Landward Flux Research Articles

Effects of Typhoon on Saltwater Intrusion in a High Discharge Estuary

AbstractSaltwater intrusion in the Changjiang Estuary occurs in winter and dry season, which may affect drinking water supply in Shanghai. An atypical saltwater intrusion event, observed under high river discharge (>20,000 m3/s) in October 2018, coincided with the passage of super typhoon Kong‐rey. The response of the estuarine salt field is reproduced here with a numerical model for analysis with a Eulerian framework. Model results show that saltwater intrusion was mainly due to the landward advection of salt (∼260 t/s), which was driven by the storm surge (∼0.4 m) and landward Ekman transport under typhoon winds. The sheared salt flux (∼145 t/s) significantly contributes to the total landward flux, especially when the surge recedes after the typhoon has passed. Northwesterly winds increased vertical shear and stratification during the event. The maximum saltwater intrusion occurred about a day after the typhoon passage, and the salinity at the freshwater reservoir intake exceeds the local drinking standard of 0.45 psu. A numerical experiment shows that if Kong‐rey had passed during spring tide, the saltwater intrusion would have been weaker due to a stronger northward transport near the coast and a landward transport in the southern channels.

Impacts of Different Dynamic Factors on the Saltwater Intrusion in the Northern Branch of the Yangtze Estuary

A 3-D baroclinic numerical model is established to study the temporal and spatial distribution of salinity in the Northern Branch (NB) of the Yangtze Estuary (YE) in response to river runoff, wind and currents. The idealized, process-oriented studies show that the lower river runoff in the dry season is the main driving factor for the spilling over from the NB to the Southern Branch (SB). The NB takes longer time to get in dynamic stable state compared with the SB, as the runoff transfers from the wet season to the dry season. The vertical stratification intensity of the NB has obvious periodic phenomena in the dry season, and the stratification strength is enhanced under the action of low runoff and north wind, which further weakens the vertical transport of water. Momentum balance analysis suggested that the landward net flux increases when considering the influence of the moderate north wind, and the landward flux will enhance if considering the boundary current during the spring tide. With the influence of the boundary current, increases the average salinity of the NB, as well as the intrusion degree from the NB to the SB. This indicates that the nearshore ocean current of the Yellow Sea is an important factor affecting the saltwater intrusion in the NB of the YE.

Current reversals in a large tidal river

A tidal river is a special zone where the basic hydrodynamic process is the interaction between riverine flow and tides. Existing studies focus mainly on the water level fluctuations, while the effort is limited regarding the flow regime between uni- and bi-directional currents. Along a tidal river, the current direction reverses where the landward flow meets with seaward flow. So far, the occurrence and movement of current reversal is not fully investigated, and the number and location of reversal points are not well understood. In this study, the water flow in the Yangtze tidal river was simulated using a hydrodynamic model, and the phenomenon of current reversals in the tidal river were investigated. It is found that in the dry season up to three reversal points may occur simultaneously, including two convergence points and one divergence point, separating the entire tidal river into two seaward-current reaches and two landward-current reaches. These multiple reversal points result from the co-existence of the preceding and present flood tidal waves. During a spring tide the peak landward flux of the preceding wave can reach up to 10,000 m3/s, about the same magnitude as the riverine discharge and the length of the landward-current occupation is as large as 120 km. It was also found that a current reversal point always coexists with a zero-gradient point of water level, and the location of this reversal is downstream of the zero-gradient point. The reversals shift landward from the river mouth consecutively, and could finally disappear farther at the upper reach of the tidal river. The bidirectional flow could extend up to 550 and 200 km upstream from the river mouth in the dry and wet seasons, respectively. The current reversals are influenced by the discharge regulation of the Three Gorges Dam. These movements of current reversals in the flow regime could furtherly have significant impact on local hydrodynamic process and material transportation.

Hydro-sedimentary processes of a shallow tropical estuary under Amazon influence. The Mahury Estuary, French Guiana

Along the Guianas coast, coastal dynamic is characterized by the migration of mud banks originating from the Amazon. This singular feature affects the dynamic and the morphology of local estuaries and can induce rapid bathymetric evolution in lower estuaries. Since 2012, the navigation channel of the Mahury Estuary (French Guiana) is enduring a severe siltation whose origin comes from a mud bank crossing the estuary mouth. This study aims to determine how the migration of a mud bank through an estuary mouth could influence the transport and fluxes in the estuary. Field measurements were performed over a year with the monitoring of the salt intrusion length, mooring surveys during spring-neap cycles and shipboard profiling surveys during semi-diurnal cycles. Salt intrusion lengths underline a significant seasonal variation characterized by the transition from a steady-state length during high river discharge and a wide range of lengths with the tidal range during low to moderate river discharge. During the rainy season, measurements indicate a fluvial-dominated condition with low suspended-sediment concentrations most of the semi-diurnal cycle. Residual sediment fluxes are usually seaward excepted when river discharge is below seasonal average. During the dry season, maximum suspended-sediment concentrations are higher in the middle part of the estuary. Residual sediment fluxes are landward along the estuary and stronger during neap tides in the estuary mouth and few kilometers upstream. In this area, a persistent density stratification traps sediments in the bottom layer and generates a gravitational circulation during neap tides, which enhances landward transports up to 2.56 t m−1 over a semi-diurnal cycle. In the middle estuary, landward fluxes are most significant during the dry season and also during the rainy season when the river discharge is below the seasonal average. Although this study includes temporal and spatial limitations, it underlines significant mud inflows in the middle part of the estuary during low to moderate river discharges. Comparison with old data suggests higher sediment loads in the estuary during the migration of a mud bank but must be confirmed by further studies during the interbank period.

Suspended-sediment dynamics in the tidal reach of a San Francisco Bay tributary

To better understand suspended-sediment transport in a tidal slough adjacent to a large wetland restoration project, we deployed continuously measuring temperature, salinity, depth, turbidity, and velocity sensors in 2010 at a near-bottom location in Alviso Slough (Alviso, California, USA). Alviso Slough is the downstream reach of the Guadalupe River and flows into the far southern end of San Francisco Bay. River flow is influenced by the Mediterranean climate, with high flows (∼90 m3 s−1) correlated to episodic winter storms and low base flow (∼0.85 m3 s−1) during the summer. Storms and associated runoff have a large influence on sediment flux for brief periods, but the annual peak sediment concentrations in the slough, which occur in April and May, are similar to the rest of this part of the bay and are not directly related to peak discharge events. Strong spring tides promote a large upstream sediment flux as a front associated with the passage of a salt wedge during flood tide. Neap tides do not have flood-directed fronts, but a front seen sometimes during ebb tide appears to be associated with the breakdown of stratification in the slough. During neap tides, stratification likely suppresses sediment transport during weaker flood and ebb tides. The slough is flood dominant during spring tides, and ebb dominant during neap tides. Extreme events in landward (salt wedge) and bayward (rainfall events) suspended-sediment flux account for 5.0 % of the total sediment flux in the slough and only 0.55 % of the samples. The remaining 95 % of the total sediment flux is due to tidal transport, with an imbalance in the daily tidal transport producing net landward flux. Overall, net sediment transport during this study was landward indicating that sediment in the sloughs may not be flushed to the bay and are available for sedimentation in the adjacent marshes and ponds.

Salinity Response to Seasonal Runoff in a Complex Estuarine System (Cochin Estuary, West Coast of India)

ABSTRACT Vinita, J.; Shivaprasad, A.; Revichandran, C.; Manoj, N.T.; Muraleedharan, K.R., and Binzy, J., 2015. Salinity response to seasonal runoff in a complex estuarine system (Cochin estuary, west coast of India). The hydrographic conditions in Cochin estuary are investigated using longitudinal measurements of salinity profiles during a 1-year period. The estuary transformed from salt wedge conditions during the wet season to a partially or well-mixed state during the dry season. During high runoff months (June–September) and moderate runoff months (October–December and May), the salinity structure is a strong function of variations in river runoff, and the estuary responds coherently to episodic inputs of freshwater. However, during the period from January to April, the dependence of the salinity field on runoff was weakened by axial diffusivity. The salt transport was a balance between seaward flux induced by river runoff and tidally induced diffusive landward flux. The latter dominated in almost all...

Variability in salt flux and water circulation in Ota River Estuary, Japan

In this study the sub-tidal and intra-tidal variations of salt fluxes in the upstream section of a shallow estuary (with a water depth of less than 3 m) were investigated. The salt fluxes were estimated based on the cross-sectional average salinity and velocity measured by the fluvial acoustic tomography system (FATS). The results indicate that the magnitude of seaward fluxes is approximately two times greater than that of landward fluxes under normal conditions. The results of short-term observation in the study area indicate that there is a phase lag of the bottom and surface salinities between the regions with the largest and smallest depths. The vertical shear flux with a peak value of −0.7 m2/s during the ebb tide indicated an important contribution to the total salt flux compared with the advective flux. A phase lag occurred between the vertical shear terms in the regions with the largest and smallest depths, which resulted from the correlation between the vertical variations of the salinity and velocity and the existence of transversal velocity circulations.

Seasonal variations in suspended-sediment dynamics in the tidal reach of an estuarine tributary

Abstract Quantifying sediment supply from estuarine tributaries is an important component of developing a sediment budget, and common techniques for estimating supply are based on gages located above tidal influence. However, tidal interactions near tributary mouths can affect the magnitude and direction of sediment supply to the open waters of the estuary. We investigated suspended-sediment dynamics in the tidal reach of Corte Madera Creek, an estuarine tributary of San Francisco Bay, using moored acoustic and optical instruments. Flux of both water and suspended-sediment were calculated from observed water velocity and turbidity for two periods in each of wet and dry seasons during 2010. During wet periods, net suspended-sediment flux was seaward; tidally filtered flux was dominated by the advective component. In contrast, during dry periods, net flux was landward; tidally filtered flux was dominated by the dispersive component. The mechanisms generating this landward flux varied; during summer we attributed wind–wave resuspension in the estuary and subsequent transport on flood tides, whereas during autumn we attributed increased spring tide flood velocity magnitude leading to local resuspension. A quadrant analysis similar to that employed in turbulence studies was developed to summarize flux time series by quantifying the relative importance of sediment transport events. These events are categorized by the direction of velocity (flood vs. ebb) and the magnitude of concentration relative to tidally averaged conditions (relatively turbid vs. relatively clear). During wet periods, suspended-sediment flux was greatest in magnitude during relatively turbid ebbs, whereas during dry periods it was greatest in magnitude during relatively turbid floods. A conceptual model was developed to generalize seasonal differences in suspended-sediment dynamics; model application to this study demonstrated the importance of few, relatively large events on net suspended-sediment flux. These results suggest that other estuarine tributaries may alternate seasonally as sediment sinks or sources, leading to the conclusion that calculations of estuary sediment supply from local tributaries that do not account for tidal reaches may be overestimates.

Composition and fluxes of particulate organic matter in a temperate estuary (Winyah Bay, South Carolina, USA) under contrasting physical forcings

To understand the role that physical processes play on the biogeochemical cycles of estuaries, we conducted intense field studies of the turbidity maximum region within a partially mixed estuary (Winyah Bay, SC, USA) under contrasting conditions of river discharge, tides and wind. Water samples and hydrographic data were collected at different depths and locations along the main channel over several tidal cycles during several cruises to Winyah Bay. Tidal variations in current speed, salinity, total suspended solid concentrations were measured within each cruise and were consistent with estuarine circulation processes. Salinity and total suspended solid concentrations ranged from 0 to 32 and from 20 to over 500 mg L −1, respectively, with the highest salinity and total suspended solid values measured during periods of low river discharge. In fact, comparison of tidally averaged salinity and total suspended solid concentrations revealed marked differences among cruises that were negatively correlated to river discharge and SW wind speed. Moreover, significant contrasts in the chemical compositions of suspended particles were evident among periods of contrasting river discharge and wind regime. For example, the weight percent organic carbon content of suspended particles ranged from 1 to over 6% and displayed a positive correlation with river discharge. Similarly, both the molar carbon to nitrogen ratios (10 to 20 mol:mol) and stable carbon isotopic compositions (−25 to −29%) of the suspended organic matter varied significantly as a function of discharge and wind. Such trends indicate that in Winyah Bay low river discharge and steady SW winds promote resuspension of bed sediments from shallow regions of the estuary. These materials contain highly altered organic matter and their incorporation into the water column leads to the observed trends in suspended particle concentrations and compositions. Furthermore, these conditions result in net landward fluxes of salt, sediment and particulate organic matter throughout most of the water column, promoting efficient trapping of materials within the estuary. Our results illustrate the fundamental connection between physical forcings, such as discharge and wind, sediment transport processes and the cycling of biogeochemical materials in estuarine environments.

Thin-layer Bloom of Diatoms Around the Mouth of the Aarakawa River in Tokyo Bay

A thin–layer diatom bloom was examined during four surveys in July and August 2005 around the mouth of the Arakawa River, Japan. During two surveys, a thin layer of concentrated chlorophyll a (thickness 1–3 m), was found under a layer of salinity 10–15 over 5 km offshore. The dominant taxa in the thin layer were Skeletonema costatum, Thalassiosira binata, and Cryptomonadales. When the thin layer was found, an estuarine circulation pattern occurred around the mouth of the river. The direction of flow was seaward in the upper layer and landward in the lower layer. The seaward flux of chlorophyll a in the upper layer was nearly the same as the landward flux of chlorophyll a in the lower layer. In addition, a surface layer was found over 5 km offshore. The surface layer (first upper layer) was formed above the halocline, which was at about 0.5–m depth. In the first upper layer, salinity was low, chlorophyll a concentration was low, and nutrient concentrations were high. Therefore, the thin layer of concentrated chlorophyll a was formed and maintained by phytoplankton remaining around the mouth of the river in response to the estuarine circulation pattern and the supply of nutrients from the first upper layer.

Sediment transfer and accumulation in two contrasting salt marsh/mudflat systems: the Seine estuary (France) and the Medway estuary (UK)

Understanding the dynamics of fine sediment transport across the upper intertidal zone is critical in managing the erosion and accretion of intertidal areas, and in managed realignment/estuarine habitat recreation strategies. This paper examines the transfer of sediments between salt marsh and mudflat environments in two contrasting macrotidal estuaries: the Seine (France) and the Medway (UK), using data collected during two joint field seasons undertaken by the Anglo-French RIMEW project (Rives-Manche Estuary Watch). High-resolution ADCP, Altimeter, OBS and ASM measurements from mudflat and marsh surface environments have been combined with sediment trap data to examine short-term sediment transport processes under spring tide and storm flow conditions. In addition, the longer-term accumulation of sediment in each salt marsh system has been examined via radiometric dating of sediment cores. In the Seine, rapid sediment accumulation and expansion of salt marsh areas, and subsequent loss of open intertidal mudflats, is a major problem, and the data collected here indicate a distinct net landward flux of sediments into the marsh interior. Suspended sediment fluxes are much higher than in the Medway estuary (averaging 0.09 g/m3/s), and vertical accumulation rates at the salt marsh/mudflat boundary exceed 3 cm/y. Suspended sediment data collected during storm surge conditions indicate that significant in-wash of fine sediments into the marsh interior can occur during (and following) these high-magnitude events. In contrast to the Seine, the Medway is undergoing erosion and general loss of salt marsh areas. Suspended sediment fluxes are of the order of 0.03 g/m3/s, and the marsh system here has much lower rates of vertical accretion (sediment accumulation rates are ca. 4 mm/y). Current velocity data for the Medway site indicate higher velocities on the ebb tide than occur on the flood tide, which may be sufficient to remobilise sediments deposited on the previous tide and so force net removal of material from the marsh.

Fluxes and sources of suspended organic matter in an estuarine turbidity maximum region during low discharge conditions

Water column concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured at three different depths in four different locations bracketing the estuarine turbidity maximum (ETM) along the main channel of a temperate riverine estuary (Winyah Bay, South Carolina, USA). Measurements were carried out over full tidal cycle (over 24 h). Salinity, temperature, current magnitude and direction were also monitored at the same time throughout the water column. Tidally averaged net fluxes of salt, TSS, POC and PN were calculated by combining the current measurements with the concentration data. Under the extreme low river discharge conditions that characterized the study period, net landward fluxes of salt were measured in the lower part of the study area, suggesting that the landward transport through the main channel of the estuary was probably balanced by export out through the sides. In contrast, the net fluxes of salt in the upper reaches of the study area were near zero, indicating a closed salt balance in this part of the estuary. In contrast to salt, the net fluxes of TSS, POC and PN in the deeper parts of the water column were consistently landward at all four sites in Winyah Bay indicating the non-conservative behavior of particulate components and their active transport up the estuary in the region around the ETM. The carbon contents (%POC), carbon:nitrogen ratios (org[C:N]a) and stable carbon isotopic compositions ( δ 13C POC) of the suspended particles varied significantly with depth, location and tidal stage. Tidally averaged compositions showed a significant increase up the estuary in the %POC and org[C:N]a values of suspended particles consistent with the preferential landward transport of carbon-rich particles with higher vascular plant debris content. The combination of tidal resuspension and flood-dominated flow appeared to be responsible for the hydrodynamic sorting of particles along the estuary that resulted in denser, organic-poor particles being transported landward less efficiently. The elemental and isotopic compositions indicated that vascular C 3 plants and estuarine algae were the major sources of the particulate organic matter of all the samples, without any significant contributions from salt marsh C 4 vegetation ( Spartina alterniflora) and/or marine phytoplankton.

Modeling the influence of settling velocity on cohesive sediment transport in Tanshui River estuary

Settling velocities of suspended cohesive sediment in estuaries vary over a range of several orders in magnitude. Variations in the suspended sediment concentration are often considered as the principal cause. Turbulence and the suspended sediment concentration, as well as other factors such as salinity, dissolved organic substances, flocculation ability, and the rate of floc growth affect setting velocities. A laterally−averaged finite difference model for hydrodynamics and cohesive sediment transport is developed and applied in the Tanshui River estuary, Taiwan. The model has been calibrated and verified with water surface elevation, longitudinal velocity, salinity, and cohesive sediment measured. The overall performance of the model is in qualitative agreement with the available data. The model is used to investigate the influence of settling velocity on cohesive sediment transport dynamics. The simulation indicates that the turbidity maximum zone is near Kuan−Du. When settling velocities increase the surface cohesive sediment concentration at Kuan−Du station trends to decrease and bottom cohesive sediment concentration increases. Both surface and bottom cohesive sediment concentrations decrease at Taipei Bridge and Pa−Ling Bridge. This implies that suspended sediment advected seaward and deposited. There is consequently a net seaward flux of suspended sediment near surface, and a net landward flux near the bed.

Floodtide pulses after low tides in shallow subembayments adjacent to deep channels

In shallow waters surface gravity waves (tides) propagate with a speed proportional to the square root of water depth ( c= g(h+η) ). As the ratio of free surface displacement to mean depth ( η/ h) approaches unity the wave will travel noticeably faster at high tide than at low tide, creating asymmetries in the tidal form. This physical process is explained analytically by the increased significance of friction and the nonlinear terms in the continuity and momentum equations. In a tidal system comprising a shallow bay adjacent to a deeper channel, tidal asymmetries will be more prevalent in the shallow bay. Thus strong barotropic gradients can be generated between the two, producing rapid accelerations of currents into the bay (relative to other bay tidal processes) and create a maximum peak in the flood tide that we describe as a floodtide pulse. These floodtide pulses can promote a landward flux of suspended-sediment into the bay. In Grizzly Bay (part of northern San Francisco Bay, USA), field observations verify the occurrence of floodtide pulses during the lowest low tides of the year. No pulses were observed in neighboring Honker Bay, which has an average depth ∼30 cm greater than Grizzly Bay. Numerical simulations of northern San Francisco Bay using realistic bathymetry demonstrated that floodtide pulses occurred in Grizzly Bay but not in Honker Bay, consistent with the observations. Both observations and numerical simulations show that floodtide pulses promote a landward flux of sediment into Grizzly Bay. Numerical simulations of an idealized bay–channel system quantify the importance of mean depth and friction in creating these floodtide pulses.

Transport of salt and suspended sediments in a curving channel of a coastal plain estuary: Satilla River, GA

This study describes the transport of salt and suspended sediment in a curving reach of a shallow mesotidal coastal plain estuary. Circulation data revealed a subtidal upstream bottom flow during neap tide, indicating the presence of a gravitational circulation mode throughout the channel. During spring tide, landward bottom flow weakened considerably at the upstream end of the channel and changed to seaward in the middle and downstream areas of the reach, suggesting the importance of tidal pumping. Salt flux near-bottom was landward at both ends of the channel during neap tide. At spring, however, the salt flux diverged along the bottom of the thalweg suggesting that tidal pumping caused a transfer of salt vertically and laterally into the intertidal zone. Thus, landward flux of salt is maintained even in the presence of subtidal seaward flow along the bottom at the downstream end of the channel. Landward bottom stress is greater than seaward stress, preferentially transporting suspended sediments upstream. Compared with salt, however, the weight of the suspended sediments causes less upward transfer of sediments into the intertidal zone. Flood flow carried more suspended sediments landward at the upstream end compared with the downstream end. We speculate that secondary flow in the curving channel picks up increasing amounts of suspended sediments along the sides during flood and adds them to the axial flow in the thalweg. Since the landward flow along the bottom of the thalweg weakens and even reverses during spring tide, there appears to be a complex re-circulation system for sediments re-suspended in curving channels that complicates the picture of a net transport of sediments landward.

Sediment Transport and Trapping in the Hudson River Estuary

The Hudson River estuary has a pronounced turbidity maximum zone, in which rapid, short-term deposition of sediment occurs during and following the spring freshet. Water-column measurements of currents and suspended sediment were performed during the spring of 1999 to determine the rate and mechanisms of sediment transport and trapping in the estuary. The net convergence of sediment in the lower estuary was approximately 300,000 tons, consistent with an estimate based on sediment cores. The major input of sediment from the watershed occurred during the spring freshet, as expected. Unexpected, however, was that an even larger quantity of sediment was transported landward into the estuary during the 3-mo observation period. The landward movement was largely accomplished by tidal pumping (i.e., the correlation between concentration and velocity at tidal frequencies) during spring tides, when the concentrations were 5 to 10 times higher than during neap tides. The landward flux is not consistent with the long-term sediment budget, which requires a seaward flux at the mouth to account for the excess input from the watershed relative to net accumulation. The anomalous, landward transport in 1999 occurred in part because the freshet was relatively weak, and the freshet occurred during neapetides when sediment resuspension was minimal. An extreme freshet occurred during 1998, which may have provided a repository of sediment just seaward of the mouth that re-entered the estuary in 1999. The amplitude of the spring freshet and its timing with respect to the spring-neap cycle cause large interannual variations in estuarine sediment flux. These variations can result in the remobilization of previously deposited sediment, the mass of which may exceed the annual inputs from the watershed.

Modelling Exchange of Natural Trace Sediments Between an Estuary and Adjacent Continental Shelf

A mathematical model is developed to investigate the exchange of natural trace sediments between an estuary and an adjacent continental shelf, assuming that the tracer originates from the shelf. The model is run for four cases representing various landward-seaward sediment transport patterns at the entrance to the estuary. The results show that landward flux of the tracer takes place independent of net sediment transport direction. This observation implies that there is some transport from the source but that the mere presence of the tracer in the estuary is not necessarily indicative of net landward sediment transport. The model also suggests that the magnitude of the tracer concentration in any estuarine stratigraphic record is controlled by 1) sediment discharge associated with the river input, 2) tracer concentration at the source, 3) processes of dispersion, 4) patterns of landward-seaward net sediment transport, and 5) the amount of time required for equilibrium to occur. Hence, net transport pathways can be defined on the basis of the magnitude of the tracer concentration in estuarine sediments, when compared against a critical level.

Longitudinal Dispersion Processes in the Upper Tamar Estuary

Measurements of velocity, salinity, and suspended solids concentration have been used to investigate the intra-tidal variation of vertical and transverse shear-induced dispersion. For the study research the interaction of the longitudinal density gradient and vertical shear during the early part of the ebb tide accounted for much of the net longitudinal dispersion of solute landward. The same mechanism also is shown to lead to a net particulate transport landward. The landward flux, however, takes place during the flood tide. The field data are also used to elucidate the tidally averaged tidal pumping mechanism.

Modelling tidal energetics of the Columbia River Estuary

The Columbia River Estuary is shallow and has a mixed diurnal and semidiurnal tide, strong riverflow, broad tidal flats and variable channel cross-section. It is weakly non-linear with respect to tidal forcing, as measured by a ratio of tidal amplitude to depth. These features, common to many shallow estuaries, were incorporated into a one-dimensional harmonic transport model. The harmonic method utilizes the fact that nearly all of the energy in the system is in a few fundamental tidal frequencies, the first overtides thereof and the mean flow. This allows the representation of the flow as a series of harmonic components whose spatial variability is determined by the model. The model provides a qualitative explantation for and accurate quantitative predictions of along-channel variations in tidal properties in terms of the momentum balance. Near the mouth of the estuary, the boundary shear stress, the pressure gradient and the acceleration terms are all important in the force balance, and the tide behaves like a damped oscillator. Far upriver the pressure gradient is primarily balanced by friction at the bed and tidal wave propagation can be described as a diffusion process. Changes in the channel width in mid-estuary cause partial reflection of the tidal wave and a maximum in tidal range. This partial reflection decreases the tidal energy flux in the upstream direction. The distribution of the time averaged energy fluxes was also determined from the model. The analysis shows that there are two regions of high energy dissipation. Energy is supplied near the mouth of the estuary by the divergence of the landward flux of tidal potential energy and far upriver by the divergence of the seaward flux of fluvial potential energy. These two regions of high dissipation are separated by a region of low dissipation, the energy flux divergence minimum. This spatial division of the estuary is mirrored in the system biology and geology. It is likely that the energy flux divergence minimum is a common feature of shallow estuaries with strong riverflow.