David A. Jay

Particle trapping in estuarine tidal flows

Particle trapping in estuarine turbidity maxima (ETM) is caused primarily by convergent mean and/or tidal fluxes of sediment. The result is an approximately bell-shaped along-channel distribution of vertically integrated, tidal cycle mean suspended sediment concentration. Observations from the Columbia River estuary suggest that (1) strong two-layer or internal along-channel residual and overtide flows are generated by time-varying stratification and (2) correlations between the near-bed velocity and the suspended sediment fields at these frequencies are important in landward transport of sediment. A new spatially and temporally integrated form of the sediment conservation equation has been derived to analyze this trapping process. Time changes in tidally averaged sediment concentration between two estuarine cross sections can be shown to be related to the divergence of the seaward, river flow transport; the divergence of velocity shear-sediment stratification correlations for the mean flow and each tidal constituent; and net erosion or deposition at the bed. Vertically integrated variables other than seaward river transport are absent from this integrated balance. Analysis of sediment fluxes using this balance supports the idea that internal residual and overtide circulations are primarily responsible for the landward sediment transport on the seaward side of ETM found near the upstream limits of salinity intrusion. The balance also shows that attempts to represent fluxes causing trapping of sediment in an ETM as a product of a time-mean, vertically integrated, along-channel gradient and a diffusivity inevitably lead to the appearance of countergradient transport and thus a negative diffusivity on the seaward side of the ETM. This result occurs because the trapping process is inherently nonlinear and at least two-dimensional and because a one-dimensional representation is physically unrealistic.

The Columbia River Plume Study: Subtidal variability in the velocity and salinity fields

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).

Circulation, density distribution and neap-spring transitions in the Columbia River Estuary

Abstract 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.

Historical changes in the Columbia River Estuary

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.

River Influences on Shelf Ecosystems: Introduction and synthesis

[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.

Green's law revisited: Tidal long‐wave propagation in channels with strong topography

Greens Law states that tidal long-wave elevation ζ and tidal transport Q vary with width b and depth h according to ζ ≌ b−1/2h−1/4 and Q ≌ b+1/2h+/4. This solution is of limited utility because it is restricted to inviscid, infinitesimal waves in channels with no mean flow and weak topography (those with topographic scale L ≫ wavelength λ). An analytical perturbation model including finite-amplitude effects, river flow, and tidal flats has been used to show that (1) wave behavior to lowest order is a function of only two nondimensional parameters representing, respectively, the strength of friction at the bed and the rate of topographic convergence/divergence; (2) two different wave equations with nearly constant coefficients can be derived that together cover most physically relevant values of these parameters, even very strong topography; (3) a single, incident wave in a strongly convergent or divergent geometry may mimic a standing wave by having a ≡ 90° phase difference between Q and ζ and a very large phase speed, without the presence of a reflected wave; (4) channels with strong friction and/or strong topography (L ≪ λ) show very large deviations from Green/s Law; and (5) these deviations arise because both frictional damping and the direct dependence of |Q| and |ζ| on topography (topographic funnelling) must be considered.

Interaction of fluctuating river flow with a barotropic tide: A demonstration of wavelet tidal analysis methods

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.

A review of recent developments in estuarine scalar flux estimation

The purpose of this contribution is to review recent developments in calculation of estuarine scalar fluxes, to suggest avenues for future improvement, and to place the idea of flux calculation in a broader physical and biogeochemical context. A scalar flux through an estuarine cross section is the product of normal velocity and scalar concentration, sectionally integrated and tidally averaged. These may vary on interannual, reasonal, tidal monthly, and event time scales. Formulation of scalar fluxes in terms of an integral scalar conservation expression shows that they may be determined either through “direct” means (measurement of velocity and concentration) or by “indirect” inference (from changes in scalar, inventory and source/sink terms). Direct determination of net flux at a cross section has a long and generally discouraging history in estuarine oceanography. It has proven difficult to extract statistically significant net (tidally averaged) fluxes from much larger flood and ebb transports, and the best mathematical representation of flux mechanisms is unclear. Observations further suggest that both lateral and vertical variations in scalar transport through estuarine cross sections are large, while estuarine circulation theory has focused on two-dimensional analyses that treatment either vertical or lateral variations but not both. Indirect estimates of net fluxes by determination of the other relevant terms in an integral scalar conservation balance may be the best means of determining scalar import-export in systems with residence times long relative to periods of tidal monthly fluctuations. But this method offers, little insight into the interaction of circulation modes and scalar fluxes, little help in verifying predictive models, and may also be difficult to apply in some circumstances. Thus, the need to understand, measure, and predict anthropogenic influences on transport or carbon, nutrient, suspended matter, trace metals, and other substances across the land-margin brings a renewed urgency to the issue of how to best carry out estuarine scalar flux determination. An interdisciplinary experiment is suggested to test present understanding, available instrument, and numerical models.

Impacts of Columbia River discharge on salmonid habitat: 2. Changes in shallow‐water habitat

[1] This is the second part of an investigation that analyzes human alteration of shallow-water habitat (SWH) available to juvenile salmonids in the tidal Lower Columbia River. Part 2 develops a one-dimensional, subtidal river stage model that explains ∼90% of the stage variance in the tidal river. This model and the tidal model developed in part 1 [Kukulka and Jay, 2003] uncouple the nonlinear interaction of river tides and river stage by referring both to external forcing by river discharge, ocean tides, and atmospheric pressure. Applying the two models, daily high-water levels were predicted for a reach from rkm-50 to rkm-90 during 1974 to 1998, the period of contemporary management. Predicted water levels were related to the bathymetry and topography to determine the changes in shallow-water habitat area (SWHA) caused by flood control dikes and altered flow management. Model results suggest that diking and a >40% reduction of peak flows have reduced SWHA by ∼62% during the crucial spring freshet period during which juvenile salmon use of SWHA is maximal. Taken individually, diking and flow cycle alteration reduced spring freshet SWHA by 52% and 29%, respectively. SWHA has been both displaced to lower elevations and modified in its character because tidal range has increased. Our models of these processes are economical for the very long simulations (seasons to centuries) needed to understand historic changes and climate impacts on SWH. Through analysis of the nonlinear processes controlling surface elevation in a tidal river, we have identified some of the mechanisms that link freshwater discharge to SWH and salmonid survival.

Columbia river estuary studies: An introduction to the estuary, a brief history, and prior studies

~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