstream advection. Well inside the river in near-bottom waters and everywhere in surface layer waters, DO appears to be decoupled from salinity in the mean, presumably due to local consumption and production processes. Occasional large intrusions may advect low DO as far as the deep hole at Castle Haven, however, such that the mean decoupling of salinity and DO does not necessarily exclude an advective component. Interannual variability in fresh water inflow to the Choptank and the Bay may change the likelihood of direct advection of low DO into shallower depths in the lower Choptank by changing the importance of the wind- driven intrusions relative to the classical gravitational circulation. Both 1986 and 1987 were years with very low freshwater inflow.

,.

Our results suggest that wind-forced intrusion of salt leads to increased gravitational circulation in the Choptank during the summer months. It is also possible that lower layer intrusion during the summer months results in a net annual import of nutrients from the Chesapeake Bay into the Choptank. A carefully designed, interdisciplinary observational program near the mouth of the river would further address these questions, and would help to place this summertime phenomenon into an appropriate annual context.

Acknowledgments. For their help in carrying out the field studies, we thank D. Pyoas, H. Griffioen, L. May, C. Moore, $. Ortiz, C. Small, C. Stenger, F. Schennermann, R. Whaley, N. Wolfe, and the captains and crews of the R/V's Aquarius, Orion, and Discovery. A. Benbow, H. Griffioen, L. May, C. Moore, J. Ortiz, C. Small, and N. Wolfe helped with data reduction and analysis. D. Kennedy helped draft some of the figures. The 300 KHz echo sounder image is courtesy of J. Halka, of the Maryland Geological Survey. This work was funded by two contracts between the State of Maryland Department of Natural Resources, Tidewater Adminis- tration, and the University of Maryland System Center for Environ- mental and Estuarine Studies, Horn Point Environmental Labora- tories. The 1986 contract was C8-86-024 and the 1987 contract was C039-87-025. UMSCEES contribution 2098.

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fjord, Estuaries, 13(1), 31-42, 1990. Carter, H.H., and D.W. Pritchard, Oceanography of the Chesa-

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W.C. Boicourt and L.P. Sanford, Center for Environmental and Estuarine Studies, Horn Point Environmental Laboratories, P.O. Box 775, Cambridge, MD 21613.

(Received November 10, 1989; accepted February 15, 1990.)

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Wind-forced salt intrusion into a tributary estuary