High River Discharge Research Articles

Dissolved barium behavior in Louisiana Shelf waters affected by the Mississippi/Atchafalaya River mixing zone

In order to better understand the constraints on the use of barium as a coastal paleo-freshwater tracer, we surveyed the dissolved Ba distribution in Louisiana Shelf waters, including the Mississippi (MR) and Atchafalaya (AR) River plumes, during May and November 2008, and June/July 2009, which represent high, low and intermediate river discharges, respectively. Dissolved Ba was found dominantly in the <0.02μm fraction, with no significant contribution from the 0.02–0.45μm colloidal size fraction. Although apparent non-conservative surface water Ba behavior was observed during all three sampling periods, there were significant differences among the distribution patterns. River–seawater mixing experiments were supportive of substantial desorptive Ba addition only during the high discharge survey. At other times, input of Ba-enriched shelf bottom water as well as river endmember variability contributed to the apparent non-conservative behavior. During at least two of our surveys (high and intermediate river discharge), shelf bottom waters were significantly enriched in dissolved Ba relative to surface waters. While the cause of this enrichment (e.g., submarine groundwater discharge, dissolution/diffusion from the sediment, and/or an anthropogenic source such as drilling muds) could not be determined, we did observe that bottom Ba enrichment correlated with diminishing dissolved oxygen during summertime shelf bottom water hypoxia. Another interesting observation was Ba depletion in some high-salinity surface waters associated with a diatom bloom during June/July 2009. In addition, different Ba concentrations in the MR and AR appear related to inputs to the AR from the Red River as well as from the wetlands in the Atchafalaya River Basin.Overall, our study of the Ba distribution in Louisiana Shelf waters implies that the seasonal variation of the surface water Ba–salinity relationship could lead to a considerable uncertainty in salinity prediction when using Ba as a proxy for paleo-salinity changes. Barium input to bottom waters and the extent to which this is natural or anthropogenically-affected is a particular source of uncertainty. Thus, as is the case with nearly all paleoceanographic proxies, the planktonic foraminiferal Ba/Ca ratio should be used in conjunction with other constraining proxies.

Piecing together the Ganges-Brahmaputra-Meghna River delta: Use of sediment provenance to reconstruct the history and interaction of multiple fluvial systems during Holocene delta evolution

Three main rivers—the Ganges, Brahmaputra, and Meghna—coalesce in the Bengal basin to form the world’s largest delta system, which serves as filter and gateway between the Himalayan collision and vast Bengal fan repository. New insights into the Holocene construction of the Ganges-Brahmaputra-Meghna delta, with a focus on river sedimentation, channel migration, and avulsion history, are presented here using the Sr geochemistry of bulk sediments as a provenance tracer. The sediment load of each river transmits a distinct Sr signature owing to differences in source rocks from the Himalaya, Tibet, and local regions, allowing for effective tracking of river channels and stratigraphic development within the delta. In the early Holocene, vigorous delta aggradation occurred under rapid sea-level rise and high river discharge and supported the construction of sand-dominated stratigraphy by laterally mobile, braided-stream channels. However, the vertically (i.e., temporally) uniform, but geographically distinct, Sr signatures from these deposits indicate that the Ganges, Brahmaputra, and Meghna fluvial systems remained isolated from one another and apparently constrained within their lowstand valleys. By the mid-Holocene, though, delta stratigraphy records spatially and temporally nonuniform Sr signatures that hallmark the onset of avulsions and unconstrained channel migration, like those that characterize the modern Ganges and Brahmaputra fluvial systems. Such mobility developed in the mid-Holocene despite declining discharge and sea-level rise, suggesting that earlier channel behavior had been strongly influenced by antecedent topography of the lowstand valleys. It is only after the delta had aggraded above the valley margins that the fluvial systems were able to avulse freely, resulting in numerous channel reorganizations from mid-Holocene to present. These channel-system behaviors and their role in delta evolution remain coarsely defined based only on this initial application of Sr-based provenance tools, but the approach is promising and suggests that a more complete understanding can be achieved with continued study.

Nutrients and carbon fluxes in the estuaries of major rivers flowing into the tropical Atlantic

Knowledge of the seasonal variability of river discharge and the concentration of nutrients in the estuary waters of large rivers flowing into the tropical Atlantic contributes to a better understanding of the biogeochemical processes that occur in adjacent coastal and ocean systems. The monthly averaged variations of the physical and biogeochemical contributions of the Orinoco, Amazon, São Francisco, Paraíba do Sul (South America), Volta, Niger and Congo (Africa) Rivers are estimated from models or observations. The results indicate that these rivers deliver approximately 0.1 Pg C yr-1 in its dissolved organic (DOC 0.046 Pg C yr-1) and inorganic (DIC 0.053 Pg C yr-1) forms combined. These values represent 27.3% of the global DOC and 13.2% of the global DIC delivered by rivers into the world’s oceans. Estimations of the air-sea CO2 fluxes indicate a slightly higher atmospheric liberation for the African systems compared with the South American estuaries (+10.67 mmol m-2 day-1 and +5.48 mmol m-2 day-1, respectively). During the high river discharge periods, the fluxes remained positive in all of the analyzed systems (average +128 mmol m-2 day-1), except at the mouth of the Orinoco River, which continued to act as a sink for CO2. During the periods of low river discharges, the mean CO2 efflux decreased to +5.29 mmol m-2 day-1. The updated and detailed review presented here contributes to the accurate quantification of CO2 input into the atmosphere and to ongoing studies on the oceanic modeling of biogeochemical cycles in the tropical Atlantic.

Semidiurnal internal tides in a Patagonian fjord

The fjords of central Chilean Patagonia (47°S) receive fresh water from both precipitation and the Baker River. This buoyancy input generates a two layer hydrographic system characterized by strong salinity stratification (∼30gkg−1 over a depth range of 7–15m), which favors baroclinic conditions in the fjord. Hourly current velocity profiles were collected with an acoustic Doppler current profiler (ADCP) moored at a depth of 40m during March–April 2009, and complemented by 11 CTD profiles and hourly sea level time series. These data allowed the detection of semidiurnal internal tidal waves for the first time in this region. Wavelength and horizontal phase speeds were determined by the dynamical mode 1 for internal waves. Maximum wavelength was 52km, travelling at a horizontal phase speed of ∼1.16ms−1. Wavelet, spectral and empirical orthogonal function (EOF) analysis techniques applied to the echo anomaly signal and to the baroclinic velocity showed largest semidiurnal amplitudes near the pycnocline. Out of three modes obtained from the EOF analysis, two modes displayed a two- or three-layer flow structure with flow direction reversing at the pycnocline. The semidiurnal internal waves were found as fluctuations near the pycnocline in sporadic packets correlated to high discharge pulses of the Baker River (r2=0.77). Additionally, internal Froude number calculations at the mouth of the Baker River indicated critical flow conditions, which allowed for generation of internal waves at the plume front. These waves are separated from the river plume after internal wave phase speeds surpassed frontal speeds. This suggests that the internal waves were modulated by pulses in high river discharge rather than the interaction of barotropic tide with bathymetry (a sill). An implication of these internal waves would be to increase vertical mixing of nutrients toward the surface, through shear instabilities, which would favor primary production.

Effects of riverbank restoration on the removal of dissolved organic carbon by soil passage during floods – A scenario analysis

Summary River restoration typically aims at improving and preserving the ecological integrity of rivers and their floodplains. Restoration projects may, however, decrease the ability of the riparian zone to remove contaminants as the river water moves into the aquifer, especially during high river discharges. The purpose of this paper is to analyze several factors involved during riverbank restoration (i.e. changes in riverbank topography and hydraulic conductivity of the upper sediments of the riverbank), with respect to their effect on enhancing dissolved organic carbon (DOC) transport from rivers into the groundwater. 3-D groundwater flow and transport with first-order decay was simulated for a typical setting of a porous groundwater aquifer near a large river. The simulations indicate that, during a 5 m flooding event, DOC concentrations in the groundwater can be 1.7–9 times higher at a restored riverbank (i.e. 250 m wide, no clogging within one meter of riverbank sediments) compared to a steep riverbank (i.e. 8 m wide, clogging within 1 m of sediments), in coarse to fine sandy gravel. 51–84 % of this increase in DOC concentration levels in the groundwater were due to an increase in submerged area of the riverbank, depending on the type of soil of the aquifer. The remaining part was caused by a change in riverbank hydraulic conductivity. The simulations further showed that the arrival times of DOC concentration peaks at 400–500 m distance from the river axis can be 18–27 days shorter at restored than at steep riverbanks. 77–100 % of the earlier arrival times of DOC concentration peaks at 400–500 m from the river axis were due to an increase in submerged area of the riverbank. The remaining part was due to a change in riverbank hydraulic conductivity. The effects of riverbank restoration on DOC concentrations and arrival times were bigger if river DOC concentrations increased than if they were assumed constant during the flood, the more the river water level increased and the closer the distance was to the river. The findings suggest that riverbank restoration projects as conducted as part of the implementation of the European Water Framework Directive, potentially, may have adverse effects on the groundwater quality near rivers. Additional monitoring strategies will therefore be needed in the future in such projects to protect alluvial ground water resources for public drinking water supply.

Impacts of dam discharge on river environments and phytoplankton communities in a regulated river system, the lower Han River of South Korea

To understand the effects of fluctuations in dam discharge due to river environments and phytoplankton communities, we monitored such environments and phytoplankton communities biweekly, from February 2001 to February 2002 and from February 2004 to February 2005, in the lower Han River (LHR), South Korea. The phytoplankton abundance during the dry season was approximately two times higher than that during the rainy season. In particular, fluctuations in diatom assemblages, which constituted over 70% of the total phytoplankton abundance, were affected severely by the changes in the discharge. When a large quantity of water in a dam was discharged into the LHR, the conductivity and the concentrations of total nitrogen (TN), total phosphorus (TP), and dissolved inorganic phosphorus (DIP) decreased rapidly, whereas the concentrations of suspended solids (SS), dissolved inorganic nitrogen (DIN), and dissolved silica (DSi) increased immediately. Time-delayed relationship also revealed that the dam discharge had an immediately significant negative relationship with phytoplankton abundance. On the whole, fluctuations in phytoplankton communities in the LHR were influenced much more by hydrodynamics such as dam discharge than by the availability of nutrients. Thus, the variability in these concentrations usually parallels the strength of river flow that is associated with summer rainfall, with higher values during periods of high river discharge.

Modeling the transport of freshwater and dissolved organic carbon in the Neuse River Estuary, NC, USA following Hurricane Irene (2011)

Numerical models are useful tools that aid in understanding complex flows and the distribution of suspended material over large geographic areas and during extreme weather events. Here we describe the use of a three-dimensional numerical model (Delft3D) to simulate freshwater and dissolved organic carbon (DOC) transport over a 3-week period, following intense precipitation that led to high river discharge into the brackish Neuse River Estuary (NRE), NC, from Hurricane Irene (Aug. 2011). The model was calibrated and validated using field measurements of water level elevations, vertical salinity profiles, and surface DOC concentrations in the estuary. DOC was simulated as a conservative tracer over the study period. Model results indicate differences in the intensity of the freshwater and DOC-laden plumes as they propagated along estuary due to a one week time lag between the maximum discharge of 540 m3 s−1 and maximum DOC concentration of 29.85 mg L−1 entering the NRE from the river. In the upper estuary, the surface DOC concentration increased by 18 mg L−1 above the pre-storm value of 7 mg L−1; the maximum concentration occurred 10 days after the passage of the storm. In the lower estuary, the outer edge of the DOC plume reached Pamlico Sound after 3 weeks with a surface DOC concentration that was 3 mg L−1 above the pre-storm value. Results also indicate cross-channel salinity differences up to 10 ppt and DOC concentration differences up to 15 mg L−1 in the upper NRE to due to wind-driven motion of the estuary. The methods described here could be applied to other coastal plain estuarine systems to simulate and characterize flow rates and DOC transport during and succeeding storm events where field measurements are often limited.

Influence of salt-wedge intrusion on ecological processes at lower trophic levels in the Yura Estuary, Japan

To examine the influence of salt-wedge intrusion on ecological processes at lower trophic levels in estuaries, we investigated seasonal variability in the nutrients, phytoplankton biomass (chlorophyll a), zooplankton density, and composition in the Yura Estuary, Japan. Phytoplankton composition was determined under two discriminative regimes: salt-wedge regime (summer) and freshwater regime (winter). Phytoplankton from two groups bloomed simultaneously under the salt-wedge regime. Freshwater and brackish phytoplankton dominated the upper freshwater layer (∼16,100 cells mL−1) while marine phytoplankton were dominant below the halocline (∼12,200 cells mL−1). Both phytoplankton groups grew using riverine nutrients. Marine phytoplankton increased closer to the tip of the salt-wedge, indicating that marine phytoplankton grew in situ in the river. The residence time of salt water increased due to the low river discharge from spring to summer, enhancing mineralization in the salt-wedge. Regenerated nutrients in the salt-wedge contribute to subsequent marine phytoplankton blooms. Copepods were more abundant in the salt-wedge (∼80.9 ind. L−1) than in the upper freshwater layer (∼14.2 ind. L−1) during summer. Phytoplankton and zooplankton densities under the freshwater regime (winter; ∼360 cells mL−1, ∼39.7 ind. L−1) were lower than under the salt-wedge regime (summer). The high river discharge under the freshwater regime flushes out the salt-wedge and discourages biological production in the river. Our results showed that salt-wedge intrusions change the physical conditions and nutrient dynamics, enhancing biological production in the salt-wedge estuary.

Analysis of the most important river plumes on the Atlantic and Mediterranean Iberian coast by means of satellite imagery

Event Abstract Back to Event Analysis of the most important river plumes on the Atlantic and Mediterranean Iberian coast by means of satellite imagery Diego F. Novoa1*, Renato Mendes2, Maite D. Rodriguez1, João Miguel Dias2 and Ramón G. Gesteira1 1 Vigo University, Spain 2 Aveiro University, Portugal Rivers discharges cause the formation of buoyant plumes in the adjacent coastal area at their mouths, which are characterized by low-salinity water and controlled by outflow inertia, rotation (Coriolis) effects, buoyancy, wind, and tide forcing. The turbid plumes influence the adjacent coastal area, since they control the patterns of nutrients, sediments and/or pollutants of fluvial origin on the coastal ocean and can promote strong physical and chemical changes on seawater. These changes affect the biological characteristics of the area, such as primary production, species composition, abundance and distribution of existing microorganism, which demonstrates its high ecological importance. The characterization of the most important river plumes along the Atlantic Iberian coast and the influence of the main forcing drivers (river discharge, wind and tide) on them, was carried out through the analysis of plume mean-state images calculated using water leaving radiance data (nLw555) obtained from the MODIS (Moderate Resolution Imaging Spectroradiometer) sensor onboard the Aqua satellite during 2003-2013. Satellite data are downloaded from Ocean Color web site (http://oceancolor.gsfc.nasa.gov). Daily high-resolution L1 files from MODIS-Aqua were processed through SeaDAS software. Composite images, interpolated to a regular pixel grid with an approximate resolution of 500m, were built for different synoptic conditions of river discharge, wind regimes and tide, in order to obtain a representative average plume image of each situation and river for the posterior analysis. Results showed that the river discharge is the main forcing factor in the river plume extension. Wind effect is noticeable under high river discharge and tide is important for the estuarine outflow regimes although with some remarkable similarities and differences between the Atlantic rivers due to their intrinsic characteristics. Keywords: Atlantic Iberian plumes, MODIS, turbidity, river discharge, wind effect, tide forcing Conference: IMMR | International Meeting on Marine Research 2014, Peniche, Portugal, 10 Jul - 11 Jul, 2014. Presentation Type: Poster Presentation Topic: BIODIVERSITY, CONSERVATION AND COASTAL MANAGEMENT Citation: Novoa DF, Mendes R, Rodriguez MD, Dias J and Gesteira RG (2014). Analysis of the most important river plumes on the Atlantic and Mediterranean Iberian coast by means of satellite imagery. Front. Mar. Sci. Conference Abstract: IMMR | International Meeting on Marine Research 2014. doi: 10.3389/conf.fmars.2014.02.00080 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 25 Apr 2014; Published Online: 18 Jul 2014. * Correspondence: Mr. Diego F Novoa, Vigo University, Ourense, Spain, diefernandez@uvigo.es Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Diego F Novoa Renato Mendes Maite D Rodriguez João Miguel Dias Ramón G Gesteira Google Diego F Novoa Renato Mendes Maite D Rodriguez João Miguel Dias Ramón G Gesteira Google Scholar Diego F Novoa Renato Mendes Maite D Rodriguez João Miguel Dias Ramón G Gesteira PubMed Diego F Novoa Renato Mendes Maite D Rodriguez João Miguel Dias Ramón G Gesteira Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Linking channel hydrology with riparian wetland accretion in tidal rivers

AbstractThe hydrologic processes by which tide affects river channel and riparian morphology within the tidal freshwater zone are poorly understood yet are fundamental to predicting the fate of coastal rivers and wetlands as sea level rises. We investigated patterns of sediment accretion in riparian wetlands along the nontidal through oligohaline portion of two coastal plain rivers in Maryland, U.S., and how flow velocity, water level, and suspended sediment concentration (SSC) in the channel may have contributed to those patterns. Sediment accretion was measured over a 1 year period using artificial marker horizons, channel hydrology was measured over a 1 month period using acoustic Doppler current profilers, and SSC was predicted from acoustic backscatter. Riparian sediment accretion was lowest at the nontidal sites (mean and standard deviation = 8 ± 8 mm yr−1), highest at the upstream tidal freshwater forested wetlands (TFFW) (33 ± 28 mm yr−1), low at the midstream TFFW (12 ± 9 mm yr−1), and high at the oligohaline (fresh‐to‐brackish) marshes (19 ± 8 mm yr−1). Channel maximum flood and ebb velocity was twofold faster at the oligohaline than tidal freshwater zone on both tidal rivers, corresponding with the differences in in‐channel SSC: The oligohaline zone's SSC was more than double the tidal freshwater zone's and was greater than historical SSC at the nontidal gages. The tidal wave characteristics differed between rivers, leading to significantly greater in‐channel SSC during floodplain inundation in the weakly convergent than the strongly convergent tidal river. High sediment accretion at the upstream TFFW was likely due to high river discharge following a hurricane.

Quantifying landscape change in an arctic coastal lowland using repeat airborne LiDAR

Increases in air, permafrost, and sea surface temperature, loss of sea ice, the potential for increased wave energy, and higher river discharge may all be interacting to escalate erosion of arctic coastal lowland landscapes. Here we use airborne light detection and ranging (LiDAR) data acquired in 2006 and 2010 to detect landscape change in a 100 km2 study area on the Beaufort Sea coastal plain of northern Alaska. We detected statistically significant change (99% confidence interval), defined as contiguous areas (>10 m2) that had changed in height by at least 0.55 m, in 0.3% of the study region. Erosional features indicative of ice-rich permafrost degradation were associated with ice-bonded coastal, river, and lake bluffs, frost mounds, ice wedges, and thermo-erosional gullies. These features accounted for about half of the area where vertical change was detected. Inferred thermo-denudation and thermo-abrasion of coastal and river bluffs likely accounted for the dominant permafrost-related degradational processes with respect to area (42%) and volume (51%). More than 300 thermokarst pits significantly subsided during the study period, likely as a result of storm surge flooding of low-lying tundra (<1.4 m asl) as well as the lasting impact of warm summers in the late-1980s and mid-1990s. Our results indicate that repeat airborne LiDAR can be used to detect landscape change in arctic coastal lowland regions at large spatial scales over sub-decadal time periods.

Po River plume pattern variability investigated from model data

The Adriatic Sea is a semi-enclosed basin located in the NE part of the Mediterranean Sea and it is strongly influenced by riverine inputs. In its northern sub-basin the freshwater plume of the Po River, together with those of numerous smaller rivers, plays a fundamental role in driving the physical and biogeochemical processes of the whole basin. In this paper we characterize the surface plume structure and identify its patterns and temporal variability on seasonal and inter-annual scales relating it to its major forcings (i.e., river discharges and winds). To perform this analysis, a 3D hydrodynamic numerical model was implemented over the whole Adriatic for the period 2003–2010 and the resulting outputs were analyzed through a series of statistical tools. The inter-annual and seasonal averages of Sea Surface Salinity (SSS) fields show that the average patterns are composed of a coastal plume, wider or narrower depending on the season, that flows southward of the Po River mouths. The first two modes of the Empirical Orthogonal Functions (EOF) analysis show a similar distribution with a cumulative explained variance up to 60%; the third mode, instead, presents a plume shape that extends well into the basin. To obtain a more detailed representation of the plume, a 2×3 Self-Organizing Map (SOM) analysis was performed over the surface salinity fields. Two antithetic patterns were depicted: (i) a small plume confined to coastal areas, typical of low discharges and/or Bora wind events and (ii) a wider plume that extends into the basin, typical of high river discharges and/or Sirocco winds. The comparison between wind regimes, riverine inputs and the time series of the SOM's Best Matching Units (BMU) suggested that, on long time scales, river discharges represent the dominant forcing in defining the plume size and surface pattern, while on time scales of few days the plume dynamics are modulated mostly by the wind structure.

Reach‐scale movements of bull trout (Salvelinus confluentus) relative to hydropeaking operations in the Columbia River, Canada

ABSTRACTDespite the widespread use of on‐demand hydropeaking operations for generating electricity from rivers, relatively little is known about how pulsed flows influence the behaviour of fishes. We studied the movements of bull trout by using radio telemetry in a hydropeaking reach of the upper Columbia River, near Revelstoke, British Columbia, Canada. Fifty‐seven bull trout were located every 12 h to evaluate the effects of discharge magnitude and rate of change on the: (1) odds of movement; (2) movement distances; and (3) movement direction. Twelve‐hour mean discharge magnitude had a negative effect on the odds of bull movement: for every 100 m3 s−1 increase in discharge, movement odds decreased by a factor of 0·91. Movement odds were unrelated to 12‐h discharge rate of change. Every 1 °C increase in water temperature increased movement odds by a factor of 1·27. Also, bull trout were more likely to move during the AM versus PM by a factor of 1·36. Movement distances were related to diel period, sex and fork length; however, these effects were not very strong. We found no evidence of downstream displacement during periods of high or changing river discharge. In fact, movement direction was unpredictable, which is consistent with the salmonid non‐migratory movement literature. Collectively, these findings provide insight into the biology of bull trout during an understudied seasonal life‐history period (i.e. autumn). It also informs river managers that bull trout movement can be modulated by operational water release from a dam. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Strong spatial differentiation of N and P deficiency, primary productivity and community composition between Nyanza Gulf and Lake Victoria (Kenya, East Africa) and the implications for nutrient management

Summary Study of phytoplankton nutrient status, biomass, productivity and species composition was carried out between March 2005 and March 2006, along a transect between north‐eastern open Lake Victoria and the large, shallow Nyanza Gulf in order to examine how the terrestrial run‐off can influence phytoplankton community and nutrient status and determine whether nutrient management of catchment run‐off has the potential to control the algal blooms in the gulf. Hydrological and nutrient differences between the open lake and the gulf create a transition from P deficiency for phytoplankton within the gulf to nitrogen deficiency in open lake. The shallow and turbid gulf was continuously dominated by non‐nitrogen‐fixing filamentous and chroococcale colonial cyanobacteria, but seasonal stratification and deeper mixing depth in the open lake favoured diazotrophic cyanobacteria and diatoms. Seston ratios and metabolic nutrient assays indicated the gulf to be sufficiently phosphorus deficient to impose P limitation on phytoplankton growth and biomass. In contrast, the open lake is not P deficient and is more likely to experience N deficiency that favours diazotrophic cyanobacteria. Because of high turbidity in the gulf, the euphotic zone is very shallow, limiting integral primary productivity compared to the less turbid open lake; high PAR extinction may also favour Microcystis blooms in the gulf. Increased P loading into the gulf may translate to higher algal biomass, mainly of the bloom‐forming and potentially toxic cyanobacteria, and therefore, reduction in P loading into the gulf should be a management priority. However, a review of historical data indicates that the greatest change in water quality in the gulf is increased turbidity that reduces light availability and may limit algal growth more than P deficiency in years of high rainfall and river discharge.

Numerical analysis on the Brisbane River plume in Moreton Bay due to Queensland floods 2010–2011

During the Queensland floods in the summer of 2010–2011, a flood-driven Brisbane River plume extended into Moreton Bay, Queensland, Australia, and then seaward, travelling in a northward direction. It covered approximately 500 km $$^{2}$$ . This paper presents a three- dimensional hydrodynamic numerical model investigation into the behaviour of the Brisbane River plume. The model was verified by using satellite observations and field measurement data. The present study concludes that the high river discharge was the primary factor determining the plume size and its seaward extensions. A notable finding was that the plume was a bottom-trapped type rather than a buoyant type. Further, the southerly winds were found to have moderately confined the alongshore extension of the plume, and had caused the plume to mix thoroughly with the ocean water.

Effects of density and size distribution on the erosion of the adult cockle &lt;i&gt;Cerastoderma edule&lt;/i&gt;

A series of experimental flume experiments were carried out to evaluate the passive transport of the adult cockle Cerastoderma edule with shell sizes from 15 to 35 mm. The purpose of the study was to measure the erosion of this bivalve under controlled laboratory conditions, reproducing the current velocities and the cockle size distribution found at the Lombos do Ulla river mouth in Spain during high river discharges. Increasing velocities (0.29, 0.31, 0.35, 0.40, 0.44 and 0.47 m s –1 ) were applied to the bivalves, analysing the influence of population density (500 and 1000 ind. m –2 ), activity (live vs dead individuals), and mollusc size and distribution. To assess the effect of bivalve size distribution on the cockle transport, uniform and non-uniform cockle size distributions were tested. The particle image velocimetry technique and the double-averaged methodology were used to determine velocity fields during the experiments. In the experiments cockle erosion rates were found to be directly related to the flow velocity and inversely related to the population density. The erosion behaviour of uniform and graded cockle size distributions showed differences similar to those found in the bedload transport of uniform and non-uniform sediment mixtures. Our results provide a mechanism to explain the observed physical transport of bivalve populations after large storm events.

Food webs in Mediterranean rivers

River food webs are subject to two regimes of longitudinally varying ecological control: productivity and disturbance. Light-limited productivity increases as channels widen downstream. Time windows for growth, however, shrink as discharge increases, substrate particle size decreases, and the frequency of flood-driven bed mobilization increases downstream. Mediterranean rivers are periodically reset by hydrologic events with somewhat predictable timing. Typically, a rainy winter with high river discharge is followed by summer drought with little or no rainfall and slowly declining river flow. The magnitude and timing of winter floods and severity of subsequent summer drought can vary considerably from year to year, however. Episodic scouring floods or prolonged periods of drought are experienced as disturbances, stressors, or opportunities by river biota. The timing, duration, and intensity of these hydrologic controls affect performances of individuals, distribution and abundances of populations, and outcomes and consequences of species interactions. These interactions in turn determine how river food webs will assemble, develop, and reconfigure after disturbance. We discuss how spatial variation in solar radiation and spatial and temporal variations in disturbance affects river food webs under Mediterranean climate seasonality, focusing primarily on long-term observations in the Eel River of northwestern California, USA.

Hydrodynamic response of Thassos Passage (N. Aegean Sea) to Nestos River discharge and meteorological forcing

The hydrodynamic response, in terms of the competitive stratification–destratification processes and the along- and cross-shore circulation in Thassos Passage, was studied. This passage is a narrow and shallow channel, located in the vicinity of Nestos River mouth, connecting the eastern and western Thracian Seas. Stratification-mixing conditions were studied in terms of the water column potential energy anomaly ϕ, and the quantification of the along- and cross-channel dynamic equation terms, utilizing the results of a regional three-dimensional model. Circulation conditions were assessed by separating the tidal and non-tidal seasonal signals recorded from a bottom-mounted ADCP, and cross-correlating residual circulation to the meteorological influence. Results showed that short-term mixing events, produced under strong south-west winds (8–14ms−1), and stratification incidents under high river discharge conditions (>100m3s−1), may alter the potential energy anomaly of the water column with a time-lag of 3–6h. Such ϕ-changes were attributed to the along- and cross-channel advective and straining terms, with significant spatial variability, due to the Thassos Island sheltering effect. The along-channel potential energy anomaly flux was computed heading westwards, having a mean value of 22.5Jm−2s−1. North-northeast winds tend to increase the sub-tidal level in the channel, while south and south-west winds are responsible for its reduction. At the northern part of Thassos Passage, the development of an elongated sandbar enhances frictional effects, favoring the development of a clockwise rotation system, supported further by southern wind events. Cross-correlations of residual flows and wind effects indicated a wind transfer coefficient of 8%, rotating the wind-induced surface flow by 14o rightwards. Overall, the impact of seasonal water column stratification on the water flow of Thassos Passage was limited, due to the reduced Nestos River runoff after its damming.

Tree-ring-based seven century long flow records of Satluj River, western Himalaya, India

River discharge provides the hydrological requirement of hydropower generation, irrigation and other day-to-day domestic needs of society. The recorded data spanning over the past few decades do not provide sufficient time window to understand the natural variations in river discharge in long-term perspective. A 711-year long composite ring width chronology of Pinus gerardiana (one site) and Cedrus deodara (three sites) was developed from moisture stressed sites in Kinnaur, the western Himalaya. The precipitation responsive chronology was used to develop 711 years (AD 1295–2005) long previous year December to current year July Satluj discharge data using linear regression model which captured 37% variance in the observed discharge series (AD 1922–2004). The reconstruction revealed 50-year low and high river discharge in the eighteenth and nineteenth century, respectively. The decreasing tendency in river discharge noticed since the 1990s is consistent with the decreasing trend in winter precipitation in the region.

Physical controls on hypoxia in Chesapeake Bay: A numerical modeling study

ABSTRACTA three‐dimensional circulation model with a relatively simple dissolved oxygen model is used to examine the role that physical forcing has on controlling hypoxia and anoxia in Chesapeake Bay. The model assumes that the biological utilization of dissolved oxygen is constant in both time and space, isolating the role that physical forces play in modulating oxygen dynamics. Despite the simplicity of the model, it demonstrates skill in reproducing the observed variability of dissolved oxygen in the bay, highlighting the important role that variations in physical forcing have on the seasonal cycle of hypoxia. Model runs demonstrate significant changes in the annual integrated hypoxic volume as a function of river discharge, water temperature, and wind speed and direction. Variations in wind speed and direction had the greatest impact on the observed seasonal cycle of hypoxia and large impacts on the annually integrated hypoxic volume. The seasonal cycle of hypoxia was relatively insensitive to synoptic variability in river discharge, but integrated hypoxic volumes were sensitive to the overall magnitude of river discharge at annual time scales. Increases in river discharge were shown to increase hypoxic volumes, independent from the associated biological response to higher nutrient delivery. However, increases in hypoxic volume were limited at very high river discharge because increased advective fluxes limited the overall length of the hypoxic region. Changes in water temperature and its control on dissolved oxygen saturation were important to both the seasonal cycle of hypoxia and the overall magnitude of hypoxia in a given year.