Tributary Confluences Research Articles

Methods and algorithms for creation of structural schemes of rivers

Methods and algorithms for creation of structural schemes of rivers

Evaluating definitions of salmonid thermal refugia using in situ measurements in the Eel River, Northern California

AbstractThermal microrefugia, sites within a landscape which are relatively protected from temperature extremes and warming trends, may be necessary for the conservation of animal species as climates warm. In freshwater environments, cold water fish species such as Pacific salmonids already rely on thermal microrefugia to persist in the southern extent of their range. Identifying sites that can provide such thermal microrefugia is essential to inform conservation and management decisions. At present, however, there is no consensus on appropriate ways to identify such sites, and multiple approaches are presented in the literature. Here, we use high resolution thermal data from two cold‐water zones created at the confluence of tributaries with a warm main channel of the South Fork Eel River in Northern California to demonstrate that the characteristics of cold zones identified as thermal microrefugia are not robust to the choice of microrefugium definition. Common definitions disagree on the existence, volume, seasonal temporal trends, and diurnal variations in microrefugia at the two confluences. The disagreements arise from the complex interaction between the confluence mixing regime with diurnal/seasonal warming patterns and how this shapes the distribution of water temperature at the confluences, and how it is classified by each definition. The disagreements cannot be resolved by simple bias correction approaches. Given that all existing definitions rely on simplifications that negate the nuanced way fish use cold water zones and respond to thermal stress, alternative observation and classification approaches may be needed to characterize cold zones in rivers as thermal microrefugia.

Metal Contamination in Seven Tributaries of the Ganga River and Assessment of Human Health Risk from Fish Consumption.

We investigated the distribution of Zn, Cu, Ni, Pb, Cr, and Cd in water, sediment, and two dietary fish (an omnivore, Labeo rohita and a benthic carnivore, Clarias batrachus) and potential health risk to human consumers during summer low flow (2017-2018) at 28 sites across 7 tributary confluences of the Ganga River. We selected Devprayag, an upper reach site, as a reference for data comparison. We found significant spatial variations in the distribution of study metals and the concentrations remained higher in tributaries, confluences, and downstream cities. The pollution load index showed all sites except Devprayag in the polluted category. Ecological risk analysis indicated 1 site with very high risk, 7 with considerable risk, and 10 with moderate-risk category. The Zn did appear the most, and Cd the least accumulated metal in the fish. The metal accumulation was higher in C. batrachus. The levels of Cd, Cr, and Pb in the study fishes were higher compared with the international standards. The health risk analysis indicated safe levels for individual metals except for Cd where the target hazard quotient (THQ) did exceed 1 for C. batrachus at the Ramganga and Varuna confluences. When all metals were considered, the THQ was > 1 (> 2 for C. batrachus), indicating the full possibility of adverse health effects to human consumers. Our study highlights the importance of tributaries in creating a mosaic of metal-rich habitats in the Ganga River and food chain associated with a health risk to human consumers.

Emerging reservoir delta‐backwaters: biophysical dynamics and riparian biodiversity

AbstractDeltas and backwater‐affected bottomlands are forming along tributary and mainstem confluences in reservoirs worldwide. Emergence of prograding deltas, along with related upstream hydrogeomorphic changes to river bottomlands in the backwater fluctuation zones of reservoirs, signals the development of new and dynamic riparian and wetland habitats. This study was conducted along the regulated Missouri River, USA, to examine delta‐backwater formation and describe vegetation response to its development and dynamics. Our research focused specifically on the delta‐backwater forming at the confluence of the White River tributary and Lake Francis Case reservoir. Objectives of the research were to: (1) describe and analyze the process of delta‐backwater formation over space and time; (2) determine by field sampling and GIS mapping how vegetation has responded to development of the delta‐backwater; and (3) compare the woody plant communities of the delta‐backwater to those along free‐flowing and regulated remnant river reaches. In response to base level changes caused by reservoir filling, the thalweg of the lower 31 km of the original White River channel and adjacent floodplain aggraded by up to 12 m between 1954 and 2011. The overall channel slope flattened from 0.70 to 0.29 m/km. Riparian Populus–Salix forests increased in area by nearly 50% during the post‐dam period by colonizing new deltaic and floodplain deposits. Many of the native woody species found along natural and regulated river reaches were also found on the delta‐backwater. Woody species sorted along a fluvial to delta gradient; wetland affiliated species (Salix spp., Typha spp.) dominated the delta‐backwater near the reservoir while riparian species (Populus, Fraxinus) dominated in upstream portions of the delta‐backwater. This habitat complex supports young stands of native riparian vegetation now in decline in remnant reaches protected from flooding.

Experiment and simulation of supersaturated total dissolved gas dissipation: Focus on the effect of confluence types

Total dissolved gas supersaturation (TDGS) downstream caused by spill discharge from high dams can easily cause fish to suffer from gas bubble disease (GBD). One potential approach to mitigate the impact of TDGS is at the confluence of a downstream tributary, where the introduction of low-TDG water might provide refuge space for fish. In this study, we carried out a series of flume experimental cases and established a three-dimensional TDGS model at confluences. The formula of the dissipation coefficient of TDGS had been obtained by parts of experiment cases. The other parts of experimental cases were carried out to validate the established TDGS model. The biggest relative error of TDG concentration between the experiment and simulation was −5.7%. The results show that the convergence of tributary water (TDG = 100%) can affect the mainstream water (TDG = 140% ∼ 150%) significantly. The two most obvious features are the presence of the separation zone and secondary flow which become more significant as the flow rate increases. The separation zone area at the bottom is smaller than that at the surface. There are two secondary circulations on transversal planes which decrease as the longitudinal distance increases. In addition, the area below 110% and 120% of TDGS in different planes of different cases were compared in detail. This study can provide scientific basis for the utilization of the low-TDG-saturation region to protect fish from the damage of TDGS at confluences during high dam discharge.

N‐Mixture Modeling of River Herring Egg Abundance and Distribution in the Tributaries of the Hudson River

AbstractN‐mixture models are often employed to estimate latent organismal abundance while concurrently accounting for detection probability. Our study offers a novel means for simultaneously measuring abundance, detection probability, and gear efficiency by focusing on a previously understudied and relatively immobile subject (river herring eggs). Custom‐designed egg mats accommodating two individual collecting surfaces were deployed in two tributaries of the Hudson River (Fall Kill and Black Creek, New York) to collect anadromous river herring eggs during the spawn. Mats were orientated approximately parallel to streamflow under a stratified random sampling design. Strata were defined as three equidistant spatial segments measured from a given tributary's confluence with the main stem of the Hudson River to its first impassable barrier to fish migration. In total, 93 sites were surveyed, with the majority of eggs being detected within the upper two‐thirds of each respective tributary. Throughout the course of the sampling season, an average of 1,585 eggs per egg mat subsampling event was recovered from the upper two strata of Black Creek, and 2,619 eggs per subsampling event were recovered from the upper two strata of the Fall Kill. In Black Creek, an egg density of 568 eggs per 58.1‐cm2 egg mat subsample was observed over an average deployment duration of 3.7 d at a detection rate of 82%. In the Fall Kill, an egg density of 1,222 eggs per 58.1‐cm2 subsample was observed over an average deployment duration of 3.9 d at a detection rate of 92%. The N‐mixture negative binomial model outperformed Poisson and zero‐inflated Poisson N‐mixture models in estimating river herring egg abundance using Akaike's information criterion model comparison indices. In terms of iteration processing time, N‐mixture modeling using the “unmarked” package in R proved to be more efficient than Bayesian‐based hierarchical modeling processed through the “jagsUI” package.

Temperature and depth profiles of Chinook salmon and the energetic costs of their long-distance homing migrations

River warming poses an existential threat to many Pacific salmon (Oncorhynchus spp) populations. However, temperature-mediated risks to salmon are often complex and addressing them requires species- and population-specific data collected over large spatial and temporal scales. In this study, we combined radiotelemetry with archival depth and temperature sensors to collect continuous thermal exposure histories of 21 adult spring- and summer-run Chinook salmon (O. tshawytscha) as they migrated hundreds of kilometers upstream in the Columbia River basin. Salmon thermal histories in impounded reaches of the Columbia and Snake rivers were characterized by low daily temperature variation but frequent and extensive vertical movements. Dives were associated with slightly cooler salmon body temperatures (~ 0.01 to 0.02 °C/m), but there was no evidence for use of cool-water thermal refuges deep in reservoirs or at tributary confluences along the migration route. In tributaries, salmon were constrained to relatively shallow water, and they experienced ~ 2–5 °C diel temperature fluctuations. Differences in migration timing and among route-specific thermal regimes resulted in substantial among-individual variation in migration temperature exposure. Bioenergetics models using the collected thermal histories and swim speeds ranging from 1.0 to 1.5 body-lengths/s predicted median energetic costs of ~ 24–40% (spring-run) and ~ 37–60% (summer-run) of initial reserves. Median declines in total mass were ~ 16–24% for spring-run salmon and ~ 19–29% for summer-run salmon. A simulated + 2 °C increase in water temperatures resulted in 4.0% (spring-run) and 6.3% (summer-run) more energy used per fish, on average. The biotelemetry data provided remarkable spatial and temporal resolution on thermal exposure. Nonetheless, substantial information gaps remain for the development of robust bioenergetics and climate effects models for adult Chinook salmon.

Vegetative and geomorphic complexity at tributary junctions on the Colorado and Dolores Rivers: a blueprint for riparian restoration

Habitat complexity in rivers is linked to dynamic fluvial conditions acting at various spatial scales. On regulated rivers in the western United States, tributaries are regions of high energy and disturbance, providing important resource inputs for riparian ecosystems. This study investigated spatial patterns and extents of tributary influence on riparian habitat complexity in the near channel zone along regulated reaches of the Colorado (> 200 km) and Dolores Rivers (~ 300 km) in the western United States. Because tributary confluences are regions of increased dynamism, we hypothesized that: (1) geomorphic and land cover complexity would be greatest close to tributary junctions and decrease with distance from tributaries; and (2) patterns in complexity would vary across different sized spatial units. Using a combination of remote sensing and spatial analysis, we classified fluvial features and land cover classes to investigate patterns longitudinally at 10-, 25-, and 100-m spatial units in the near channel zone of two regulated rivers. Using change point analysis and randomization tests, we detected shifts in riparian habitat complexity closer to tributary junctions. Patterns varied across 10-, 25-, and 100-m spatial units in the near channel zone, with significance (p ≤ 0.05) recorded for 10- and 25-m spatial units. Tributary junctions deliver critical resource inputs on regulated systems, providing for increased geomorphic and land cover diversity upstream and downstream of tributaries. We found that patterns of response were non-linear and discontinuous, varying across spatial units and potentially influenced by the degree of mainstem flow regulation.

Thermal exposure of adult Chinook salmon and steelhead: Diverse behavioral strategies in a large and warming river system

Rising river temperatures in western North America have increased the energetic costs of migration and the risk of premature mortality in many Pacific salmon (Oncorhynchus spp.) populations. Predicting and managing risks for these populations requires data on acute and cumulative thermal exposure, the spatio-temporal distribution of adverse conditions, and the potentially mitigating effects of cool-water refuges. In this study, we paired radiotelemetry with archival temperature loggers to construct continuous, spatially-explicit thermal histories for 212 adult Chinook salmon (O. tshawytscha) and 200 adult steelhead (O. mykiss). The fish amassed ~500,000 temperature records (30-min intervals) while migrating through 470 kilometers of the Columbia and Snake rivers en route to spawning sites in Idaho, Oregon, and Washington. Spring- and most summer-run Chinook salmon migrated before river temperatures reached annual highs; their body temperatures closely matched ambient temperatures and most had thermal maxima in the lower Snake River. In contrast, many individual fall-run Chinook salmon and most steelhead had maxima near thermal tolerance limits (20–22 °C) in the lower Columbia River. High temperatures elicited extensive use of thermal refuges near tributary confluences, where body temperatures were ~2–10 °C cooler than the adjacent migration corridor. Many steelhead used refuges for weeks or more whereas salmon use was typically hours to days, reflecting differences in spawn timing. Almost no refuge use was detected in a ~260-km reach where a thermal migration barrier may more frequently develop in future warmer years. Within population, cumulative thermal exposure was strongly positively correlated (0.88 ≤ r ≤ 0.98) with migration duration and inconsistently associated (-0.28 ≤ r ≤ 0.09) with migration date. All four populations have likely experienced historically high mean and maximum temperatures in recent years. Expected responses include population-specific shifts in migration phenology, increased reliance on patchily-distributed thermal refuges, and natural selection favoring temperature-tolerant phenotypes.

Three-dimensional flow structure, morphodynamics, suspended sediment, and thermal mixing at an asymmetrical river confluence of a straight tributary and curving main channel

Past field studies of confluence dynamics have focused mainly on experimental or small stream confluences and confluences of large rivers. Few studies have explored fluvial processes and forms at confluences of medium-size rivers, especially those where river planform varies from a straight alignment. This study examines changes in flow structure, bed morphology, sediment suspension, and mixing with changing incoming flow conditions at a river confluence where a straight tributary joins a curving main channel at the upstream end of a bend. Data include five sets of field measurements of three-dimensional velocity components, bed topography, acoustic backscatter intensity, and water-surface temperatures at the confluence of the curving Wabash River and its straight tributary, the Embarras River. Results show that, in contrast to large river confluences, channel-scale helical motion is a prominent feature of flow structure at this confluence. Dual counter-rotating helical cells, separated by a distinct shear layer, develop over a region of scour within the confluence and contribute to maintenance of scour by sweeping sediment away from this region. Helical motion develops through channel curvature and flow deflection within the confluence. The location and geometry of the scour hole are subject to change depending on the relative momentum fluxes of the two rivers with the zone of scour shifting outward and enlarging as the momentum flux of the tributary increases relative to the momentum flux of the main river. Patterns of backscatter intensity, used here as a surrogate for suspended sediment concentration, confirm that near-bed suspended sediment moves around the scour hole. The thermal mixing interface between the two rivers corresponds to the position of the shear layer, and thermal mixing at the surface within the immediate vicinity of the confluence is limited. The hydrodynamics, morphodynamics, and patterns of sediment suspension and thermal mixing at this river confluence reflect a combination of confluence and bend dynamics.

Strontium isotopic composition of the Po river dissolved load: Insights into rock weathering in Northern Italy

Strontium isotopic composition (87Sr/86Sr) of river water provides insight on the nature of rocks involved in the weathering processes and important constraints for reconstructing silicate versus carbonate weathering in large scale basins. New data, presented in the context of a geochemical map of the Po river basin, highlight that the Po river water evolves from the upper part of the catchment, where 87Sr/86Sr approaching 0.7097 relates to the weathering of igneous and metamorphic silicate rocks, to the middle part, where less radiogenic compositions (87Sr/86Sr ∼ 0.7089) are driven by the confluence of the tributaries draining Mesozoic carbonate rocks of the South Alpine domain and from inflow of Apennine streams characterized by less radiogenic Sr signatures due to weathering of marly sediments and mafic magmatic rocks. Down flow, the Sr isotopic compositions of Po river water rises at 0.7091 either by the confluence of the last Apennine tributary (Panaro, 87Sr/86Sr ∼ 0.7095) and/or by the possible contribution from hyporheic exchanges. Po river isotopic signatures reveal slightly more radiogenic 87Sr/86Sr in comparison to other important Alpine river systems (e.g. Danube, Rhine and Rhône), likely suggesting that silicate rocks are comparatively contributing more weathering byproducts in the Po basin.

Decadal scale geomorphic changes and tributary confluences within the Ganga River valley in Varanasi region, Ganga Plain, India

Allahabad to Buxar, the Ganga River shows distorted menders, huge meander cut-offs, ox-bow lakes, scroll bars and abandoned linear channel belts, all suggesting to a fault system-related tectonic control on the river. The confluence dynamics of northern tributaries Gomati and Varuna Rivers which have shifted progressively from north to south, and confluences of southern tributary Jirgo which shifted from south to north, further corroborate the view and also suggest that the presence of a NE-SW trending dextral strike slip fault having a gravity component.The Ganga River came close to Varanasi around 40 ka ago following a tectonic event that caused extensive liquefaction of the sediments due to a seismic pulse and upheaval in the area. Between 40 to 7ka, the Ganga River was freely oscillating within its valley when another tectonic event took place and the Ramnagar cliff was raised due to upheaval and channel incision along NE-SW trending fault. After 7ka, the Ramnagar cliff was degraded before it was inhabited by man at around 3800 years BP, which have been contemporaneous to antiquity of Varanasi.Based on available remote sensing data, the morphometry has shown that in 1972 active channel covered 12% area in Ganga River Valley and aerial extent remained 8% in 2010. The decrease of sand bar areas (14%–11%) in the studied stretch reflects the decrease in monsoon strength over the last four decades. However, the decline in the aerial extent of relict valley features, viz. meander scars, meander cut-offs and alluvial islands, is quite characteristic and may be related to sediment recycling by migrating channels present within the active channel belt and floodplain areas of the Ganga River.

Community managed forests dominate the catchment sediment cascade in the mid-hills of Nepal: A compound-specific stable isotope analysis

Soil erosion by water is critical for soil, lake and reservoir degradation in the mid-hills of Nepal. Identification of the nature and relative contribution of sediment sources in rivers is important to mitigate water erosion within catchments and siltation problems in lakes and reservoirs. We estimated the relative contribution of land uses (i.e. sources) to suspended and streambed sediments in the Chitlang catchment using stable carbon isotope signature (δ13C) of long-chain fatty acids as a tracer input for MixSIAR, a Bayesian mixing model used to apportion sediment sources. Our findings reveal that the relative contribution of land uses varied between suspended and streambed sediment, but did not change over the monsoon period. Significant over- or under-prediction of source contributions could occur due to overlapping source tracer values, if source groups are classified on a catchment-wide basis. Therefore, we applied a novel deconvolutional framework of MixSIAR (D-MixSIAR) to improve source apportionment of suspended sediment collected at tributary confluences (i.e. sub-catchment level) and at the outlet of the entire catchment. The results indicated that the mixed forest was the dominant (41 ± 13%) contributor of sediment followed by broadleaf forest (15 ± 8%) at the catchment outlet during the pre-wet season, suggesting that forest disturbance as well as high rainfall and steep slopes interact for high sediment generation within the study catchment. Unpaved rural road tracks located on flat and steep slopes (11 ± 8 and 9 ± 7% respectively) almost equally contributed to the sediment. Importantly, agricultural terraces (upland and lowland) had minimal contribution (each <7%) confirming that proper terrace management and traditional irrigation systems played an important role in mitigating sediment generation and delivery. Source contributions had a small temporal, but large spatial, variation in the sediment cascade of Chitlang stream. D-MixSIAR provided significant improvement regarding spatially explicit sediment source apportionment within the entire catchment system. This information is essential to prioritize implementation measures to control erosion in community managed forests to reduce sediment loadings to Kulekhani hydropower reservoir. In conclusion, using compound-specific stable isotope (CSSI) tracers for sediment fingerprinting in combination with a deconvolutional Bayesian mixing model offers a versatile approach to deal with the large tracer variability within catchment land uses and thus to successfully apportion multiple sediment sources.

Simulation of the channel capacity for Tisza river considering backwater curve during flood.

The following sequence of curve calculation is proposed in the point of confluence of main river and tributaries during flood of 1% probability on the example of Tisza River. The method of calculation was used to determine flood zones during flood of 1% probability and assessment of effectiveness of the proposed active and passive flood protection structures in the Basin of Tisza River.

Performance of a groyne in controlling flow, sediment and morphology around a tributary confluence

In a middle stream reach, irreversible morphological changes are observed such as growth of a huge sandbar in front of the confluence, thalweg migration from the left to right bank, erosion of the main channel and sedimentary deposit on the floodplain. As a countermeasure against such degradation of river morphology, a groyne was constructed beneath the tributary confluence. Performance of a groyne in controlling flood flow, sediment transport and river morphology was investigated by a twodimensional hydrodynamic model. The analysis on twenty years of morphological change indicates that the groyne has an excellent performance in improving quality of river morphology such as reduction of the sandbar development and migration of the thalweg to the original position.

Evaluation of geogenic and anthropogenic impacts on spatio-temporal variation in quality of surface water and groundwater along Cauvery River, India

Assessment of groundwater and surface water quality along a river is important as it directly affects the agricultural, industrial activities and population. The objective of the study is to assess the quality of the Cauvery river water and adjacent groundwater for drinking and irrigational purposes and to identify the infuence of geogenic and anthropogenic sources. Groundwater and surface water samples were collected along the course of the river at approximate intervals of 25 km. The samples were analysed for electrical conductivity, pH, sodium, calcium, magnesium, potassium, bicarbonate, chloride and sulphate. Sodium was identified as the dominant cation and bicarbonate was the dominant anion for both river water and groundwater. These values were compared with limits recommended by the Bureau of Indian Standards for drinking purposes. The total dissolved solids were found to exceed the permissible limits for drinking water in most of the groundwater samples, and it was below the permissible limits in river water samples. Most of the river water samples were found to be suitable as per the drinking water quality standards, but most of the groundwater samples were unsuitable based on the concentration of major ions. Irrigation water quality was also assessed based on magnesium hazard, residual sodium carbonate, sodium percentage, sodium adsorption ratio, permeability index and salinity hazard. Most of the river water samples collected were suitable for irrigation, whereas most of the groundwater samples collected were doubtful for irrigation based on residual sodium carbonate and sodium percentage. Drinking water and irrigation water quality indices were also computed to assess the characteristics of water. Groundwater quality in locations nearer to the confluence of tributaries and industrial areas was of poor quality, while both river water and groundwater near the coast were poor, both for drinking and irrigation purposes. Comparison of the dissolved load with other rivers of the world was also made, which reveals that the Cauvery River yields comparatively higher dissolved load per area than most of the rivers. The chemical load in the river is due to natural and anthropogenic sources. Therefore, it is necessary to enforce the existing norms for the discharge of treated effluents by industries and townships along the river so as to reduce the chemicals contributed by anthropogenic sources.

Species inventory of aquatic macrophytes in the last undammed stretch of the Upper Paraná River, Brazil

Abstract The last undammed stretch of the Upper Paraná River in the Brazilian territory is, to date, not completely inventoried, which rises concerns given the rapid rates of species extinctions. Aim Here, we provide an inventory of macrophyte taxa recorded in the last undammed 230 km stretch of the Upper Paraná River. Methods We inventoried macrophyte taxa in 27 sampling stations including lakes, the main channel of the river, and the tributary confluences in November 2013 and in May and November 2014. Macrophyte were sampled in littoral zones. We explored taxonomic and life form aspects of the recorded species. We used species accumulation curves and ordination techniques to summarize the variation in richness and composition of macrophyte species. Results We recorded 71 macrophyte taxa in 35 families. Most species were emergent, amphibian, free-floating, and rooted submerged. At the main channel of the Upper Paraná River, Eichhornia azurea, E. crassipes, and Paspalum repens represented the most frequent species. The most frequent species in the tributary confluences were Ludwigia sp., E. azurea, and Polygonum ferrugineum, while in lakes, Ludwigia sp., Oxycaryum cubense, and E. azurea were the most representative taxa. In regard to macrophyte composition, the invasive Hydrilla verticillata mostly influenced sites in the Upper Paraná River, in the ordination space, while floodplain lakes were mostly characterized by the presence of species such as Scleria melaleuca, Panicum maximum, and Thelypteris sp., among others. Conclusions Our study indicates that the last undammed stretch of the Upper Paraná River contains a large number of macrophyte species, suggesting that this stretch is an important area for the conservation of macrophyte species. In addition, the occurrence of invasive species (e.g., Hydrilla verticillata) within the investigated stretch deserves attention in terms of potential impacts for which management action may be necessary.

Catastrophic flood disturbance and a community's response to plant resilience in the heart of the Texas Hill Country

The Blanco River, which flows through the limestone Balcones Canyonlands of central Texas (USA), experienced catastrophic flooding in May 2015 that resulted in significant biogeomorphic disturbance to its riparian corridor. High-resolution aerial and satellite imagery from pre- and post-flooding for a 55-km reach of river were used to map and categorize patterns of disturbance by degree of severity ranging from complete floodplain stripping to no disturbance. The most severe disturbance occurred within the floodway near the channel and decreased with lateral distance into the 100- and 500-year floodplains. Disturbance patterns previously identified in the literature including meander scour, parallel chute scour, convex bank erosion, and macroturbulent scour were all present following this event, as well as substantial disturbance proximal to tributary confluences. In the aftermath of this event, TreeFolks, a local nonprofit organization, engaged with the community to actively replant and restore the riparian corridor of the Blanco River on public and private lands. These reforestation efforts supplement the natural passive recovery of the riparian corridor, enabling the system to recover more quickly and be resilient to future flood events.

CO2 Outgassing from an Urbanized River System Fueled by Wastewater Treatment Plant Effluents.

Continuous underway measurements were combined with a basin-scale survey to examine human impacts on CO2 outgassing in a highly urbanized river system in Korea. While the partial pressure of CO2 (pCO2) was measured at 15 sites using syringe equilibration, 3 cruises employing an equilibrator were done along a 30 km transect in the Seoul metropolitan area. The basin-scale survey revealed longitudinal increases in surface water pCO2 and dissolved organic carbon (DOC) in the downstream reach. Downstream increases in pCO2, DOC, fluorescence index, and inorganic N and P reflected disproportionately large contributions from wastewater treatment plant (WWTP) effluents carried by major urban tributaries. Cruise transects exhibited strong localized peaks of pCO2 up to 13 000 μatm and 13CO2 enrichment along the confluences of tributaries at an average flow, whereas CO2 pulses were dampened by increased flow during the monsoon period. Fluctuations in pCO2 along the eutrophic reach downstream of the confluences reflected environmental controls on the balance between photosynthesis, biodegradation, and outgassing. The results underscore WWTP effluents as an anthropogenic source of nutrients, DOC, and CO2 and their influences on algal blooms and associated C dynamics in eutrophic urbanized river systems, warranting further research on urbanization-induced perturbations to riverine metabolic processes and carbon fluxes.

River tributaries as sediment sinks: Processes operating where the Tapajós and Xingu rivers meet the Amazon tidal river

AbstractThe Amazon River is the largest fluvial source of fresh water and sediment to the global ocean and has the longest tidally influenced reach in the world. Two major rivers, the Tapajós and Xingu, enter the Amazon along its tidal reach. However, unlike most fluvial confluences, these are not one‐way conduits through which water and sediment flow downstream towards the sea. The drowned‐river valleys (rias) at the confluences of the Tapajós and Xingu with the Amazon River experience water‐level fluctuations associated not only with the seasonal rise and fall of the river network, but also with semidiurnal tides that propagate as far as 800 km up the Amazon River. Superimposed seasonal and tidal forcing, distinct sediment and temperature signatures of Amazon and tributary waters, and antecedent geomorphology combine to create mainstem–tributary confluences that act as sediment traps rather than sources of sediment. Hydrodynamic measurements are combined with data from sediment cores to determine the distribution of tributary‐derived and Amazon‐derived sediment within the ria basins, characterize the sediment‐transport mechanisms within the confluence areas and estimate rates of sediment accumulation within both rias. The Tapajós and Xingu ria basins trap the majority of the sediment carried by the tributaries themselves in addition to ca 20 Mt year−1 of sediment sourced from the Amazon River. These findings have implications for the interpretation of stratigraphy associated with incised‐valley systems, such as those that dominated the transfer of sediment to the oceans during lowstands in sea level.