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Changes in tidal range due to anthropogenic impacts exacerbate nuisance flooding in estuaries along the U.S. coastline., Nuisance flooding (NF) is defined as minor, nondestructive flooding that causes substantial, accumulating socioeconomic impacts to coastal communities. While sea-level rise is the main driver for the observed increase in NF events in the United States, we show here that secular changes in tides also contribute. An analysis of 40 tidal gauge records from U.S. coasts finds that, at 18 locations, NF increased due to tidal amplification, while decreases in tidal range suppressed NF at 11 locations. Estuaries show the largest changes in NF attributable to tide changes, and these can often be traced to anthropogenic alterations. Limited long-term measurements from estuaries suggest that the effects of evolving tides are more widespread than the locations considered here. The total number of NF days caused by tidal changes has increased at an exponential rate since 1950, adding ~27% to the total number of NF events observed in 2019 across locations with tidal amplification.

An analysis of water level time series from 20 tide gauges in Southeast Asia finds that diurnal and semidiurnal astronomical tides exhibit strong seasonal variability of both amplitude and phase, which is not caused by known modulations of the astronomical tide-generating forces. Instead, it is found that the tidal properties are coherent with the western North Pacific monsoon index (WNPMI), indicating that monsoonal mechanisms are the likely cause. The study domain includes the Malacca Strait, Gulf of Thailand, the southern South China Sea, and Java Sea. The character of the geography and the tidal variability is different in each of these subregions. A new barotropic regional tide model is developed that incorporates the coupling between geostrophic currents, wind-driven (Ekman) currents, and tidal currents in the bottom boundary layer in order to examine the influence of these factors on tides. The dynamics thus preserve the frictional nonlinearities while neglecting advective nonlinearities and baroclinic tides, approximations that should be valid on the wide and shallow continental shelves in the study region. The model perturbation approach uses the climatological seasonal variability of wind stress and geostrophic currents, which are prescribed singly and in combination in the model, to explain the observed tidal variability. Results are most successful in the southern Gulf of Thailand and near Singapore, where it is found that the combined effect of geostrophic and Ekman currents shows increased skill in reproducing the tidal variability than individual models. Ambiguous results at other locations suggest more localized processes such as river runoff.

Ocean tides are changing worldwide for reasons unrelated to astronomical forcing. Changes in tidal properties coupled with altered mean sea level (MSL) may yield higher peak water levels and increased occurrence of short-term exceedance events such as storm surge and nuisance flooding. Here we investigate the hypothesis that changes in relative sea-level are correlated with alterations in tidal amplitudes. Our approach focuses on the correlation between short-term (monthly to interannual) fluctuations in sea-level with changes in tidal properties of major ocean tides (M2, and K1; S2 and O1) at 152 gauges. Results suggest that sea-level variability is correlated to inter-annual tidal variability at most (92%) of tide gauges in the Pacific, with statistically significant rates between ±10 and ±500 mm per meter sea-level rise observed. These tidal anomalies, while influenced by basin-scale climate processes and sea-level changes, appear to be locally forced (in part) and not coherent over amphidromic or basin-wide scales. Overall, the Western Pacific shows a greater concentration of tide/sea level correlations at interannual time scales than the Eastern Pacific; 44% and 46% of gauges are significant in K1 and O1 in the west compared to 29% and 30% in the east, and 63% and 53% of gauges in the west are significant in M2 and S2 versus 47% and 32% in the east. Seasonal variation in tidal properties is less apparent in the empirical record, with statistically significant seasonal variations observed at only 35% of all gauges, with the largest concentrations in Southeast Asia.

Scientists and engineers have observed for some time that tidal amplitudes at many locations are shifting considerably due to non-astronomical factors. Here we review comprehensively these important changes in tidal properties, many of which remain poorly understood. Over long geological time-scales, tectonic processes drive variations in basin size, depth, and shape, and hence the resonant properties of ocean basins. On shorter geological time-scales, changes in oceanic tidal properties are dominated by variations in water depth. A growing number of studies have identified widespread, sometimes regionally-coherent, positive and negative trends in tidal constituents and levels during the 19th, 20th and early 21st centuries. Determining the causes is challenging because a tide measured at a coastal gauge integrates the effects of local, regional, and oceanic changes. Here, we highlight six main factors that can cause changes in measured tidal statistics on local scales, and a further eight possible regional/global driving mechanisms. Since only a few studies have combined observations and models, or modelled at a temporal/spatial resolution capable of resolving both ultra-local and large-scale global changes, the individual contributions from local and regional mechanisms remain uncertain. Nonetheless, modelling studies project that sea-level rise and climate change will continue to alter tides over the next several centuries, with regionally coherent modes of change caused by alterations to coastal morphology and ice sheet extent. Hence, a better understanding of the causes and consequences of tidal variations is needed to help assess the implications for coastal defense, risk assessment, and ecological change.

This study explores the spatial and temporal character of turbidity maxima in the Columbia River Estuary (CRE) using satellite observations. Surface reflectance data measured by the Moderate Imaging Spectroradiometer (MODIS) were calibrated against in situ measurements of surface turbidity (R 2 = 0.85 for 205 measurements). More than 1500 satellite images from 2000 to 2015 were then conditionally sampled to explore the physical processes that drive the spatial distribution of the turbidity field. We find satellite measurements are able to describe seasonal, spring–neap, and spatial features of the estuarine turbidity maxima (ETM) that are not easily observable by other means. System-wide levels of turbidity are most sensitive to river flow and spring–neap tidal range, with a weaker correlation to wind and waves. Maximum surface turbidity is observed in winter during elevated flow from coastal tributaries and remains elevated during the spring freshet of the main stem Columbia. Two ETM with asymmetric along-channel profiles are observed, one in the North Channel and another in the South Channel. Turbidity distributions migrate downstream as tidal range and river flow increase but appear to become topographically trapped near topographic holes at river kilometers 15–20. Hence, depth-sensitive circulation processes like internal asymmetry and gravitational circulation are likely important mechanisms for trapping particles and determining ETM location. These conclusions confirm the theoretical result that along-channel distributions of turbidity should have an asymmetric distribution and emphasize the role of bottom topography.

Reliable estimation of freshwater inflow to the ocean from large tidal rivers is vital for water resources management and climate analyses. Discharge gauging stations are typically located beyond the tidal intrusion reach, such that inputs and losses occurring closer to the ocean are not included. Here, we develop a method of estimating river discharge using multiple gauges and time-dependent tidal statistics determined via wavelet analysis. The Multiple-gauge Tidal Discharge Estimate (MTDE) method is developed using data from the Columbia River and Fraser River estuaries and calibrated against river discharge. Next, we evaluate the general applicability of MTDE by testing an idealized two-dimensional numerical model, with a convergent cross-sectional profile, for eighty-one cases in which nondimensional numbers for friction, river flow, and convergence length scale are varied. The simulations suggest that MTDE is applicable to a variety of tidal systems. Model results and data analyses together suggest that MTDE works best with at least three gauges: a reference station near the river mouth, and two upstream gauges that respond strongly to distinct portions of the observed range of flow. The balance between tidal damping and amplifying factors determines the favorable location of the gauges. Compared to previous studies, the MTDE method improves the time resolution of estimates (from 2.5 to

Tidal rivers are a vital and little studied nexus between physical oceanography and hydrology. It is only in the last few decades that substantial research efforts have been focused on the interactions of river discharge with tidal waves and storm surges into regions beyond the limit of salinity intrusion, a realm that can extend inland hundreds of kilometers. One key phenomenon resulting from this interaction is the emergence of large fortnightly tides, which are forced long waves with amplitudes that may increase beyond the point where astronomical tides have become extinct. These can be larger than the linear tide itself at more landward locations, and they greatly influence tidal river water levels and wetland inundation. Exploration of the spectral redistribution and attenuation of tidal energy in rivers has led to new appreciation of a wide range of consequences for fluvial and coastal sedimentology, delta evolution, wetland conservation, and salinity intrusion under the influence of sea level rise and delta subsidence. Modern research aims at unifying traditional harmonic tidal analysis, nonparametric regression techniques, and the existing understanding of tidal hydrodynamics to better predict and model tidal river dynamics both in single-thread channels and in branching channel networks. In this context, this review summarizes results from field observations and modeling studies set in tidal river environments as diverse as the Amazon in Brazil, the Columbia, Fraser and Saint Lawrence in North America, the Yangtze and Pearl in China, and the Berau and Mahakam in Indonesia. A description of state-of-the-art methods for a comprehensive analysis of water levels, wave propagation, discharges, and inundation extent in tidal rivers is provided. Implications for lowland river deltas are also discussed in terms of sedimentary deposits, channel bifurcation, avulsion, and salinity intrusion, addressing contemporary research challenges.

Here, we demonstrate that reductions in the depth of inlets or estuary channels can be used to reduce or prevent coastal flooding. A validated hydrodynamic model of Jamaica Bay, New York City (NYC), is used to test nature-based adaptation measures in ameliorating flooding for NYC's two largest historical coastal flood events. In addition to control runs with modern bathymetry, three altered landscape scenarios are tested: (1) increasing the area of wetlands to their 1879 footprint and bathymetry, but leaving deep shipping channels unaltered, (2) shallowing all areas deeper than 2 m in the bay to be 2 m below Mean Low Water, (3) shallowing only the narrowest part of the inlet to the bay. These three scenarios are deliberately extreme and designed to evaluate the leverage each approach exerts on water levels. They result in peak water level reductions of 0.3%, 15%, and 6.8% for Hurricane Sandy, and 2.4%, 46% and 30% for the Category-3 hurricane of 1821, respectively (bay-wide averages). These results suggest that shallowing can provide greater flood protection than wetland restoration, and it is particularly effective at reducing "fast-pulse" storm surges that rise and fall quickly over several hours, like that of the 1821 storm. Nonetheless, the goal of flood mitigation must be weighed against economic, navigation, and ecological needs, and practical concerns such as the availability of sediment.

River-tide dynamics remain poorly understood, in part because conventional harmonic analysis (HA) does not cope effectively with nonstationary signals. To explore nonstationary behavior of river tides and the modulation effects of river discharge, this work analyzes tidal signals in the Yangtze River estuary using both HA in a nonstationary mode and continuous wavelet transforms (CWT). The Yangtze is an excellent natural laboratory to analyze river tides because of its high and variable flow, its length, and the fact that there are do dams or reflecting barriers within the tidal part of the system. Analysis of tidal frequencies by CWT and analysis of subtidal water level and tidal ranges reveal a broad range of subtidal variations over fortnightly, monthly, semiannual, and annual frequencies driven by subtidal variations in friction and by variable river discharges. We employ HA in a nonstationary mode (NSHA) by segregating data within defined flow ranges into separate analyses. NSHA quantifies the decay of the principal tides and the modulation of M4 tide with increasing river discharges. M4 amplitudes decrease far upriver (landward portion of the estuary) and conversely increase close to the ocean as river discharge increases. The fortnightly frequencies reach an amplitude maximum upriver of that for over tide frequencies, due to the longer wavelength of the fortnightly constituents. These methods and findings should be applicable to large tidal rivers globally and have broad implications regarding management of navigation channels and ecosystems in tidal rivers.

Interaction of barotropic tides with subsurface topography is vital to ocean mixing. Yet the behavior of large-amplitude, nonlinear, internal solitary waves (ISWs) that can cause strong mixing remains poorly understood, especially that of higher-mode and multimodal internal waves. Therefore, a 2.5-dimensional, nonhydrostatic model with adjustable vertical resolution was developed to investigate effects of upper-ocean stratification on tidally induced multimodal internal waves and to show how they are generated by the subcritical ridge where only upward-propagating internal wave beams (IWBs) are present. The effects of the stratification on properties and characteristics of the excited IWBs and on the energy partition of the radiated mode-1 and mode-2 internal waves were investigated based on the model results. Higher modes of internal waves can also be effectively generated in the IWBs by the subcritical topography, and the contribution to IWBs from higher modes increases with the upper-ocean stratification. Mode-2 ISWs can be excited from the IWBs if both the tidal Froude number and the contribution to IWBs from mode-2 waves are sufficiently high (U0 is the tidal current speed, and c2 is the phase speed of mode-2 waves). In a moderately stratified upper ocean, both mode-1 and mode-2 ISWs can be produced, but for weak (strong) stratification, only mode-1 (mode-2) ISWs are generated. Further, it is found that the distance between two successive mode-1 or mode-2 ISW trains increases linearly with the upper-ocean stratification. The ratio of the kinetic energy to the available potential energy for the mode-2 ISWs increases with the upper-ocean stratification in a strongly stratified ocean.

Hourly sea level is examined at 25 open-ocean stations in the Pacific Ocean with records longer than 30 yr. A search for trends finds that the amplitude of the dominant semidiurnal tide M2 is increasing at 12 of the 13 sites where a statistically significant trend can be identified. It is also found that nontidal variance in the neighborhood of M2 is decreasing at all 12 of the sites where a significant increase in M2 tide is occurring. The trend in amplitude of the dominant diurnal tide K1 is significant at six stations, and it is both increasing (four stations) and decreasing (two stations). The trend in semidiurnal-band variance suggests the hypothesis that increases in M2 could be caused by improvements in time keeping or data processing that would reduce the apparent phase variability of the tide. This possibility is examined and found to be the likely explanation for tidal trends at four stations. Local changes are found to explain the trend at two stations, Johnston Island and Mokuoloe, as diagnosed from correlated changes in nonlinear overtides and site history. Changes in M2 at the equatorial station of Kanton are significant and unexplained by either timing errors or local morphodynamics; although, strong nonlinear overtides are present. Changing tides at five stations in the western Pacific, Malakal, Yap, Saipan, Kapingamarangi, and Pohnpei cannot be explained by the above and suggest a region where a connection between climate and tides may yet be identified.

This study examines connections between mean sea level (MSL) variability and diurnal and semidiurnal tidal constituent variations at 17 open-ocean and 9 continental shelf tide gauges in the western tropical Pacific Ocean, a region showing anomalous rise in MSL over the last 20 years and strong interannual variability. Detrended MSL fluctuations are correlated with detrended tidal amplitude and phase fluctuations, defined as tidal anomaly trends (TATs), to quantify the response of tidal properties to MSL variation. About 20 significant amplitude and phase TATs are found for each of the two strongest tidal constituents, K1 (diurnal) and M2 (semidiurnal). Lesser constituents (O1 and S2) show trends at nearly half of all gauges. Fluctuations in MSL shift amplitudes and phases; both positive and negative responses occur. Changing overtides suggest that TATs are influenced by changing shallow water friction over the equatorial Western Pacific and the eastern coast of Australia (especially near the Great Barrier Reef). There is a strong connection between semidiurnal TATs at stations around the Solomon Islands and changes in thermocline depth, overtide generation, and the El Nino Southern Oscillation (ENSO). TATs for O1, K1, and M2 are related to each other in a manner that suggests transfer of energy from M2 to the two diurnals via resonant triad interactions; these cause major tidal variability on sub-decadal time scales, especially for M2. The response of tides to MSL variability is not only spatially complex, it is frequency dependent; therefore, short-term responses may not predict long-term behavior.

This two-part paper provides comprehensive time and frequency domain analyses and models of along-channel water level variations in the 234-km-long Lower Columbia River and Estuary (LCRE) and documents the response of floodplain wetlands thereto. In Part I, power spectra, continuous wavelet transforms, and harmonic analyses are used to understand the influences of tides, river flow, upwelling and downwelling, and hydropower operations (“power-peaking”) on the water level regime. Estuarine water levels are influenced primarily by astronomical tides and coastal processes and secondarily by river flow. The importance of coastal and tidal influences decreases in the landward direction, and water levels are increasingly controlled by river flow variations at periods from ≤1 day to years. Water level records are only slightly nonstationary near the ocean, but become highly irregular upriver. Although astronomically forced tidal constituents decrease above the estuary, tidal fortnightly and overtide variations increase for 80–200 km landward, both relative to major tidal constituents and in absolute terms. Near the head of the tide at Bonneville Dam, strong diel and weekly fluctuations caused by power-peaking replace tidal daily (diurnal and semidiurnal) and fortnightly variations. Tides account for 60–70 %, river flow and seasonal processes 5–20 %, and weather 2–4 % of the total variance in the seaward 60 km of the system. In the landward 70 km of the LCRE, seasonal-fluvial variations account for 80–90 % of the variance, power-peaking 1–6 %, and tides

Three of the nine highest recorded water levels in the New York Harbor region have occurred since 2010 (March 2010, August 2011, and October 2012), and eight of the largest twenty have occurred since 1990. To investigate whether this cluster of high waters is a random occurrence or indicative of intensified storm tides, we recover archival tide gauge data back to 1844 and evaluate the trajectory of the annual maximum storm tide. Approximately half of long-term variance is anticorrelated with decadal-scale variations in the North Atlantic Oscillation, while long-term trends explain the remainder. The 10 year storm tide has increased by 0.28 m. Combined with a 0.44 m increase in local sea level since 1856, the 10 year flood level has increased by approximately 0.72 ± 0.25 m, and magnified the annual probability of overtopping the typical Manhattan seawall from less than 1% to about 20–25%.

On 11 March 2011, the Tohoku Tsunami overtopped a weir and penetrated 49 km up the Kitakami River, the fourth largest river in Japan [1]. Similarly, the 2010 Chile tsunami propagated at least 15 km up the Maule River [2]. In the Pacific Northwest of the United States, large tsunamis have occurred along the Cascadia subduction zone, most recently the ‘orphan tsunami’ of 1700 (Atwater et al. [3]). The expected future occurrence of a Cascadia tsunami and its penetration into the Lower Columbia River became the subject of “the Workshop on Tsunami Hydrodynamics in a Large River” held in Corvallis, Oregon, 2011. We found that tsunami penetration into the Columbia River is quite different from a typical river. The tsunami enters the vast river estuary through the relatively narrow river mouth of the Columbia, which damps and diffuses its energy. The tsunami transforms into a long period, small amplitude wave that advances to Portland, 173 km from the ocean. Understanding this unique tsunami behavior is important for preparing a forthcoming Cascadia tsunami event.

We compute rates of sediment removal from the Columbia River, USA plume near field using a control volume technique. By using data from repeated transect passes along the plume axis, we account for variations in plume width, depth, and velocity, and we construct a framework for determining the dominant terms in the complete sediment balance. The contribution due to turbulent sediment flux is estimated using collocated measurements of turbulent shear and suspended-sediment concentration. The maximum sediment clearance rate on the landward end of the transect, 4 km from the river mouth, is approximately 0.1 g m−2 s−1, which corresponds to an effective removal velocity of 10 mm s−1. This is significantly higher than the settling velocities expected for single-grain and aggregate particles observed in the estuary. The plume clearance rate decreases seaward, with values of 0.01 g m−2 s−1 6.5 km from the river mouth. We investigate several potential mechanisms to explain the along-axis variability in sediment removal, including gravitational settling of several particle-size distributions and removal by turbulent mixing. A simple model that accounts for the settling of the distribution of particles observed in the estuary with the addition of a very fine sand settling class best explains the observed magnitude and the seaward decrease in the sediment removal rate. Although it is unlikely that fine sands are retained in the plume from the estuary due to their high settling speeds, we hypothesize that they may be locally resuspended landward of the transect. The turbulent flux of sediment out of the plume is almost an order of magnitude smaller than the total removal rates observed on the landward end of the transect. However, the turbulent flux generally is greater than the removal rate predicted for unflocculated fine silt, indicating that it may be important for smaller particle size distributions or farther from shore.

Columbia River tidal plume dynamics can be explained in terms of two asymmetries related to plume-front depth and internal wave generation. These asymmetries may be an important factor contributing to the observed greater primary productivity and phytoplankton standing crop on the Washington shelf. The tidal plume (the most recent ebb outflow from the estuary) is initially supercritical with respect to the frontal internal Froude number FR on strong ebbs. It is separated from the rotating plume bulge by a front, whose properties are very different under upwelling vs. downwelling conditions. Under summer upwelling conditions, tidal plume fronts are sharp and narrow ( FR and plume depth are key parameters in distinguishing the upwelling and downwelling situations, and these two asymmetries can be explained in terms of potential vorticity conservation. The divergence of the tidal outflow after it leaves the estuary embeds relative vorticity in the emerging tidal plume water mass. This vorticity controls the transition of the tidal plume front to a subcritical state and consequently the timing and location of internal wave generation by plume fronts.

The Columbia River plume is typical of large-scale, high discharge, mid-latitude plumes. In the absence of strong upwelling winds, freshwater from the river executes a rightward turn and forms an anticyclonic bulge before moving north along the Washington coast. In addition to the above dynamics, however, the river plume outflow is subject to large tides, which modify the structure of the plume in the region near the river mouth. Observations based on data acquired during a summer 2005 cruise indicate that the plume consists of four distinct water masses; source water at the lift-off point, and the tidal, re-circulating and far-field plumes. In contrast to most plume models that describe the discharge of low-salinity estuary water into ambient high-salinity coastal water, we describe the Columbia plume as the superposition of these four plume types. We focus primarily on a conceptual summary of the dynamics and mutual interaction of the tidal and re-circulating plumes. The new tidal plume flows over top of the re-circulating plume and is typically bounded by strong fronts. Soon after the end of ebb tide, it covers roughly 50–100% of the re-circulating plume surface area. The fronts may penetrate well below the re-circulating plume water and eventually spawn internal waves that mix the re-circulating plume further. The re-circulating plume persists throughout the tidal cycle and corresponds to a freshwater volume equivalent to 3–4 days of river discharge. Finally, the plume water masses are distinguished from one another in term of surface chlorophyll concentration, suggesting that the above classification may also describe different biological growth regimes. The low-salinity re-circulating plume serves as an extension of the estuary into the coastal ocean, or an “estuary at sea”, because residence times during periods of high river flow are greater than those in the estuary.

River plume front-generated internal solitons play an important role in the interaction between the plume and coastal waters. The internal solitons drive a non-harmonic velocity field, resulting in a horizontal transport that carries plume water seaward and redistributes nutrients and sediments. In this study, we present observations of internal solitons generated at the Columbia River plume front that separates the new, tidal plume, older plume and coastal waters. Scale analyses suggest that the plume front-generated internal solitons are highly non-linear waves, and their dynamic properties do not conform to any weakly non-linear theory. Thus, a high-order Korteweg–de Vries (KdV) theory is used to analyze the internal solitons. The comparison between theoretical values and cruise data shows that the high-order KdV model is much better than the weakly non-linear theories for prediction of the soliton dynamic parameters. Based on the model, we develop theoretical and numerical solutions of the soliton-induced upper layer horizontal transport and Lagrangian water parcel transport distance, which shows that the water particle drift, during the internal soliton passage, is as far as 1 km, and demonstrates the role of the internal solitons on the exchange between the plume and ambient coastal water. Energy fluxes caused by the internal solitons are estimated using the high-order KdV theory. The leading soliton fluxes 2.0×10 3 W m −1 per unit crest length, and carries energy of 4.2×10 5 J m −1 . The total energy carried by the eight internal solitons is 1.6×10 6 J m −1 , about 70% of the total frontal energy.

Traditional harmonic analysis of tides is highly sensitive to omnipresent environmental noise. Robust fitting is an extension of the ordinary least squares calculation of harmonic analysis that is more resistant to broad spectrum noise. Since the variance of the amplitude and phase is calculated from the power spectrum of the residual, a calculation that filters broad spectrum noise and reduces the residual variance will increase the confidence of the computed parameters, and also allows more low-amplitude constituents to be resolved from the background noise. Improvement in confidence and resolution of more constituents has obvious benefits to the resolution of both seasonal and long-term variation of amplitude and phase of tidal constituents. Using a 6 month calculation window, confidence intervals were systematically reduced by 30–85% over results calculated with standard methods, with an increase in resolved constituents of 20–75%. The analysis was carried out with Matlab, using the t-tide package [Pawlowicz, R., Beardsley, B., Lentz, S., 2002. Classical tidal harmonic analysis with errors in matlab using t-tide. Computers and Geosciences 28, 929–937], with modifications to accommodate Matlab's implementation of the Iteratively Reweighted Least Squares algorithm.

The long-term response of circulation processes to external forcing has been quantified for the Columbia River estuary using in situ data from an existing coastal observatory. Circulation patterns were determined from four Acoustic Doppler Profilers (ADP) and several conductivity–temperature sensors placed in the two main channels. Because of the very strong river discharge, baroclinic processes play a crucial role in the circulation dynamics, and the interaction of the tidal and subtidal baroclinic pressure gradients plays a major role in structuring the velocity field. The input of river flow and the resulting low-frequency flow dynamics in the two channels are quite distinct. Current and salinity data were analyzed on two time scales—subtidal (or residual) and tidal (both diurnal and semidiurnal components). The residual currents in both channels usually showed a classical two-layer baroclinic circulation system with inflow at the bottom and outflow near the surface. However, this two-layer system is transient and breaks down under strong discharge and tidal conditions because of enhanced vertical mixing. Influence of shelf winds on estuarine processes was also observed via the interactions with upwelling and downwelling processes and coastal plume transport. The transient nature of residual inflow affects the long-term transport characteristics of the estuary. Effects of vertical mixing could also be seen at the tidal time scale. Tidal velocities were separated into their diurnal and semidiurnal components using continuous wavelet transforms to account for the nonstationary nature of velocity amplitudes. The vertical structure of velocity amplitudes were considerably altered by baroclinic gradients. This was particularly true for the diurnal components, where tidal asymmetry led to stronger tidal velocities near the bottom.

Surface elevation and current records contain non-tidal variance, often dismissed as noise. The processes responsible for the non-tidal component may also modulate the tidal signal, altering its strength and frequency structure. Because of their manner of generation and propagation, internal tides are inherently irregular. The non-stationary character of these and other tidal processes provides an integral and useful property of tidal records, because it provides an opportunity to obtain insights into tidal dynamics and the interaction of tidal and non-tidal processes. It is, moreover, productive to use multiple approaches in analyzing coastal and estuarine tidal processes so that both the time-varying and average frequency content are determined. Only by confronting the causes of non-stationary behaviour in this way can some of the remaining challenges in tidal analysis and prediction be overcome, e.g. shelf and estuarine currents, river tides, internal tides, tide-surge interactions and tidally influenced ecological processes. Several examples illustrate the utility of non-stationary tidal analysis methods.

Tidal variations in estuarine stratification are revealed by the depth and thickness of the density interface. The depth of the interface may be predicted using an inviscid two-layer model that combines baroclinic estuarine circulation with barotropic tidal currents [Helfrich, 1995]. Here we present results from a two-layer model modified to include the effects of bottom friction and interfacial mixing. Modeled layer thickness and speed compare favorably with prior analytic studies [Farmer and Armi, 1986; Pratt, 1986]. We use a bulk Richardson number criterion to estimate the thickness of the pycnocline from two-layer model results; the predicted pycnocline depth and thickness compare remarkably well with observations. We also investigate the effects of changing bottom friction and barotropic currents on the pycnocline thickness.

Continuous wavelet transforms (CWTs) provide an approach to understanding the numerous tidal phenomena that deviate markedly from an assumed statistical stationarity or exact periodicity inherent in traditional tidal methods. Use of wavelets allows determination of the degree of non-stationarity present in time series, such as estuarine and shelf currents, usually treated as stationary. Wavelets also provide a consistent analysis of tidal and non-tidal variance, a feature often essential for dynamical analyses of non-stationary tides. We summarize basic notions of the wavelet transform, also known as a perfect reconstruction filter bank or a multire solution analysis, contrast them with those of harmonic analysis and Fourier transforms, construct a continuous wavelet transform basis with a scale selection especially adapted to tidal problems, describe possibilities for analysis of scalar and vector quantities, define a criterion for knowledge of independence of process between adjoining scales, and illustrate use of wavelet tools with several examples. In contrast to the nearly periodic barotropic tide typical of coastal stations, this paper analyses processes that are in part tidally driven but non-stationary, e.g. baroclinic tidal currents, river tides, continental shelf internal tides, and some kinds of biological activity in the coastal ocean. In all cases, wavelet analysis provides a consistent, linear analysis of tidal and non-tidal variance and reveals features that harmonic analysis on a Fourier transform approach could not elucidate.

Many tidal phenomena, including river tides, estuarine currents, and shelf and fjord internal tides, are non-stationary. These tidal processes are poorly understood and largely beyond the realm of practical prediction, even when the perturbing phenomenon causing the non-stationary behavior is itself fairly predictable. Our inadequate understanding of these phenomena has been exacerbated by an absence of a self-consistent procedure for analysis of the entire spectrum of non-stationary motions, tidal and non-tidal. The most difficult methodological situation occurs when the disturbing non-tidal signal is stronger than the tidal one and has an event-like character. Such sharp changes in forcing are multi-scale, containing energy at tidal frequencies, as well as at larger sales. Because of the distinct response of different parts of the tidal spectrum to non-tidal perturbations, multi-scale forcing events have the potential to provide a valuable new generation of tests of tidal dynamics models. This paper compares three techniques for the analysis of such signals, using artificial tidal records of known frequency content to ascertain which method most accurately represents evolving frequency content. The methods are: (a) conventional least-squares, short-term harmonic analysis (STHA); (b) a modified STHA (or mSTHA) that uses a smoothing window and augments the frequency structure of the analysis wave; and (c) linear convolution analysis in the form of continuous wavelet transforms (CWT). Results show that STHA and mSTHA lack a definable frequency response and mix energy between tidal and non-tidal signals in an unpredictable manner. STHA also effectively imposes a boxcar window on the data, the effects of which can be serve for short records. In general, STHA and mSTHA results using short windows will be least reliable in the circumstances where short windows are most desired—when the signal is highly non-stationary. There is, moreover, no simple way to set a minimum window length for STHA/mSTHA that will produce stable results, except to make the window too long to capture the fluctuating variance being sought. In contrast, CWT correctly recovers both tidal and non-tidal variance, as long as resolution limits set by the Heisenberg uncertainty principle are respected. Whatever method is chosen, use of window lengths less than ∼4–6 d requires great care, unless diurnal and subtidal energy are insignificant.

A comprehensive study of the strongly wind driven midlatitude buoyant plume from the Columbia River, located on the U.S. west coast, demonstrates that the plume has two basic structures during the fall/winter season, namely, a thin (∼5–15 m), strongly stratified plume tending west to northwestward during periods of southward or light northward wind stress and a thicker (∼10–40 m), weakly stratified plume tending northward and hugging the coast during periods of stronger northward stress. The plume and its velocity field respond nearly instantaneously to changes in wind speed or direction, and the wind fluctuations have timescales of 2–10 days. Frictional wind-driven currents cause the primarily unidirectional flow down the plume axis to veer to the right or left of the axis for northward or southward winds, respectively. Farther downstream, currents turn to parallel rather than cross salinity contours, consistent with a geostrophic balance. In particular, during periods when the plume is separated from the coast, currents tend to flow around the mound of fresher water. At distances exceeding about 20 km from the river mouth, the along-shelf depth-averaged flow over the inner to midshelf is linear, and depth-averaged acceleration is governed to lowest order by the difference between surface and bottom stress alone. In this region, along-shelf geostrophic buoyancy-driven currents at ∼5 m (calculated from surface density) and along-shelf geostrophic wind-driven currents (computed from a depth-averaged linear model) are comparable in magnitude (∼10–25 cm s−1).

Wavelet transforms provide a valuable new tool for analysis of tidal processes that deviate markedly from an assumption of exact periodicity inherent in traditional harmonic analysis. A wavelet basis adapted to nonstationary tidal problems is constructed and employed to analyze the modulation of the external tide in a river by variations in streamflow. Interaction of a surface tide with river flow is the best available demonstration of the continuous wavelet transform (CWT) methods developed. It is the simplest and perhaps the only nonstationary tidal process for which both sufficient data and dynamical understanding exist to allow detailed comparisons between CWT analyses and analytical predictions of the response of tides to nontidal forcing. Variations at upriver locations of low-frequency elevation (river stage ZR) and three tidal species are deduced from cross-sectionally integrated equations. For landward propagation in a channel of constant cross section with quadratic friction, the log of the amplitude of the diurnal (D1), semidiurnal (D2) and quarterdiurnal (D4) elevations should vary at far upriver locations with the square root of the river flow (QR;), and river stage (ZR) should depend on the square of river flow. Convergent geometry and species-species frictional interactions modify these predictions somewhat. CWT analyses show that the predicted amplitude behavior for the tidal species is approximately correct. Best results are obtained for the dominant, dynamically simplest processes (ZR and D2). In the past, further progress in understanding river tides has been limited by a lack of data analysis tools. Data analysis tools are now clearly better than the available analytical solutions.

[1] Large rivers represent gateways for the transport of terrigenous and anthropogenic material to the coastal ocean. Here we document a ∼700 km2 recirculation or bulge associated with the Columbia River plume that retains recently discharged river water sufficiently to create a regional bioreactor. Fueled by a fluvial nitrate source, this feature stimulated growth across three trophic levels and may buffer this gateway system during periods of increased warming and stratification that lead to decreased ocean productivity, potentially enhancing production at multiple trophic levels and enriching surface waters far from the river mouth.

[1] River Influences on Shelf Ecosystems (RISE) is the first comprehensive interdisciplinary study of the rates and dynamics governing the mixing of river and coastal waters in an eastern boundary current system, as well as the effects of the resultant plume on phytoplankton standing stocks, growth and grazing rates, and community structure. The RISE Special Volume presents results deduced from four field studies and two different numerical model applications, including an ecosystem model, on the buoyant plume originating from the Columbia River. This introductory paper provides background information on variability during RISE field efforts as well as a synthesis of results, with particular attention to the questions and hypotheses that motivated this research. RISE studies have shown that the maximum mixing of Columbia River and ocean water occurs primarily near plume liftoff inside the estuary and in the near field of the plume. Most plume nitrate originates from upwelled shelf water, and plume phytoplankton species are typically the same as those found in the adjacent coastal ocean. River-supplied nitrate can help maintain the ecosystem during periods of delayed upwelling. The plume inhibits iron limitation, but nitrate limitation is observed in aging plumes. The plume also has significant effects on rates of primary productivity and growth (higher in new plume water) and microzooplankton grazing (lower in the plume near field and north of the river mouth); macrozooplankton concentration (enhanced at plume fronts); offshelf chlorophyll export; as well as the development of a chlorophyll ‘‘shadow zone’’ off northern Oregon.

[1] Analysis of the Columbia River tidal plume using Lagrangian frontal equations provides a concise description of the evolution of frontal depth H, velocity U, reduced gravity g′, and frontal internal Froude number FR. Because the estuary mouth is narrow, the initial radial plume motion is supercritical (FR > 1) for up to 12 hours. Understanding this supercritical phase is vital, because plume properties change rapidly, with strong ecosystem impacts. To analyze this expansion, analytical and numerical models (the latter with three mixing formulations) were tested. Model results are compared to synthetic aperture radar images to verify that the predicted frontal properties are realistic. Lagrangian theory provides especially simple constraints (independent of the mixing model) on spatial variations in FR and Ug′H. For parameters representative of the Columbia River plume, the plume spreads 10–35 km and thins to 25–60% of its initial depth before becoming subcritical. After liftoff, FR increases as the front accelerates and thins, it then decreases to unity; g′ decreases, but more slowly than H and U. H, controlled by a balance between spreading and mixing, first decreases then increases. The strength of vertical mixing and the mixing efficiency EF (ratio of buoyancy flux to dissipation) both play a significant role in determining plume properties, and it is important to include both the horizontal gradient in g′ and surface slope component of the pressure gradient. A model with an Ellison-Turner type entrainment scheme predicts the plume trajectory better than one that assumes a constant interfacial gradient Richardson number.

[1] Measurements of suspended sediment concentration, velocity, salinity, and turbulent microscale shear in the near-field region of the Columbia River plume are used to investigate the mechanisms of sediment resuspension and entrainment into the plume. An east-west transect was occupied during spring and neap tide periods in August 2005 and May 2006, corresponding to low and high river discharge conditions, respectively. During the high-discharge period the plume is decoupled from the bottom, and fine sediment resuspended from the bottom does not leave the benthic boundary layer. The primary modes of sediment transport associated with the plume are advection of sediment from the estuary and removal of sediment from the plume by gravitational settling and turbulent mixing. In contrast, the plume is much less stratified during low-discharge conditions, and large resuspension events are observed that entrained sediment through the water column and into the plume. Our measurements indicate that two factors control the magnitude and timing of sediment resuspension and entrainment: the supply of fine sediment on the seabed and the relative influence of tidal turbulence compared with buoyancy input from the river. The latter is quantified in terms of the estuary Richardson number RiE. The magnitude of vertical turbulent sediment flux is correlated with RiE during the low-flow period when there is a sufficient supply of bottom sediment in the near-field region. Such sediment resuspension may be an important mechanism for the delivery of bioavailable micronutrients to the plume during the summer.

[1] Large-amplitude nonlinear internal waves (NLIWs) are frequently observed propagating away from Columbia River tidal plume fronts. They are generated because of the deceleration of the frontal bulge. During the River Influences on Shelf Ecosystems project cruises, the velocity, density structure and acoustic backscatter of the plume fronts and frontal NLIWs were observed using a towed vehicle, vessel-mounted instrumentation, and a vessel X band radar. These observations indicate that in the presence of strong ambient velocity shear, the NLIWs with maximum amplitudes occur well below the density interface and at a depth deeper than in the absence of shear. This deepening of the NLIW shear is associated with intensified vertical mixing below the interface. This is consistent with a dynamic analysis of NLIWs under the influence of sheared ambient flow, obtained from a high-order KdV model. There are two mechanisms responsible for the vertical turbulent mixing intensification. The nonlinear interaction between ambient shear and the NLIWs increases total velocity shear and causes the gradient Richardson number Rig to decrease below a critical value in certain depth ranges. Also, because of the presence of the ambient shear, the maximum NLIW velocity shear occurs at a depth below the density interface where there is less stratification.

[1] Global sea level (GSL) rise is well documented. However, changes in high waters, including the tidal contribution, are sometimes more relevant than GSL rise. Analysis of 34 long tidal records from the Eastern Pacific Ocean shows that K1 and M2 amplitudes (∣K1∣ and ∣M2∣) are increasing, except for ∣M2∣ in the Gulf of Panama. North of 18°N, ∣K1∣ and ∣M2∣ are both growing at 2.2% century−1. The mean increase in total tidal amplitude (0.59 mmyr−1) is less than the present GSL rise (1.7 mmyr−1). However, mean sea level is nearly constant in the NE Pacific, so tidal evolution plays a major regional role in changes in high water levels. The spatial pattern of tidal evolution suggests the influence of large-scale processes, and the similarity in spatial patterns for ∣K1∣ and ∣M2∣ excludes mechanisms with strong frequency dependence. Increasing tidal amplitudes may impact ocean mixing, nutrient supply, primary production, fisheries, and coastal erosion.

Understanding tidal and mean flow transport mechanisms that maintain the estuarine salt balance against seaward transport by river flow is one of the fundamental questions of estuarine oceanography. Previous salt transport studies have failed to resolve this problem for two principal methodological reasons, in addition to the inherent variability of estuaries. First, salt transport expansions used to represent the salt balance have included a large number of terms that could not be related to any theory of estuarine circulation and whose physical meaning was thus unclear. Second, it has proven difficult to measure small mean velocities in the presence of much larger tidal variations. A new analysis method that expresses Lagrangian scalar property transport in terms of time and space variations of Eulerian variables is much simpler than expansions previously used and yields new insights into estuarine transport processes. This approach (like previous expansions) is valid for narrow systems in which cross-channel transport processes are weak and the ratio of tidal amplitude to mean depth is small. A surface boundary condition is used to eliminate one class of troublesome terms. The equivalence of the tidal-cycle average transport of tidal variations in salinity and the Stokes transport of mean salinity is then employed to show that the vertically integrated tidal flow plays no role in the overall salt balance. That is, seaward transport of salt caused by the river flow can only be balanced by landward transport resulting from correlations between shear and stratification in the mean, tidal and overtide flows. In a hypothetical inviscid estuary, tide and overtide flows would be vertically uniform, and outward fluvial salt transport could only be balanced by shear and stratification in the mean gravitational circulation. In a more realistic example with strong friction, the gravitational circulation would be severely damped, and inward transport could only be accomplished by correlations of shear and stratification in the tide and overtide flows.

The Columbia River Estuary is an energetic, sand-bedded system. Mixed tides with a spring tidal range of about 3.6m and a large freshwater discharge produce bottom shear stresses capable of transporting nearly all the sizes of sediment present in the estuary. As a result, the morphology of the estuary is closely related to the fluxes of tidal and fluvial energy. To investigate the relationship between energetics and sedimentary processes, an energy budget based on a one-dimensional harmonic tidal model was developed for the estuary-tidal river system and has been compared with the results of geological studies of the bedforms, large-scale morphology, sediment distribution, and suspended sediment field. The pattern of energy fluxes predicted by the model suggests the division of the system into three regimes: a tidally dominated lower estuary, a mid-estuary energy flux divergence (EFD) minimum region, and a fluvially controlled, tidal-fluvial reach. Model results also show that non-linear interactions between the tidal and fluvial flows are responsible for the suppression of the tides in the tidal-fluvial reach during periods of high flow. Finally, changes in tidal amplitude at the mouth result in a less than proportionate response in tidal elevations at points inside the estuary, because the cubic dependence of dissipation on tidal amplitude damps system responses to such changes. Many features of the observed sediment transport patterns and sedimentary environments can be related to the energy budget. Most of the medium to coarse sands entering the system from the river are permanently retained within the EFD minimum. Much of this deposition takes place upstream of the limits of salinity intrusion and is not, therefore, related to baroclinic circulatory effects. Most of the fine sands and the silts and clays entering the system are not permanently retained. Some of the silts and clays are, however, temporarily retained in a turbidity maximum, whose mean position is near the lower end of the EFD minimum. This position is dictated by the inability of salinity intrusion to extend up the fluvial potential energy gradient. Although some aspects of sedimentary processes cannot readily be related to the energy budget, our model provides a valuable conceptual approach to the dominant long-term sedimentary processes of sand transport and deposition in the Columbia River Estuary that structure the observed sedimentary environments. Biological processes are in turn strongly influenced by these geological patterns.

This paper has two purposes. The first is to use tidal-monthly variations in the density and velocity fields and the salt and water transports as key to understanding the circulation of the Columbia River Estuary and other river estuaries. The Columbia River Estuary is a good natural laboratory in this regard, because the flushing time of the system (a few days) is short relative to the tidal month during all seasons. This allows the occurrence of distinct transitions from a strongly to a weakly stratified water column (and back) during the tidal month. Furthermore, because atmospheric processes are secondary to riverflow and tidal influence in determining the circulation, most of the energy in circulatory phenomena is confined to distinct tidal, tidal-monthly and seasonal frequency bands. Observations of salt transport and neap-spring transitions reported herein should provide important constraints on future theoretical studies of estuarine circulation. The second purpose is to describe the circulation and density field of the Columbia River Estuary as background for understanding the geologic and biological investigations discussed in other papers in this volume. Previous investigations have focused on seasonal variations in riverflow as governing the turbidity maximum and biological productivity. Studies reported in this volume show that tidal monthly variations in circulatory processes are of comparable importance. With regard to the velocity field, the influence of stratification causes the tidal flow to show the largest vertical variations in phase and amplitude in the lower estuary. The vertical distribution of the mean current is controlled by the ebb-flood asymmetry in the time-dependent flow, vertical mixing processes, the baroclinic pressure gradient, and interaction of the flow with topography. Net upstream bottom flow is weak or absent during periods of weak stratification; it is substantially only when the system is highly stratified. Net upstream bottom flow tends to occur downstream of, and net downstream bottom flow upstream of, topographic highs. This pattern of convergences in the mean flow tends to preserve these topographic highs and lows and is not consistent with the traditional view that the baroclinic mean flow and mean salinity distribution structure one another; the former results instead primarily from ebb-flood asymmetry in the time-dependent flow. Comprehensive salt transport calculations were carried out, because knowledge of salt transport variations during the tidal month is vital to understanding the causes of the observed transitions in the density and velocity fields that occur between extremes of tidal range. These calculations show that neap-spring and seasonal variability of the gross features of the salinity intrusion in the lower estuary is limited by compensating adjustments of the tidal advective and mean-flow salt transports under most riverflow conditions. In contrast, the absence of such balancing mechanisms in combination with substantial neap-spring changes in vertical mixing allow large variations in the density field and salinity intrusion length in the upper estuary. Neap-spring variability is much higher during the low-flow season than during the high-flow season. Under the lowest riverflow conditions permitted by the present dam system, large tidal monthly changes in stratification occur throughout the system. The salt transport calculations also show that salt is carried into the estuary near mid-depth by tidal mechanisms acting primarily in the North Channel. Salt is transported out of the estuary closer to the surface of the South Channel by the strong mean flow. Mean velocities near the bottom are weak, and near-bottom, upstream salt transport by the mean flow is small. It is likely that inward tidal transport of salt occurs at mid-depth in many estuaries, as this is where salinity variations during the tidal cycle are usually greatest. Finally, the dynamical observations presented here, together with the known input of sediment from the river to the South Channel suggest that the concentration of fine sediment by the tidal asymmetry in a turbidity maximum should be stronger in the South than the North Channel. Limited observations confirm this.

~Fisheries Research Institute, WH-IO, University of Washington, Seattle, WA 98195, USA ZCollege of Oceanography, Oregon State University, Corvallis, OR 97331, USA SDepartment of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331, USA 4Geophysics Program, AK-50, University of Washington, Seattle, WA 98195, USA 5School of Oceanography, WB-I O, University of Washington, Seattle, WA 98195; current affiliation - Battelle, Pacific Marine Science Laboratories, Sequim, WA 98382, USA

Historical changes in the hydrology, sedimentology, and physical oceanography of the Columbia River Estuary have been evaluated with a combination of statistical, cartographic, and numerical-modelling techniques. Comparison of data digitized from US Coast and Geodetic Survey bathymetric surveys conducted in the periods 1867–1875, 1926–1937, and 1949–1958 reveals that large changes in the morphology of the estuary have been caused by navigational improvements (jetties, dredged channels, and pile dikes) and by the diking and filling of much of the wetland area. Lesser changes are attributable to natural shoaling and erosion. There has been roughly a 15% decrease in tidal prism and a net accumulation of about 68 × 106m3 of sediment in the estuary. Large volumes of sediment have been eroded from the entrance region and deposited on the continental shelf and in the balance of the estuary, contributing to formation of new land. The bathymetric data indicate that, ignoring erosion at the entrance, 370 to 485 × 106m3 of sediment has been deposited in the estuary since 1868 at an average rate of about 0.5 cm y−1, roughly 5 times the rate at which sea level has fallen locally since the turn of the century. Riverflow data indicate that the seasonal flow cycle of the Columbia River has been significantly altered by regulation and diversion of water for irrigation. The greatest changes have occurred in the last thirty years. Flow variability over periods greater than a month has been significantly damped and the net discharge has been slightly reduced. These changes in riverflow are too recent to be reflected in the available in the available bathymetric data. Results from a laterally averaged, multiple-channel, two-dimensional numerical flow model (described in Hamilton, 1990) suggest that the changes in morphology and riverflow have reduced mixing, increased stratification, altered the response to fortnightly (neap-spring) changes in tidal forcing, and decreased the salinity intrusion length and the transport of salt into the estuary. The overall effects of human intervention in the physical processes of the Columbia River Estuary (i.e. decrease in freshwater inflow, tidal prism, and mixing; increase in flushing time and fine sediment deposition, and net accumulation of sediment) are qualitatively similar to those observed in less energetic and more obviously altered estuarine systems. A concurrent reduction in wetland habitats has resulted in an estimated 82% reduction in emergent plant production and a 15% reduction in benthic macroalgae production, a combined production loss of 51,675 metric tons of organic carbon per year. This has been at least partially compensated by a large increase in the supply of riverine detritus derived from freshwater phytoplankton primary production. Comparison of modern and estimated preregulation organic carbon budgets for the estuary indicates a shift from a food web based on comparatively refractory macrodetritus derived from emergent vegetation to one involving more labile microdetritus derived from allochthonous phytoplankton. The shift has been driven by human-induced changes to the physical environment of the estuary. While this is a relatively comprehensive study of historical physical changes, it is incomplete in that the sediment budget is still uncertain. More precise quantification of the modern estuarine sediment budget will require both a better understanding of the fluvial input and dredging export terms and a sediment tranport model designed to explain historical changes in the sediment budget. Oceanographic studies to better determine the mechanisms leading to the formation of the turbidity maximum are also needed. The combination of cartography and modelling used in this study should be applicable in other systems where large changes in morphology have occurred in historical time.

[1] Internal solitons generated at the Columbia River plume front during ebb tide are displayed on a synthetic aperture radar (SAR) image taken on 26 July 2004 at 14:24:22 UTC. Scale analyses suggest that these internal solitons belong to the finite-depth category. A theoretical model, relating the radar backscatter cross section to dynamic parameters of internal solitons in the continuous stratification, is developed. Using the model and background stratification data collected by the River Influences on Shelf Ecosystem (RISE) project in July 2004, we extract the soliton dynamic parameters of the half-width, amplitude, phase speed, and average energy flux per crest length along a typical section across the internal solitons. The observed stratification is simulated by using a theoretical model parameterized by pycnocline depth, thickness, and maximum buoyancy frequency. The half-width, amplitude, phase speed, and energy flux of the leading soliton at the section are 119.3 m, 9.3 m, 0.69 m s−1, and 417.4 W m−1 with the uncertainty ranges of ±14.7 m, ±2.6 m, ±0.10 m s−1, and ±281.6 W m−1, respectively. Sensitivity analyses reveal that the extraction of the soliton half-width from the SAR image is not affected by the stratification. The amplitude is less influenced by the pycnocline thickness than the depth and is independent of the maximum buoyancy frequency.

[1] We present shipboard observations of very strong convergence, vertical velocities and mixing, and near-bed impacts associated with the leading-edge front of the tidally-pulsed Columbia River plume. With upwelling-favorable winds and riverflow of 4900 m3s−1, the plume propagates as a buoyant gravity current with a rotary, bore-like vertical frontal circulation and downwelling as strong as 0.35 m s−1. In waters as deep as 65 m, near-bed currents intensify to as much as 1.0 m s−1 after frontal passage, and are often associated with elevated acoustic backscatter. Mixing is locally strong, with an eddy diffusivity of O(0.2 m2s−1) 50 m behind the front, and T-S diagrams imply plume mixing with 10 m deep ocean water. These observations indicate that the leading-edge front of a surface-advected plume can cause exchanges of (a) nutrients between cold subsurface shelf waters and the river plume, and (b) nutrients and sediments across the sediment-water interface.

[1] This article is part 2 of a set of papers addressing mixing in a highly stratified estuary. In Part 2, turbulence and mean flow measurements are together used to understand the role of mixing in the overall dynamics of the system. Estimates of the partition of energy dissipation between bed-generated and internal mixing suggest that internal mixing accounts for about two thirds of the total turbulent kinetic energy (TKE) produced in the salt-wedge reach of the Columbia River estuary during the neap-tide, moderate river-flow observation period. This suggests that internal mixing plays a larger role in the energetics of such systems than has previously been recognized. The lack of near-bed mixing on the neap is also likely a major factor in the accumulation of sediment in the estuary on neap tides. Profiles of estimated turbulent kinetic energy (TKE) dissipation were obtained using a new approach, which we call the “method of constrained differences,” that allows the calculation of all major terms in the momentum and density balances during ebb mixing events from data obtained by a single vessel. The method of constrained differences uses a local equilibrium turbulence closure and measured velocity and density profiles to determine the density flux divergence necessary for internal consistency in the momentum and mass equations. The calculated mixing rates from the iterative procedure compare favorably with measured dissipation rates, tending to validate both the measurements and the procedure. Although this method involves a constraint of the form of a known or inferred stress at some point in the water column, it has the potential for wide application, especially if point measurements of TKE dissipation rates are available.

[1] This article is Part I of a set of papers addressing mixing in a highly stratified estuary. Measurements of interfacial turbulence were made in Columbia River estuary under conditions of moderate river flow and neap tides. A flux correlation method was used to determine buoyancy fluxes, and fits to theoretical velocity and temperature variance spectra were used to measure turbulent kinetic and potential energy dissipation rates. At the measurement site in the interior of the estuary, mixing along the top of the salt wedge is found to occur only on ebb, during periods when the internal Froude number is supercritical. The wedge cannot, therefore, be in a quasiequilibrium state, and vertical mixing plays a dominant role in its ebb retreat. The turbulent Froude number FrT, which represents the ratio of effects of shear and stratification on the mixing, was found to be close to FrT ≈ 1 during mixing episodes. This suggests an interplay between shear and buoyancy effects that corresponds to a condition of maximum mixing efficiency Γ. Γ is measured using both direct calculations of the buoyancy and dissipation terms in the turbulent kinetic energy (TKE) balance (this estimate of Γ is referred to as Γ0), and by estimating the buoyancy term B by its relationship to χT, the dissipation of temperature variance (this estimate of Γ is referred to as Γd). Averages over several mixing events give Γ0 ≈ 0.22 and Γd ≈ 0.36. Sources of bias in these measurements result from (1) approximations to the full TKE and available potential energy (APE) balances, (2) empirical constants used in fits of measured spectra in inertial subranges, (3) frame motion, and (4) high-pass filtering. The sign of the biases suggest that 0.22 < Γ < 0.36. Flood-ebb asymmetry in mixing also results in a time dependent force balance (discussed further by Kay and Jay [2003]). This asymmetry is analyzed using a new method to determine profiles of the momentum balance during ebb mixing events.