High River Flow Research Articles

In search of non-stationary dependence between estuarine river discharge and storm surge based on large-scale climate teleconnections

Compound floods may happen in low-lying estuarine environments when sea level above normal tide co-occurs with high river flow. Thus, comprehensive flood risk assessments for estuaries should not only account for the individual hazard arising from each environmental variable in isolation, but also for the case of bivariate hazard. Characterization of the dependence structure of the two flood drivers becomes then crucial, especially under climatic variability and/or change that may affect their relationship. In this article, we demonstrate our search for evidence of non-stationarity in the dependence between river discharge and storm surge along the East and Gulf coasts of the United States, driven by large-scale climate variability, particularly El-Niño Southern Oscillation and North Atlantic Oscillation (NAO). Leveraging prolonged overlapping observational records and copula theory, we recover parameters of both stationary and dynamic copulas using state-of-the-art Markov Chain Monte Carlo methods. Physics-informed copulas are developed by modeling the magnitude of dependence as a linear function of large-scale climate indices, i.e., Oceanic Niño Index or NAO index. After model comparison via suitable Bayesian metrics, we find no strong indication of such non-stationarity for most estuaries included in our analysis. However, when non-stationarity due to these climate modes cannot be neglected, this work highlights the importance of appropriately characterizing bivariate hazard under non-stationarity assumption. As an example, we find that during a strong El-Niño year, Galveston Bay, TX, is much more likely to experience a coincidence of abnormal sea level and elevated river stage.

Pollarding May Relieve Drought Stress in Black Poplars

Pollarding has historically been used in broadleaf tree species across European woodlands. However, despite pollarding enhances vigor growth in the short term, it is still unclear how long this effect lasts and whether it can alleviate drought stress in seasonally dry regions. We compared the radial growth and wood δ13C (13C/12C), a proxy of intrinsic water-use efficiency (iWUE), of trees pollarded 10 and 20 years ago in two black poplar (Populus nigra L.) riparian stands located in North Eastern Spain and subjected to different ecohydrological conditions. We also assessed if pollarded trees showed different leaf phenology as compared with uncut trees of coexisting white poplar (Populus alba L.) trees. The relationships between growth, climate variables, drought severity and river flow were quantified. Pollarded and uncut trees showed a similar leaf phenology with a trend towards earlier leaf unfolding as springs become warmer. Pollarding increased growth rates by 54% (ratio between trees pollarded 10 and 20 years ago, respectively), but this enhancement was transitory and lasted ca. 10 years, whereas wood δ13C decreased −5%. The growth of black poplar increased in response to high precipitation in the previous winter, cool wet conditions, and a higher river flow in summer. Pollarding improves growth and relieves drought stress.

Long-term observations of the turbid outflow plume from the Russian River, California

Understanding the mechanisms that spread freshwater away from small river systems and form turbid, low-salinity coastal plumes is crucial for assessing water quality in coastal waters. We present an analysis of 15 years (January 2004 to December 2018) of daily MODIS Aqua satellite data and in situ instrument data on the turbid freshwater plume that forms off the Russian River (California, USA), a prototypical Mediterranean-climate, small mountainous river system (SMRS). We present per-pixel statistical metrics and regression analyses to identify and quantify the controls on the extent and configuration of the plume exerted by river discharge, waves, winds, and tides. While freshwater outflow exhibits a persistent signal in nearshore waters, a large-scale plume only extends offshore into coastal waters during high river flow, when plume turbidity can be detected more than 10 km offshore from the river mouth. Our results show times when wave radiation stress exceeds outflow inertia, confining the plume within the surf zone and leading to an absence of detectable plume turbidity in coastal waters. Although tidal currents significantly influence the plume near the inlet, wind forcing is the primary control on plume shape and extent in coastal waters, deflecting the turbid outflow more than 30 km upcoast or downcoast of the river mouth with respective wind directions. Coriolis forcing is also significant and observed most clearly during periods of high river discharge and low wind forcing. In addition to introducing novel remote sensing methodology for SMRS plume analyses, these findings highlight the complex interplay of forcing related to tides, river discharge, winds, and waves in shaping the behavior of SMRS plumes. New insights include the impact of tides on larger discharges, the role of Coriolis forcing in SMRS plumes, and the effect of cross-shore winds on plume compression. Further, by considering the Russian River as a model for SMRS, this study can be used to ground-truth existing numerical models of small river plumes and to contribute to understanding critical for managing coastal water quality and nearshore ecosystems.

Rainfall-runoff modeling using artificial neural networks in the Mono River basin (Benin, West Africa)

Hydrological models are developed to simulate river flows over a watershed for many practical applications in the field of water resource management. However, the rainfall-runoff models mostly used in the Mono river basin struggle to better simulate high river flows. This paper presents a modeling approach based on Artificial Neural Networks (ANN) under different input meteorological parameters in the Mono River basin to better take into account the non-linearity of the relationship between rainfall and runoff. To this end, precipitation, potential evapotranspiration, and previously observed flow have been used for the daily flow simulation. The Levenberg-Marquardt algorithm is used to train the ANN rainfall-runoff models over the other optimization training algorithms mostly implemented in the study area. The analysis of the rainfall-runoff variability allowed us to show the strong correlation between rainfall and runoff and the impact of the Nangbéto dam on the flows at Athiémé. The results obtained after the training, validation, and testing of the ANN models are satisfactory (e.g., the coefficient of correlation varies between 0.93 and 0.99). The most efficient model has been identified and implemented in the Mono river basin at Nangbéto. The satisfactory results obtained show that ANN models can be considered good alternatives for traditional rainfall-runoff modeling approaches.

Rainfall-runoff modeling using artificial neural networks in the Mono River basin (Benin, West Africa)

Hydrological models are developed to simulate river flows over a watershed for many practical applications in the field of water resource management. However, the rainfall-runoff models mostly used in the Mono river basin struggle to better simulate high river flows. This paper presents a modeling approach based on Artificial Neural Networks (ANN) under different input meteorological parameters in the Mono River basin to better take into account the non-linearity of the relationship between rainfall and runoff. To this end, precipitation, potential evapotranspiration, and previously observed flow have been used for the daily flow simulation. The Levenberg-Marquardt algorithm is used to train the ANN rainfall-runoff models over the other optimization training algorithms mostly implemented in the study area. The analysis of the rainfall-runoff variability allowed us to show the strong correlation between rainfall and runoff and the impact of the Nangbéto dam on the flows at Athiémé. The results obtained after the training, validation, and testing of the ANN models are satisfactory (e.g., the coefficient of correlation varies between 0.93 and 0.99). The most efficient model has been identified and implemented in the Mono river basin at Nangbéto. The satisfactory results obtained show that ANN models can be considered good alternatives for traditional rainfall-runoff modeling approaches.

On the role of small estuaries in retaining buoyant particles

Estuaries, as connectors between land and ocean, have complex interactions of river and tidal flows that affect the transport of buoyant materials like floating plastics, oil spills, organic matter, and larvae. This study investigates surface-trapped buoyant particle transport in estuaries by using idealized and realistic numerical simulations along with a theoretical model. While river discharge and estuarine exchange flow are usually expected to export buoyant particles to the ocean over subtidal timescales, this study reveals a ubiquitous physical transport mechanism that causes retention of buoyant particles in estuaries. Tidally varying surface convergence fronts affect the aggregation of buoyant particles, and the coupling between particle aggregation and oscillatory tidal currents leads to landward transport at subtidal timescales. Landward transport and retention of buoyant particles is greater in small estuaries, while large estuaries tend to export buoyant particles to the ocean. A dimensionless width parameter incorporating the tidal radian frequency and lateral velocity distinguishes small and large estuaries at a transitional value of around 1. Additionally, higher river flow tends to shift estuaries toward seaward transport and export of buoyant particles. These findings provide insights into understanding the distribution of buoyant materials in estuaries and predicting their fate in the land-sea exchange processes.

Climate variability and hydrology impacts in east Africa’s Rwenzori Mountains

Study regionThe eastern flank of the 4 km Rwenzori Mountains and the Mobuku catchment 0.25–0.4 N, 29.85–30.1E are the geographic area for detailed analysis. Research focusHydro-climate variability is studied using high resolution satellite- and model- assimilated products in the period 1980–2023. The Mobuku catchment receives rainfall of 3–6 mm/day which generates an eastward discharge of 100 m3/s that declines rapidly downstream, thereby limiting hydro-power availability. New insightsLong-term trends in cloud fraction and potential evaporation reveal a tendency for drying associated with increasing easterly winds, subsidence near the mountain top, and warming of +.04 C/year that is melting glaciers. These constrain runoff on the eastern flank of the Rwenzori Mountains. Low river flows in Dec-Mar correspond with dry air intrusions from the northeast. High river flows in Jul-Nov are modulated by sea temperatures in the Indian Ocean that oscillate east-west at ∼3 year interval. Improved understanding of climate variability will contribute to better management of Uganda’s hydro-power resources.

Joint Occurrence of Extreme Water Level and River Flows in St. Lawrence River Coasts Under Present and Sea Level Rise Conditions

AbstractIn low‐lying coastal regions, the joint occurrence of high river flow and high water levels can cause coastal flooding with substantial economic and social implications. Recent studies over Canada's coasts have shown that neglecting the interdependency between flood drivers can underestimate the risk of flooding by up to 50%. However, to date, such interdependency has not been investigated for the coasts of the St. Lawrence River, Estuary and Gulf system (StL), where Sea Level Rise (SLR), along with intensified river peaks, are already threatening these communities. In this study, a copula‐based bivariate frequency analysis was applied to quantify the likelihood of occurrence of flooding events under dependent and independent assumptions, for 26 sites along the StL. Furthermore, to quantify the impact of anthropogenic climate change, the joint return period in historical period was compared with that of projected SLR associated with RCP 8.5 for the year 2100. Results show that (a) the independence assumption can underestimate the likelihood of occurrence of flooding event in the Fluvial Section of the StL by up to 30 times and (b) the SLR can increase the likelihood of occurrence of flooding event by up to 50 times in the Estuary and the Gulf and by up to 5 times in the Fluvial Section of the StL. This study highlights the need for explicit consideration of the dependence between flood drivers and of SLR in the delineation of flood maps along the coast of the St. Lawrence.

Investigation of compound occurrence of storm surge and river flood in Mikawa Bay, Japan, using typhoon track ensemble experiments

The risk of high river flow and peak storm surge superimposing in the estuary of small-and medium-sized rivers can be more severe during typhoon landfalls than at other times. In this study, 30 cases of typhoon track ensemble experiments for five rivers in Mikawa Bay, Japan, were evaluated using an atmospheric-river-ocean model. The results showed that storm surges coincided with high river flows when typhoons crossed the western side of Mikawa Bay, with the highest potential observed when typhoons passed northwest of the Bay. The average time discrepancy for the five rivers between the storm surge occurrence and increased river flow in the minimum time difference case was 48 min at the river mouth. Distinct trends in the time difference between a storm surge and river flow were observed for each river mouth. The order of time differences corresponded to the drainage area scale: Toyo (188 min), Umeda (112 min), Otowa (114 min), Sana (96 min), and Yagyu (88 min). Time difference trends also varied based on the distance between the typhoon center and each river mouth. For Toyo River, storm surges and high river flows did not substantially overlap, regardless of distance. However, for medium-and small-sized rivers, the time difference increased proportionally with the distance from the typhoon. The Yagyu River showed the highest potential for compound occurrence of a storm surge and river flood because of the synchronization of storm surge peaks and high river flow peaks, with a time difference of 1–2 h for small-and medium-sized rivers within 100 km2 of the basin.

Typology and classification of water quality in an intermittent river in a semi-arid Mediterranean climate.

The typology and classification of rivers are highly relevant concepts in the field of limnology and freshwater ecology. Water body typology systematically categorizes water bodies based on their natural attributes, while water body classification groups them based on specific criteria or purposes for management, regulatory, or administrative reasons. Both concepts play important roles in understanding and managing water resources effectively. This scientific article focuses on the ZAT River in Morocco as a model for studying low-flow and intermittent rivers. The objective is to develop an accurate model for the typology and classification of small, low-flow rivers into homogeneous classes based on natural and anthropogenic factors. The study also investigates the impact of human activities on altering the uniformity and reference nature of the water body. The typology of water bodies is carried out according to the European methodology specified in The European Commission's Water Framework Directive (WFD) in 2000. The classification of water bodies is conducted by assessing their chemical and biological quality using the weighted index (WI), the Iberian Biological Monitoring Working Group (IBMWP) index, and multivariate statistical methods such as principal component analysis (PCA) for confirming water quality assessment. The results indicate the possibility of dividing the basin into four water bodies. Water bodies show homogeneity in terms of chemical quality when human influence is minimal or during periods of high river flow. However, increased human influence and decreased river flows lead to heterogeneity in chemical quality, indicating an unstable state. This study is the first of its kind in arid and semi-arid intermittent rivers, where such an approach could be suggested to determine their typology and classification.

Finding Navigable Paths through Tidal Flats with Synthetic Aperture Radar

Tidal flats are some of the most dynamic coastal environments in the world, where traditional coastal mapping and monitoring provide insufficient temporal resolution to reliably map channels and sand flats. Satellite-based Synthetic Aperture Radar (SAR) enables regular cloud-penetrating detection of water flowing through channels within the tidal flats, referred to as tidal channels. This paper presents a method for detecting a path through tidal channels, using satellite imagery, that supports our understanding and safe exploitation of this valuable coastal environment. This approach is the first proposed to identify navigable paths in all conditions, with SAR images susceptible to variation due to weather and tidal conditions. Tidal channels are known to vary in SAR presentation, and we find that tidal flat presentation is also influenced by conditions. The most influential factor is the wind, with high winds causing an inversion in how both tidal flats and tidal channels present in SAR images. The presented method for the automatic detection of tidal channels accounts for this variability by using previous channel paths as a reference to reliably correct imagery and detect the latest path. The final algorithm produces paths with minor errors in 17.6% of images; the error rate increases to 71.7%, with an almost tenfold increase in errors, when the SAR image and paths are not adjusted to account for conditions. This capability has been used to support the Nith Inshore Rescue in attending call-outs from their base in Glencaple, UK, while the insights from monitoring tidal channels for a year demonstrate how periods of high river flow preceded major changes in the channel path.

Combined impacts of climate and land-use change on future water resources in Africa

Abstract. Africa depends on its water resources for hydroelectricity, inland fisheries and water supply for domestic, industrial and agricultural operations. Anthropogenic climate change (CC) has changed the state of these water resources. Land use and land cover have also undergone significant changes due to the need to provide resources to a growing population. Yet, the impact of the land-use and land cover change (LULCC) in addition to CC on the water resources of Africa is underexplored. Here we investigate how precipitation, evapotranspiration (ET) and river flow respond to both CC and LULCC scenarios across the entire African continent. We set up a Soil and Water Assessment Tool (SWAT+) model for Africa and calibrated it using the hydrological mass balance calibration (HMBC) methodology detailed in Chawanda et al. (2020a). The model was subsequently driven by an ensemble of bias-adjusted global climate models to simulate the hydrological cycle under a range of CC and LULCC scenarios. The results indicate that the Zambezi and the Congo River basins are likely to experience reduced river flows under CC with an up to 7 % decrease, while the Limpopo River will likely have higher river flows. The Niger River basin is likely to experience the largest decrease in river flows in all of Africa due to CC. The Congo River basin has the largest difference in river flows between scenarios with (over 18 % increase) and without LULCC (over 20 % decrease). The projected changes have implications for the agriculture and energy sectors and hence the livelihood of people on the continent. Our results highlight the need to adopt policies to halt global greenhouse gas emissions and to combat the current trend of deforestation to avoid the high combined impact of CC and LULCC on water resources in Africa.

The impact of floods on plastic pollution

Abstract Non-Technical Summary Plastic harms ecosystem health and human livelihood on land, in rivers, and in the sea. To prevent and reduce plastic pollution, we must know how plastics move through the environment. Extreme events, such as floods, bring large amounts of plastic into rivers around the world. This article summarizes how different flood types (excessive rainfall, high river flow, or floods from the sea) flush or deposit plastic pollution, and how this impacts the environment. Furthermore, this paper also discusses how improved resilience to floods is important to prevent and reduce plastic pollution. Technical Summary Plastic pollution is ubiquitous in the environment and threatens terrestrial, freshwater, and marine ecosystems. Reducing plastic pollution requires a thorough understanding of its sources, sinks, abundance, and impact. The transport and retention dynamics of plastics are however complex, and assumed to be driven by natural factors, anthropogenic factors, and plastic item characteristics. Current literature shows diverging correlations between river discharge, wind speed, rainfall, and plastic transport. However, floods have been consistently demonstrated to impact plastic transport and dispersal. This paper presents a synthesis of the impact of floods on plastic pollution in the environment. For each specific flood type (fluvial, pluvial, coastal, and flash floods), we identified the driving transport mechanisms from the available literature. This paper introduces the plastic-flood nexus concept, which is the negative feedback loop between floods (mobilizing plastics), and plastic pollution (increasing flood risk through blockages). Moreover, the impact of flood-driven plastic transport was assessed, and it was argued that increasing flood resilience also reduces the impact of floods on plastic pollution. This paper provides a perspective on the importance of floods on global plastic pollution. Increasing flood resilience and breaking the plastic-flood nexus are crucial steps toward reducing environmental plastic pollution. Social Media Summary Floods have a large impact on plastic pollution transport, which can be reduced through improved flood resilience

Assessment of compound occurrence of storm surge and river flood in Ise and Mikawa Bays, Japan using a framework of atmosphere–ocean–river coupling

This study evaluated the compound flood risk of 11 different-sized rivers in the estuaries of the Ise and Mikawa Bays, Japan using an integrated framework of atmosphere–ocean–river developed in this study (one-way coupling). First, the framework was developed by incorporating the river channel into a coupled model of surge-wave-tide to include the interaction of the storm surge runup and river flow. In addition, the framework was validated by the Typhoon Trami (2018)-induced meteorological field, discharge, and storm surge with high accuracy. Second, the time difference between the storm surge and discharge at the estuary (ΔT) was investigated, assuming six typhoons with different tracks and similar distributions of intensity and precipitation using Typhoon Hagibis (2019) as a case study. The ΔT was highly positively correlated with the length of the river channel (correlation coefficient: 0.90). Moreover, the smaller rivers were more prone than large rivers to simultaneous storm surges and high river flow. The average ΔT for the smaller rivers was 180.4 min (normalized S.D. = 0.31) with a minimum of 15 min in the most severe case, while the average ΔT for the large-scale rivers was 614.1 min (normalized S.D. = 0.39). We clarified that the storm surge and high river flow occurred simultaneously (within 15 min) in the most severe river case (Yagyu River). These results infer that small rivers have a more significant impact on the co-occurrence of storm surge and high-river flow than large-scale rivers.

Predicted variable river response at high and low flows due to groundwater abstraction changes in Chalk catchments

Improving low flows in Chalk streams by relocating groundwater abstraction further downstream is an idea that has become popular in the UK in recent years. Simulations using the Environment Agency's Hertfordshire Chalk Groundwater Model predict that reducing groundwater abstraction close to ephemeral or intermittent Chalk streams is more likely to increase river flows during high-flow conditions than during low flows. This finding helps to explain the apparent lack of observed benefit to river flows during drought periods in catchments where abstractions have been reduced. If abstraction reductions are predicted to result in more of an increase to high river flows than to low flows, they may risk contributing to increased flood likelihood downstream, without providing significant habitat protection during low flows. Conversely, it has also been found that, under certain circumstances, preferential benefits can be predicted for low flows. Simulations show that such benefits are most likely to manifest by relocating abstraction to downstream locations where groundwater levels are already below the base of the riverbed (e.g. due to existing abstraction or artificial channel modifications). Here, the varying degree of hydraulic connection between groundwater and river can result in preferential benefits to downstream low flows and reduced downstream flood risk. Thematic collection: This article is part of the Karst: Characterization, Hazards & Hydrogeology collection available at: https://www.lyellcollection.org/topic/collections/karst

Reconnaissance Survey of Organic Contaminants of Emerging Concern in the Kabul and Swat Rivers of Pakistan.

The Swat and Kabul rivers of northern Pakistan are within an important regional watershed that supports river-based livelihoods and is impacted by untreated effluent discharges and municipal solid waste. Evidence indicates that fish populations are decreasing in these rivers. One potential cause of poor aquatic health is pollution; therefore, we investigated the presence of contaminants of emerging concern (CECs) in the river systems. Water samples were collected in the Kabul River (n = 9) and Swat River (n = 10) during seasons of high (summer 2018) and low (winter 2019) river flow. Agrochemicals, pharmaceuticals, plasticizers, chemicals in personal care products, and hormones were quantified via liquid chromatography high-resolution mass spectrometry. In the Swat River, caffeine (18-8452 ng/L), N,N-diethyl-meta-toluamide (DEET; 16-56 ng/L), and plasticizers (13-7379 ng/L) were detected at all sites during both seasons, while butachlor (16-98 ng/L) was detected only during high flow. In the Kabul River, caffeine (12-2081 ng/L) and several plasticizers (91-722 ng/L) were detected at all sites during both seasons, while DEET (up to 97 ng/L) was detected only during high flow. During low flow, pharmaceuticals (analgesics and nonsteroidal anti-inflammatory drugs) were quantified in both rivers (up to 823 ng/L), with detection frequencies from 70% to 100% and 0% to 78% in the Swat and Kabul Rivers, respectively. Intermittent-use and natural seasonal processes (increased runoff and dilution from rainfall and snowmelt) yielded higher agrochemical concentrations and lower concentrations of continuous-use compounds (e.g., caffeine) during high flow. The present study provides the first insight into CEC concentrations in the Swat River, additional insight into the Kabul River stressors, and, overall, contaminant risks to aquatic life. Environ Toxicol Chem 2023;42:2599-2613. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

A measurement-to-modelling approach to understand catchment-to-reef processes: sediment transport in a highly turbid estuary

As sediments carried by rivers enter coastal waters, fine particles can reduce the amount of light that reaches the reef through light attenuation. The Fitzroy Estuary - Keppel Bay (FE-KB), being the second-largest source of sediments to the Great Barrier Reef (GBR) poses a significant threat to the GBR ecosystem such as coral reefs and seagrass meadows, and biogeochemical cycles that influence water clarity. While monitoring and modelling capabilities for catchment and marine settings are now well-developed and operational, a remaining key gap is to better understand and model the transport, dynamics and fate of catchment derived material through tidally influenced sections of rivers that discharge into the GBR. This study aims to reveal sediment transport in the FE-KB estuary by continuously monitoring the seasonal variability over a year-long period and build a high-resolution model to predict sediment budgets under different scenarios of physical forcing and river conditions. Multiple data sources, including field surveys, historical data, and numerical modelling were used to obtain a detailed understanding of the sediment transport processes during wet (high river flow) and dry (low-to-no river flow) seasons. The use of high-resolution bathymetry and survey data for sediment model parameterization allowed for accurate mapping of the morphological changes, while numerical modeling provided insights into the hydrodynamic and sediment transport processes in the estuary. Observation and model data confirm the existence of a Turbidity Maximum Zone (TMZ) in the FE-KB (approximately 35 – 40 km from estuary head), where the topography plays a critical role in trapping sediments. By utilizing the model, a closed sediment budget was calculated under varying flow conditions and the results were used to determine the estuarine trapping coefficient that ranges from 28% (during extreme wet condition) to 100% (during dry condition) of the total catchment loads. Morphodynamic modelling demonstrated a persistent erosion pattern in the upper reach of the FE. The lower FE and southern tidal creeks serve as a large sediment storage basin during both wet and dry seasons, and sediment is exported and deposited offshore during high river flow conditions.

Simulating synergistic impacts of climate change and human induced stressors on a northern Gulf of Mexico estuarine food web

Apalachicola Bay, an estuary located in northwest Florida, is likely to experience a continuing increase in the severity of the effects of changing climate and human-induced stressors, such as sea level rise and changes in freshwater inflow. A coupled hydrodynamic and food web modeling approach was used to simulate future scenarios of freshwater input and sea level rise in Apalachicola Bay from 2020 to 2049 to demonstrate the range of temporal and spatial changes in water temperature, salinity, fisheries species biomasses, total food web biomass and upper trophic level diversity. Additionally, a survey of Apalachicola Bay stakeholders was conducted concurrently with model development to assess stakeholder knowledge and concerns regarding species and environmental changes within the system. Results of the model simulations indicated an increase in water temperature across all scenarios and an increase or decrease in salinity with scenarios of low or high river flow, respectively. These results aligned with the impacts anticipated by stakeholders. White shrimp biomass increased with low river flow and decreased with high river flow, while Gulf flounder biomass decreased across all scenarios. The simulated trends in white shrimp biomass contrasted with stakeholder perceptions. The food web model results also showed an increase in total food web biomass and decrease in upper trophic level diversity across all future scenarios. For all modeled simulations, the largest differences in future environmental variables and species biomasses were between scenarios of low and high river flow, rather than low and high sea level rise, indicating a stronger influence of river flow on the abiotic and biotic characteristics of the estuary. Stakeholders anticipated a future reduction in river flow and increase in sea level rise as negatively impacting the Franklin County economy and stakeholders’ personal interaction with the Apalachicola Bay ecosystem. The use of the ensemble modeling approach combined with the stakeholder survey highlights the use of multiple knowledge types to better understand abiotic and biotic changes in the estuarine system. Results provide insight on the synergistic effects of climate change and human-induced stressors on both the estuarine food web and human community of Apalachicola Bay.

Interannual variability of air-water CO2 flux in a large eutrophic estuary

Air-water CO2 fluxes in estuarine environments are characterized by high interannual variability, in part due to hydrological variability that alters estuarine carbonate chemistry through multiple physical and biogeochemical processes. To understand the relative contributions of these varied controls on interannual air-water CO2 fluxes in the mainstem Chesapeake Bay, we implemented both hindcast and scenario simulations using a coupled physical-biogeochemical model. Significant spatiotemporal variability in bay-wide fluxes was found over a 10-year period (1996-2005), where the mainstem Bay was primarily a net CO2 sink, except in drought periods. Sensitivity scenario results suggested substantial effects of riverine nutrient and organic matter (OM) inputs to CO2 flux variations. The high correlations between riverine inputs and upper-Bay fluxes were due to elevated respiration under increased OM inputs. The interannual flux variations in the lower Bay was mostly regulated by the nutrient inputs. Both nutrient and OM inputs contributed to the flux variability in the mid Bay. It is found that the interannual CO2 flux of the mainstem was most sensitive to riverine nutrient inputs associated with the hydrological changes. For each hindcast simulation we computed the ratio of organic carbon turnover time to water residence time (λ), a proxy for CO2 efflux potential, and found that the wetter periods had a relatively lower λ. The variability of mainstem CO2 fluxes can be well represented using a generic function of λ. The model results showed that higher river flows would lead to enhanced CO2 sinks into a large eutrophic estuary by promoting net autotrophy.

Water diversion structures and river discharge dictate salinity dynamics on tidal flats of the Fraser River delta, British Columbia

Salinity in estuaries affects numerous ecological processes, and is strongly driven by river outflow interacting with the tide. We used an exponential decay function to model the relationship between salinity and freshwater flow (i.e., river discharge) to characterize and map salinity dynamics over the tidal flats of the Fraser River delta, British Columbia, Canada. The model has three parameters: the horizontal asymptote Asym that represents the average weekly salinity at very high river flows; the theoretical intercept R0 that represents salinity conditions during very low river flows; and the natural log of the rate constant lrc that reflects the relationship between river discharge into the estuary and resultant average salinity. Using data from 47 salinity-monitoring stations deployed from 2015 to 2021, we found that Fraser River discharge had a strong and inverse relationship with average weekly salinity on the outlying tidal mudflats, which is expressed by the lrc value of −8.56. Model estimates indicate that weekly salinity (on the Practical Salinity Scale) at very low discharge rates (R0) had a value of 22.4, which decreased at high discharge rates (Asym) to a value of 3.7. This predictable decrease in salinity for increasing river discharge means that the salinity regime of the Fraser River delta tidal flats is poikilohaline, and undergoes a regular and biologically relevant transition from brackish to nearly fresh water as the river outflow increases and abates seasonally. The resulting spatial gradients in salinity dynamics were associated with distance to river outflows and existing water diversion structures (i.e., jetties and causeways), meaning the timing and extent of this transition varied across the tidal flats. As estuaries become increasingly modified by coastal development and climate change, this simple model can be used to evaluate management interventions and river discharge scenarios that affect salinity gradients over tidal flats.