River Flow Regulation Research Articles

The Effects of High River Discharges on the Morphodynamics of the Guadiana ebb-tidal delta

López-Ruiz, A.; Garel, E., and Ferreira, Ó., 2020. The effects of high river discharges on the morphodynamics of the Guadiana ebb-tidal delta. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 558-562. Coconut Creek (Florida), ISSN 0749-0208.This work analyses the role of high river discharges upon the morphodynamics of an ebb-tidal delta at a jettied inlet (the Guadiana river mouth, at the southern border between Portugal and Spain). The analysis is performed using a numerical model to simulate the effects of synthetic single high river discharge events with return periods ranging from 3 to 670 years. This model was calibrated and validated both for hydro- and morphodynamics. Results show that the area with mobile sediment over the delta varies strongly with the magnitude of the river discharge. Events peaking over 7,500 m3/s produce significant bathymetric variations at the delta, including scouring of the inlet channel, along with the offshore migration and volumetric growth of the outer shoal. Observations have indicated that the delta does not recover from these sudden morphological changes. However, the recent strong river flow regulation by dams has limited the maximum river discharges to 2,500 m3/s. Such vanishing of unusually high discharge events has affected both the short-term and long-term morphodynamics of the Guadiana ebb-tidal delta.

ANALYSIS OF SOIL DEGRADATION AND DESERTIFICATION IN SOUTHERN BALKASH DESERT BASED ON GIS

Desertification and degradation provide a clear picture of global environmental and socio-economic issues. Most of Kazakhstan is located in a desert region, including the suburbs of South Balkhash. The reason is that desertification of the region has a strong influence on natural and anthropogenic factors. To consider the geomorphological state of the region and the problem of desertification of the territory, it is necessary to determine the importance of the process of relief of geological structure and relief of tectonics. In recent years, the environmental situation in Balkhash has deteriorated sharply not only as a result of river flow regulation, but also as a result of non-commercial economic activities. Therefore, it is very important to assess the situation of desertification and degradation in the Balkhash region. Desert vegetation has been identified, since information in the spectral range is often insufficient to describe the state of plants, plant indices often develop by combining two or more spectral bands. Land cover index is the percentage of vegetation over a given surface area. Remote sensing information was used to detect the entire land cover. Remote sensing with time and space limitations is widely used to classify vegetation cover. In this work, the proportion of vegetation was estimated by NDVI. The proportion of land cover is based on the relationship between NDVI (NDVIS) and NDVI (NDVIV) in the soil. Using the NDVI index, land cover zones were determined based on satellite images of 2006 and Landsat-5 from 2011. TCT (Tasseled Cap Transformation) coefficients are used in the widest range of problems solved using Earth remote sensing data: from recognition of the coastline of water bodies to determination of forest disturbances. Stressful vegetation may be an indirect sign of the presence of salt in soils. Saline soils are usually characterized by poorly planted areas. A normalized differential salinity index (NDSI) was also determined.

Floristic Description of Parts of the Khoper River Upstream and Downstream of the Potlovskaya Dam and Water Quality Assessment by Hydrochemical Indicators

In the modern world, increasing importance is attached to the formation and preservation of surface water quality, which is recognized as one of the leading factors in maintaining human health and well-being. In the 2000s the water quality in the Khoper river was considered as one of the best in Europe, but now the waters have become more polluted, and flow regulation (even with a low-pressure dam) may cause declension of their condition due to disruption of the water exchange process. The aim of the study was to evaluate the river flow regulation effect on the floristic diversity of vascular aquatic plants and the water quality in the vicinity of the Potlovskaya Dam near the source of the Khoper River (Penza region, Russia). The content of nutrients in water was estimated since their concentration and ratio determine the trophic status, water quality, and the state of aquatic vegetation, which is important in assessing the status of the environment of a particular region. Water chemistry and plants were sampled at two sites, one being upstream of the Dam, the other one being downstream. We have found that flow regulation by this small hydraulic installation causes some changes in the hydrochemical regime but does not affect the floristic diversity.

Functional and trophic diversity of fishes in the Mekong-3S river system: comparison of morphological and isotopic patterns

The Lower Mekong River and its three major tributaries, the Sekong, Sesan, and Srepok rivers, are systems with high aquatic biodiversity that now face impacts from new hydroelectric dams. Despite the ecological, economic, and cultural importance of the freshwater biota, knowledge about aquatic ecology in this region remains poor. We used morphological and stable isotope data to explore how fish functional and trophic diversity vary between the four rivers that comprise the Mekong-3S river system. During our field surveys, the Sesan had experienced greatest flow alteration from dams and had lowest taxonomic and functional diversity, with species less packed and less evenly dispersed within morphological space compared to the other rivers. The Sekong had greatest functional diversity, with species less packed in morphological space. Species in the Mekong and Srepok were more evenly distributed in morphological space and had intermediate levels of functional diversity. Isotopic niche diversity in the Sesan did not appear to be significantly different from the Srepok and Sekong rivers. Conversely, the more species-rich Mekong fish assemblage encompassed a greater isotopic space with species less packed and evenly distributed. Greater trophic redundancy was observed amongst fishes of the 3S rivers than the Mekong. Species functional redundancy could buffer river food webs against species loss caused by dams; however, additional traits and niche dimensions should be evaluated to test this hypothesis. Overall, morphological and isotopic evidence from the Mekong and 3S rivers indicate that river impoundment and flow regulation function as an environmental filter that reduces fish functional diversity.

ИСПОЛЬЗОВАНИЕ ДИНАМИЧЕСКОГО ПОДХОДА ПРИ РАЙОНИОВАНИИ БЕРЕГОВ АЗОВСКОГО МОРЯ

The purpose of this work is to comprehensively study the ecological and geological conditions and results of monitoring studies of the coastal territories of the Bay of Taganrog and the Sea of Azov to identify indicators for assessing natural risks. The object of research is a complex system of the coastal zone, consisting of subsystems of land, sea and coast. The subject of the study is the assessment of the risk of loss of territory as a result of various natural factors, both constant over time (geological structure, height of the coastal cliff) and changing (dynamics of the level, waves, wind conditions). Within this framework, we show the use of a dynamic approach for zoning the shores of the Sea of Azov, performed based on a comprehensive analysis of natural and anthropogenic factors that determine the development of dangerous coastal processes (DCP). All factors divided into two groups: those that increase the intensity of coastal processes and those that contribute to reducing their manifestations. It established that hydrodynamic factors in the development of DCP are the determining factors. Anthropogenic impact can be associated with both global and regional impacts, through ecosystem changes, regulation of river flow, environmental pollution, and local – irrational coastal protection, industrial and agricultural activities in the coastal zone, unauthorized removal of beach material, etc.

Environmental issues in sustainable use of water from minor and medium rivers in the Yenisei River Basin District

Minor and medium rivers of the Russian North regions are vulnerable to anthropogenic impact. The article deals with the water use problems in the Yenisei River Basin where about 90% of the rivers used for water supply are minor and medium. Aassessment of the main sources of anthropogenic impact at the rivers of the region is given. The article substantiates the main problems in sustainable use of water in the Yenisei River Basin, such as water pollution, regulation of river flow and water resource depletion. The methods of estimation of acceptable water intake are analyzed, empirical and analytical probability curves of water intake values for each season for eleven rivers are built. Environmental flow for eleven rivers, as the average month and average annual values, are calculated.

Новый подход в расчете наибольших расходов взвешенных наносов грязевых селей с учетом влияния основных природных факторов (на примере рек Малого Кавказа)

The work is devoted to the methodology for calculating the highest flow rates of suspended sediment mudflows of the rivers of the Minor Caucasus. The research materials were the data from stationary observations of the highest suspended sediment load presented by the Department of Hydrometeorology under the Ministry of Natural Resources of the Republic of Azerbaijan until 2000 and data from expeditionary operations until 2016. For extension of the series of the highest suspended sediment load rates, the relationships Qmax. = f (Rvax.) from 2000 to 2015. The parameters of physical and geographic factors influencing the high flow rates of suspended sediments are analyzed separately and in a complex. As main natural factors that determine the volume of mudflow sediment load, the values of the highest water discharge, the coefficient of natural regulation of river flow and forest cover of the catchment area were taken. The study resulted in the methodology for calculating the largest flow rate of suspended sediment. It mostly reflects conditions for the formation of mudflows and has a calculation error of ± 25%. Recommended formulas can be used to study and describe parameters of mudflows of unexplored rivers of the Minor Caucasus.

FlowRegEnvCost: An R Package for Assessing the Environmental Cost of River Flow Regulation

FlowRegEnvCost is an R package developed for evaluating the environmental costs of river flow regulation. The methods for calculating the costs are separated in three steps: (i) measuring the admissible range of variability of regulated river flow based on flow data during the period before the impact, (ii) measuring the environmental impact of regulated flows on a daily basis according to the resulting hydrological change in terms of the magnitude, duration, and frequency of the impact, and (iii) estimating the environmental costs associated to flow regulation subject to spatiotemporal characteristics. The method is based on the “polluter pays” principle, i.e., the estimated cost should be proportional to the resulting impact on the environment. This paper presents FlowRegEnvCost and applies the methodology in the Esla River case study, in Spain. FlowRegEnvCost enlarges the current recognition of the environmental costs and represents an easy-to-use and transparent management tool for accomplishing the goals of the Water Framework Directive.

Detecting Changes in River Flow Caused by Wildfires, Storms, Urbanization, Regulation, and Climate Across Sweden

AbstractChanges in river flow may appear from shifts in land cover, constructions in the river channel, and climatic change, but currently there is a lack of understanding of the relative importance of these drivers. Therefore, we collected gauged river flow time series from 1961 to 2018 from across Sweden for 34 disturbed catchments to quantify how the various types of disturbances have affected river flow. We used trend analysis and the differences in observations versus hydrological modeling to explore the effects on river flow from (1) land cover changes from wildfires, storms, and urbanization; (2) dam constructions with regulations for hydropower production; and (3) climate‐change impact in otherwise undisturbed catchments. A mini model ensemble, consisting of three versions of the S‐HYPE model, was used, and the three models gave similar results. We searched for changes in annual and daily stream flow, seasonal flow regime, and flow duration curves. The results show that regulation of river flow has the largest impact, reducing spring floods with up to 100% and increasing winter flow by several orders of magnitude, with substantial effects transmitted far downstream. Climate changed the total river flow up to 20%. Tree removal by wildfires and storms has minor impacts at medium and large scales. Urbanization, on the contrary, showed a 20% increase in high flows also at medium scales. This study emphasizes the benefits of combining observed time series with numerical modeling to exclude the effect of varying weather conditions, when quantifying the effects of various drivers on long‐term streamflow shifts.

Assessing the impacts of reservoir operation on downstream water diversions using a simplified flow model

ABSTRACT The operation of the Three Gorges Reservoir (TGR) affects the evolution of the interactions between the Yangtze River and Dongting Lake in China and water diversions from the river, which are essential to water resources management in this large river–lake system. Due to the lack of up-to-date and detailed channel topographic/bathymetric data, a simplified flow model based on rating curves was developed to simulate discharges in the river system, and to further quantify and differentiate the contributions of river erosion and flow regulation of the TGR at a seasonal scale. The results indicate that the effect of channel alteration counteracts the effect of reservoir regulation in the high-flow periods. The impacts of TGR regulation on water diversions for both pre- and post-flood seasons were significant, but no obvious changes in the discharge diversion ratios were observed on an annual time scale.

Geomorphometric Assessment of the Impacts of Dam Construction on River Disconnectivity and Flow Regulation in the Yangtze Basin

Rivers are under increasing pressure from anthropogenic impacts with incremental dam construction, experiencing global and regional alteration due to river disconnectivity, flow regulation, and sediment reduction. Assessing the cumulative impacts of dams on river disconnectivity in large river basins can help us better understand how humans disintegrate river systems and change the natural flow regimes. Using the Yangtze basin as the study area, this study employed three modified metrics (river connectivity index, RCI; basin disconnectivity index, BDI; and the degree of regulation for each river section, DOR) to evaluate the cumulative impacts on river disconnectivity over the past 50 years. The results indicated that the Yangtze had experienced strong alterations, despite varying degrees and spatial patterns. Among the major tributaries, the greatest impact (lowest RCI value) happened in the Wu tributary basin due to the construction of cascade dams on the main stem of the tributary, while the lowest impact (highest RCI value) happened in the Fu tributary basin, which still has no dams on its main stem. Collectively, rivers in the upper Yangtze reaches experienced more serious disturbances than their counterparts in the middle and lower reaches. The BDI results displayed that a substantial part of the Yangtze River, especially the Wu, Min, Jialing, and Yuan tributaries, only maintain connectivity among one to three representative river systems. No part of the Yangtze connects all the 12 representative river systems. This study also revealed that small dams can also exert significant impacts in flow regulation on regional river systems through their sheer number and density. The study results can help promote more environmentally sustainable river management policies in the Yangtze basin.

Boreal river impoundments caused nearshore fish community assemblage shifts but little change in diversity: a multiscale analysis

River flow regulation, fragmentation, and changes in water quality caused by dams have varying effects on aquatic biodiversity and ecosystem functions, but are not clearly resolved in boreal ecosystems. We adopted a multiscale approach to quantify fish community trajectories over 20 years using a network of sites spread across four reservoirs in two hydroelectricity complexes in northern Quebec, where other anthropogenic factors have been negligible. Across three spatial scales, we found little evidence of directional temporal trends in diversity relative to reference sites. Using beta-diversity analyses, we also detected a high degree of stability in fish composition over time and space at the complex and reservoir scales. However, changes in species assemblage following impoundment were detected at the scale of the sampling station. At this scale, we found that some species consistently benefited (coregonids and pikes) from impoundment, whereas others were detrimentally affected (suckers and one salmonid). Overall, we conclude that examining different scales is key when trying to understand the impacts of humans on biodiversity and in formulating management recommendations.

River habitat homogenisation enhances trophic competition and promotes individual specialisation among young of the year fish

Abstract In large rivers, fish ontogenic development success is mainly influenced by resource availability and by the possibility of species to adapt their diet (i.e. trophic niche). Humans have drastically modified freshwater habitats, notably for navigation purposes. Such modifications may reduce food availability for young of the year (YOY) fish and, consequently, influence their ability to reach the adult age. In the Meuse River, decrease of fish abundance is thought to be linked to a drastic reduction of phytoplankton biomass. In this context of decreasing phytoplankton biomass, we studied trophic niches of three cyprinid species (common bleak Alburnus alburnus, chub Squalius cephalus, and roach Rutilus rutilus) and one percid species (European perch Perca fluviatilis) at various stages of development, using stable isotope analysis in order to compare intra‐ and interspecific competition between sites differing in degree of channelisation. Two reaches of the Meuse River differing by their degree of regulation were investigated. We hypothesised that habitat homogenisation would (1) decrease food resource availability and diversity and (2) increase trophic competition, particularly among earlier ontogenic stages, and promote individual specialisation. Our study provides evidence that in the context of low planktonic biomass, most YOY relied on benthic food sources. Furthermore, the Meuse River flow and depth regulation significantly impacted the abundance and species richness of YOY. In the heavily channelised reach, between‐stages competition and low resource diversity lead to an increase in diet partitioning between cyprinid larvae, as well as consumption of non‐optimal energetic food sources such as aquatic vegetation by some individuals. By contrast, in the less channelised reach, larvae displayed a generalist feeding habit focusing on high energy content prey such as different taxa of macroinvertebrates, suggesting that the diversity of habitat reduces the food competition within and between stages. Intraspecific resource repartition is a key point for YOY fish having to cope with plankton‐depleted conditions. Younger cyprinid stages seem more affected by intra‐ and inter‐specific competition in the more channelised reach. YOY fish communities were also less diversified and abundant in the more altered site, which highlight the importance of limiting channelisation to better conserve fish communities.

Macroecology of fish community biomass – size structure: effects of invasive species and river regulation

The biomass of organisms of different sizes is increasingly being used to explore macroscale variation in food-web and community structure. Here we examine how invasive species and river flow regulation affect native fish biomass and fish community log10biomass – body mass scaling relationships in Australia’s largest river system, the Murray–Darling. The log10biomass – body mass scaling exponent (scaling B) of invasive fishes (95% CI: −0.14 to −0.18) was less negative than for native fishes (95% CI: −0.20 to −0.25), meaning that invasive species attained a higher biomass in larger size-classes compared to native species. Flow alteration and invasive common carp (Cyprinus carpio) biomass were correlated with severe reductions in native fish biomass ranging from −47% to −68% (95% CI). Our study provides novel evidence suggesting that invasive and native communities have different biomass – body mass scaling patterns, which likely depend on differences in their trophic ecology and body size distributions. Our results suggest that restoration efforts using environmental flows and common carp control has potential to boost native fish biomass to more than double the current level.

Computer implementation of the mathematical model for water flow management by a hydro complex

In this article computer implementation of the river flow regulation model for a hydro complex with a hydroelectric power plant on the main river during extreme water discharges is considered. Modes of operation for hydroelectric power plant with a reservoir are appointed using design dispatch schedules for annual flow regulation. The use of the proposed model makes it possible to determine the operating modes of the hydro complex, taking into account modern economic and environmental requirements, make adjustments to their parameters, assess the energy-economic and environmental effects.

Changes in short term river flow regulation and hydropeaking in Nordic rivers

Quantifying short-term changes in river flow is important in understanding the environmental impacts of hydropower generation. Energy markets can change rapidly and energy demand fluctuates at sub-daily scales, which may cause corresponding changes in regulated river flow (hydropeaking). Due to increasing use of renewable energy, in future hydropower will play a greater role as a load balancing power source. This may increase current hydropeaking levels in Nordic river systems, creating challenges in maintaining a healthy ecological status. This study examined driving forces for hydropeaking in Nordic rivers using extensive datasets from 150 sites with hourly time step river discharge data. It also investigated the influence of increased wind power production on hydropeaking. The data revealed that hydropeaking is at high levels in the Nordic rivers and have seen an increase over the last decade and especially over the past few years. These results indicate that increased building for renewable energy may increase hydropeaking in Nordic rivers.

MATERIALS FOR THE SUBSTANTIATION OF HYGIENIC STANDARD OF MICROCYSTIN-LR IN WATER OF WATER OBJECTS

Introduction. The growth of the anthropogenic pollution of water bodies and the regulation of river flow has led to an almost universal intensification of eutrophication of water bodies, which contributes to the mass development (“blooming “) in the summer cyanobacteria - blue-green algae. The main danger of “ blooming “ is the ability of cyanobacteria to produce toxins to adversely affect human health, among which hepatotoxin microcystin-LR takes the first place in prevalence and toxicity. To date, microcystin-LR in drinking water is regulated by WHO and 22 States, but in Russia, microcystin-LR in water is not standardized. The purpose of this work is to substantiate the MAC of microcystin-LR in water of water bodies and drinking water of the Russian Federation. Material and methods. The data of the world practice of the toxicity assessment of microcystin-LR, including the acute and chronic effects of microcystin-LR on animals and humans in natural conditions; the results of the study of acute, subacute and chronic toxic effects, effects on reproductive function, mutagenic, genotoxic and carcinogenic effects in experiments on laboratory animals were analyzed. The methodology of substantiation and harmonization of hygienic standards of substances in water was used. Results. Analysis of the information about the toxic effect of microcystin-LR, the experience of WHO and 22 countries on the regulation of its permissible content in water, as well as domestic experience of justification and harmonization of hygienic standards makes it possible to recommend the MAC microcystin-LR in water of water bodies and drinking water at the level of 0.001 mg/l marked “k” (carcinogen), 1st hazard class, limiting harmfulness index - sanitary-toxicological. Discussion. The results of the study showed that to justify the MAC of microcystin-LR in water, one index of harmfulness is sufficient - sanitary-toxicological, the study of the effect on organoleptic and general sanitary indices is not advisable because of the specific properties of the toxin. Conclusion. In the conditions of non-decreasing pollution of water bodies with cyanotoxins of blue-green algae, the introduction of MAC microcystin-LR will be important for the prevention of its adverse effects on public health.

Ухудшение ситуации с подтоплением, связанное с антропогенным влиянием на примере города Тюкалинска

Актуальность работы обусловлена увеличением влияния антропогенного фактора в развитии процессов подтопления освоенных территорий. В последние годы в Омской области подтоплению подвергается большое количество населенных пунктов, в том числе и местности с благополучными в целом условиями. Одним из таких населенных пунктов является город Тюкалинск, где проблема подтопления серьезно ощущалась в последние четыре года, что и повлияло на выбор объекта исследования. Цель работы: выявление причин обострения ситуации с подтоплением, поиск решений по стабилизации проблемы с учетом влияния реки, осуществляющей естественный дренаж территории. Методы исследования: анализ многолетних наблюдений за подземными водами на скважинах режимной сети мониторинга состояния недр с учетом количества выпавших осадков, выявление причинно-следственных связей с использованием графоаналитических зависимостей. Результаты. Проанализированы многолетние изменения уровня подземных вод, разгрузка которых осуществляется в реку Тюкалка. Изучена интенсивность выпавших осадков на данной территории за периоды обследований. Выявлены причины медленного ухода талых вод весной и накопления подземных вод в многолетний период. Несмотря на достаточно высокую упругость приходно-расходного баланса (атмосферные осадки - поверхностные - подземные воды) и высокую способность к отводу избыточной воды за счет транзитной (пересекающей город) реки Тюкалка, многочисленные антропогенные сооружения, такие как: водопроводящие трубы, плотины, насыпные мосты, дороги и мн. др., вносят существенный вклад в естественный дренаж, понижая расходную часть водного баланса. Регулирование стока реки приводит к ухудшению отвода как поверхностных, так и подземных вод, увеличивая риски развития подтопления и продолжительность весеннего половодья.

Modern condition and analysis of anthropogenous-climatic transformation of vegetation of lakes of the northern Steppe land

In this article, the current and former distribution of higher aquatic vegetation has been analyzed for floodplain lakes, arenas lakes and third terraces lakes in the valleys of large and medium North-Steppe Dnieper rivers. The article is devoted to the current state analysis of the higher aquatic vegetation at North-Steppe Dnieper lakes, its dynamics over a long-term period, as well as the determination of the nature and extent of anthropogenic-climatic changes in vegetation. Anthropogenic influence is a major threat to the development and functioning of most aquatic ecosystems. Since the twentieth century, it has been intensified by trends to long-term climate changes, which are also largely result of human activity. Increasing temperature of the winter season does not contribute to snow accumulation. Reduction of snow accumulation (frequent thaws during the winter), regulation of river flow (formation of a reservoirs cascade and ponds) and accumulation of melt water in artificial reservoirs led to the smoothing of the peak of the spring flood. Thus, the factor that provided spring washing of floodplain lakes, limited their overgrowing by air-water vegetation and their waterlogging disappeared. The anthropogenic factors that influence negatively include: intensification of agriculture, plowing of coastal areas, unreasonable land reclamation, overgrazing, development of transport and engineering infrastructure, urbanization, recreation, and chemical pollution. The presented data was obtained on the basis of processing our own research materials of 2009–2018 and literary and archival materials analysis (the herbarium of the Dnipropetrovs’k National University and the archive of the Research Institute of Biology). Natural Northern Steppe Dnieper lakes are located mainly in river valleys, so the study area was conventionally divided into sections: the large river valley (Dnieper) and the middle rivers valleys (Samara and Orel). Three ecological groups of macrophytes were reviewed and compared: hydatofites (submerged species), pleistophytes (species with floating leaves) and helophytes (air-water species). The vegetation of Dnieper floodplain lakes practically did not change for all three formation groups. The number of immersed plants communities within the floodplains of medium-sized rivers has decreased by three. The pleistophytes and helophytes associations decreased to fragments of associations. The lakes vegetation within the sandy Dnieper terrace practically did not change for all three formation groups. The submerged lakes plants associations within the sandy medium-sized rivers terraces have been reduced by two. As part of the lakes vegetation on the Dnipro saline terraces, fragments of associations of the two species are considered extinct. A new association of southern adventive species Ruppia maritima L. has appeared within the limits of the middle rivers saline terrace. Changes in higher aquatic vegetation are characteristic of all types of lakes. Changes occur in the direction of crowding out higher aquatic vegetation communities by airborne plant communities. The consequence of the anthropogenic-climatic transformation of aquatic ecosystems is increased mineralization, siltation, and, as a result, intensive overgrowing of lakes by aboriginal and adventive species with a wide ecological amplitude (replacement of sensitive to environmental changes species).

Improvement of Cascade Regulation of River Flow of Reservoirs

The article is devoted to the problem of imperfect regulation of river flow in a cascade of reservoirs exemplified by the Kama and Votkinsk hydroelectric power complexes. The reservoir filling and emptying storage-outflow curve makes it possible to increase the regulating capacity of the hydro complexes for solving problems of downstream navigation.