Lake Outflow Research Articles

Density effects on the hydrodynamics and mixing at a large confluence with a compound channel tributary

Abstract Confluences are marked by converging streamlines, complex hydrodynamics features and mixing processes. Understanding such intricate flow patterns and their effects on the mixing at natural river junctions is difficult, particularly for large confluences with a compound channel tributary, due to the scarcity of field data. This research aims to assess the effects of density differences on hydrodynamics and mixing at the large river confluence between the Yangtze River and its tributary, the Poyang Lake, whose outflow channel has a large inner-side floodplain. Field data were collected during three surveys under various flow conditions to analyse how the alternate flow mechanism (compound channel vs single channel) of the Poyang Lake outflow channel influences the confluence mixing process. In high flow and relatively high flow conditions, the Poyang Lake outflow channel functioned as a compound channel and switched to a single channel in low flow. Acoustic Doppler current profiling (ADCP) was used in all three surveys to explore the features of hydrodynamics and the mixing process at the confluence, supplemented by water quality sampling to characterise the mixing patterns. Secondary flows observed during the field surveys were found to be affected by alterations in the flow mechanism (compound channel vs single channel) of the Poyang Lake outflow channel and the density effects, which were characterised using the densimetric Froude number Frd. Ultimately, the findings obtained in these field surveys confirm the role of density differences between the tributaries in significantly affecting hydrodynamics features and the mixing process at river confluences.

Attribution of hydrological droughts in large river-connected lakes: Insights from an explainable machine learning model

Large lakes play an important role in water resource supply, regional climate regulation, and ecosystem support, but they face threats from frequent extreme drought events, necessitating an understanding of the mechanisms behind these events. In this study, we developed an explainable machine learning (ML) model that combines the Bayesian optimized (BO) long short-term memory (LSTM) model and the integrated gradients (IG) interpretation method to simulate and explain lake water level variations. In addition, the hydrological drought trends and extreme drought events in Poyang Lake from 1960 to 2022 were identified using the standardized water level index (SWI) and run theory. The analysis revealed that the frequency of hydrological droughts in Poyang Lake increased from 1960 to 2022, especially in the autumn after 2003. By selecting the flows of the catchment and the Yangtze River as the input features, the BO-LSTM model accurately predicted the water level of Poyang Lake. The IG method was then used to interpret the prediction results from three aspects: the importance ranking of the input features, their roles in the seasonal drought trends, and their roles in extreme drought events. The results indicate that (1) the most influential factor affecting the water level of Poyang Lake was the inflow of the Ganjiang River in the catchment. (2) The increase in the lake outflow caused by the Yangtze River's draining effect was the reason for the intensification of the autumn drought in Poyang Lake. (3) The extreme hydrological drought events were primarily caused by low catchment inflows. Overall, this research provides a new approach that balances prediction accuracy with interpretability for predicting and understanding the hydrological processes in large river-connected lakes. Moreover, this method was also applied to the attribution analysis of hydrological drought in Poyang Lake, providing theoretical support for regional water resource management.

Growth of two invasive cichlids (Perciformes: Cichlidae) in a natural thermal water habitat of temperate Central Europe (Lake Hévíz, Hungary)

The outflow of the natural thermal Lake of Hévíz is habitat of several fish species, with conservation relevance. In the past few years, numerous thermophile (tropically originated) fishes were reported in this waterbody, from which two species Parachromis managuensis (Günther, 1867), Vieja melanurus (Günther, 1862) characterized with strong, self-sustaining population. The aim of our research was to provide basic population data and to study their individual growth. The standard length of jaguar cichlid ranged from 37 to 283 mm (mean SL = 110.21 ± 65.4 mm), the redhead cichlid standard length varied between 30 and 203 mm (mean SL = 93.91 ± 40.0 mm). Slightly positive allometry (b > 3) was found in the case of both species. The von Bertalanffy Growth Function can be described as the following Lt = 343.6[1 − e−0.196(t+0.973)] in jaguar cichlid and Lt = 298.9[1 − e−0.113(t+0.997)] in the case of redhead cichlid. The Bertalanffy growth equations show slow growth for both species. Fulton’s condition factor (K) values varied between 1.376 and 2.11 (mean K = 1.701 ± 0.17) in the case of jaguar cichlid, and between 1.391 and 3.033 (mean K = 2.237 ± 0.24) for redhead cichlid. These baseline population biology data from the first known self-sustaining, temperate-zone populations of two tropical cichlids provide information e.g., for future ecological risk assessments or comparative growth analyzes.

Investigating limnological processes and modern sedimentation at Lake Żabińskie, northeast Poland: a decade-long multi-variable dataset, 2012–2021

Abstract. Here, we present a dataset of a decade-long monitoring project of Lake Żabińskie, a hardwater eutrophic lake in northeast Poland. The lake contains annually laminated (varved) sediments that form a unique archive of past environmental variability. Regular measurements of the lake water physical and chemical characteristics were done using a multiparameter sonde and a set of temperature sensors deployed in the water column. Seasonal variability in sediment fluxes was documented by a sediment trap. Field sampling provided information about the hydrochemistry of incoming streams and of the lake outflow. The overall monitoring program was designed to assess relationships among meteorological conditions, limnological processes, and modern sedimentation and to answer the question of whether meteorological and limnological phenomena can be precisely tracked with varves. However, this dataset can also be used for modeling physical and biogeochemical processes in lakes. The dataset is archived at https://doi.org/10.34808/w8w7-9x34 (Tylmann et al., 2023).

Fish community traits near a large confluence: Implications for its nodal effects in the river ecosystem

River confluences are nodes where the unique hydrological processes of two rivers meet, resulting in complex flow structure and water quality mixing processes. Thus, greater food availability and habitat complexity can occur at a confluence, making it a hotspot for fish productivity and diversity. Nonetheless, studies that relate fish community traits to specific habitat characteristics at large river confluences are limited. Two field surveys were conducted at the large confluence between the Yangtze River and the Poyang Lake outflow channel involving fish hydroacoustic detection, environmental DNA, and acoustic velocity profiling. The discharge ratios of this confluence in two surveys were similar, but the water quality conditions represented by turbidity differed greatly. The results demonstrated high spatial heterogeneity of fish density, size, and species near the confluence. In the Yangtze River with high flow velocity, the abundance of small-sized fish was substantially higher than that in Poyang Lake outflow channel with low velocity, while large-sized fish chose their habitats more freely between the two tributaries and post-confluence channel. The convergence of two tributaries created prominent spatial heterogeneity of habitat conditions in the post-confluence channel, thus highest fish abundance and species biodiversity occurred there. The species assemblage structure of the local fish community was greatly affected by the change of water quality, e.g., the intrusion of a turbid flow from Poyang Lake into the Yangtze River, and thus the exchange of fish communities between two tributaries was weakened. The present study highlights the ecological importance of river confluence in improving regional fish abundance and species diversity and provides the theoretical foundation for the conservation and management of the aquatic environment at the confluence and in the whole river ecosystem.

Developing a stochastic hydrological model for informing lake water level drawdown management

Winter drawdown (WD) is a common lake management tool for multiple purposes such as flood control, aquatic vegetation reduction, and lake infrastructure maintenance. To minimize adverse impacts to a lake’s ecosystem, regulatory agencies may provide managers with general guidelines for drawdown and refill timing, drawdown magnitude, and outflow limitations. However, there is significant uncertainty associated with the potential to meet management targets due to variability in lake characteristics and hydrometeorology of each lake’s basin, making the use of modeling tools a necessity. In this context, we developed a hydrological modeling framework for lake water level drawdown management (HMF-Lake) and evaluated it at 15 Massachusetts lakes where WDs have been applied over multiple years for vegetation control. HMF-Lake is based on the daily lake water balance, with inflows simulated by a lumped rainfall-runoff model (Cemaneige-GR4J) and outflow rate calculated by a modified Target Storage and Release Based Method (TSRB). The model showed a satisfactory performance of simulating historical water levels (0.53 ≤ NSE ≤ 0.86), however, uncertainties from meteorological inputs and TSRB determined lake outflow rate affected the result accuracy. To account for these uncertainties, the model was executed stochastically to assess the ability of study lakes to follow the Massachusetts’ general WD guidelines: drawdown by Dec 1 and fully refilled by Apr 1. By using the stochastic HMF-Lake, the probabilities of each lake to reach the drawdown level by Dec 1 were calculated for different drawdown magnitudes (1–6 ft). The probability results suggest it was generally less possible for most of study lakes to achieve a drawdown of 3 ft or more by Dec 1. Moreover, we employed the stochastic model to derive the annual latest refill starting dates that ensure a 95 % probability of reaching the normal water level by Apr 1. We found starting a refill in March for drawdowns up to 6 ft was feasible for most of study lakes. These results provide lake managers with a quantitative understanding of the lake’s ability to follow the state guidelines. The model may be used to systematically evaluate current WD management strategies at state or regional scales and support adaptive WD management under changing climates.

The Potential Use of Global Evapotranspiration Products and Models to Estimate Lake Residence Time at Large Scale

AbstractWater residence time (WRT) is an important lake attribute that influences almost all in‐lake nutrient processes. However, information on WRT is often missing or outdated, especially in regions lacking lake inflow or outflow observations. Recent advances in global precipitation and evapotranspiration (ET) products and calibration‐free ET models provide the possibility of estimating lake WRT at large scales using estimated runoff from the water balance over the lake catchment. Here, we tested four global ET products and two calibration‐free ET models for estimating the mean annual lake outflow and WRT of 67 globally distributed lakes using precipitation inputs from four global precipitation products. The results show that the WRT estimated using the water balance method and global ET estimates generally correlates well with reference WRT (R = 0.90 ∼ 0.99) when using reliable precipitation inputs and is slightly more accurate than the estimates from the existing global WRT dataset and methods (R = 0.79 ∼ 0.94). Despite the large relative error (RE) in the estimates of the WRT, the RE of the phosphorus retention derived from the estimated WRT is acceptable (RE < ±30%). The simplicity and versatility of this approach underline its potential applications to large‐scale studies that require a fair estimation of WRT.

Influences of increasing water release of the Three Gorges Reservoir during dry season on water regimes of downstream lake

AbstractThe severe and frequent drought in Poyang Lake, the largest freshwater lake in China, has seriously affected the local water resources security and wetland ecological security. Especially, the impoundment of the Three Gorges Reservoir (TGR) in the Yangtze River further aggravated the drought in lake. To explore the approaches to alleviate drought in Poyang Lake, this study simulated and quantified the influences of increasing water release of the TGR during the dry periods on water regimes of Poyang Lake, including the lake water level, duration of low water, flow velocity, flow field of lake, as well as potential suitable habitat area of migratory birds, based on a hydrodynamic model. Major results showed that the increase of water release of the TGR raised the lake water level, especially in the northern lake area, with the average lake level increase of 0.2–1.08 m and the lowest lake level increase of 0.16–0.68 m. Meanwhile, the duration of low water was significantly shortened, from 127 days to 87–116 days (threshold lake level of 12.0 m) at Xingzi station. In addition, the hydrodynamic simulations revealed that the increasing water release of the TGR reduced the lake flow velocity and discharge of Poyang Lake, which decreased the outflow of lake by 1.66–17.87 × 108 m3, accounting for about 0.47%–5.11% of the total outflow in the dry season. Finally, the potential suitable habitat area with 0.2–0.6 m water depth increased by 1.7–27.9 km2 in December–February, and showed a significant expansion trend in the central area of Poyang Lake. Outcomes of the study are useful for understanding the influences of water release of the TGR on water regimes of downstream lakes, and can provide valuable references for mitigating lake droughts in complex river–lake system.

Outflows from lakes as ecotones - stable conditions maintain macroinvertebrates biodiversity

Outflows from the lakes are rarely considered as an ecotones. The main research direction on invertebrates of lake outflows most often are functional feeding groups, especially filter-feeders as a dominant group in that environment. Our goal was to describe the biodiversity of macroinvertebrates in lake-river ecotones in lowlands of Central Europe, found the environmental factors that shape the biodiversity of that environment and indicate directions for further biodiversity conservation. 40 outflows from lakes with different parameters were selected for the research. During the research, 57 taxa were found at the study sites, of which 32 taxa reached a frequency at least 10 %. The Multiple linear regression showed only one significant relationships between fluvial model and biodiversity. Of the components of this model, only depth of the outflow correlated significantly. There was observed significant differences in the Shannon-Wiener index, which was significantly higher in deeper outflows. Depth of the outflow indirectly affects the preservation of the biodiversity of the ecotone, which results from the greater stability of the water conditions in this place. It should be pay special attention to the water conditions of the catchments in order to minimize water level fluctuations and their negative effect on biodiversity in lake-river ecotones.

Effect of water depth and waterway obstructions on the divergence and confluence areas of Dongting Lake and the Yangtze River after the operation of the Three Gorges Project

AbstractClimate change and human activities have profoundly changed the evolutionary processes and developmental trends of the hydrology and geomorphology of rivers and lakes. Such changes have affected the evolution of bars and channels at the intersections of rivers and lakes and the utilization of water depth resources in waterways, which, in turn, have affected the sustainable development of waterways, such as the “Golden Waterway” of the Yangtze River. Considering the confluence of Dongting Lake and the Yangtze River as the present study area, we analyzed the effect of the hydrology of Dongting Lake and the evolution of bars and channels in the Jingjiang reach of the Yangtze River on the depth of the waterways. Our results indicated that from before the operation of the Three Gorges Project (1960–2002) to after its operation (2003–2021), the flow and sediment volume of the three inlets of Dongting Lake demonstrated a continuous decreasing trend. Furthermore, the relative increase in the runoff of the Jingjiang reach increased channel erosion. Channel erosion at the river–lake confluence from 2003 to 2021 created favorable conditions for increasing the dimensions of the waterways. The total length of the river–lake confluence that did not meet the requirements for safe navigation [4.5 × 200 m (water depth × width)] was 12.6 km, accounting for 68.35% of the total length of the Jingjiang reach with obstructed navigation. We observed that a decrease in the low‐water level and “steep slope and rapid flow” were the major factors hindering the navigation of the reach (from Zhicheng to Dabujie) affected by the Songzikou diversion of Dongting Lake. This was mostly reflected by the width of the 4.5 m waterway being less than 200 m. A declining water level, shrinking sandbanks and tidal flats, and alternating branches that destabilize the low‐water route were the major factors hindering navigation in the reach (from Taipingkou) affected by the Taipingkou diversion of Dongting Lake. Consequently, alternative waterways were implemented during low‐water periods with the water depth decreasing below 4.5 m. The obvious collapse of sandbanks and tidal flats and their erosion and migration were the major factors hindering navigation of the Ouchikou diversion of Dongting Lake. In addition, the scattered submerged shoals in the navigation channel restricted the width of the 4.5 m channel to less than 200 m. The reach affected by the Dongting Lake confluence (from Xiongjiazhou–Chenglingji reach) was subject to the erosion and silting of bends and the jacking effect of lake outflow, causing shoals with a water depth of less than 4.5 m in the channel. However, the jacking effect slowed the current, thereby assisting in improving channel conditions.

Water depth resources and driving mechanism of navigation hindrance of Jingjiang Reach watershed after Three Gorges Project operation

系统解析长江中游河湖交汇区域航道水深资源的变化,可支撑“黄金航道”的可持续发展。本研究以洞庭湖-荆江交汇段为对象,研究洞庭湖分汇区域水沙条件、荆江河段滩槽演变与河湖交汇段航道水深资源的关系。研究表明:三峡工程运行以来,洞庭湖三口分流量和分沙量延续了三峡工程运行前的减少态势,伴随洞庭湖分流量减少,相对增加的长江干流径流量增强了河床冲刷强度,反馈使得洞庭湖三口分流量和分沙量均处于较低值;荆江河段河床冲刷给航道尺度提升奠定了有利基础,河湖分汇区域不满足4.5 m×200 m(水深×宽度)的长度为12.6 km,占荆江河段碍航总长度的68.35%。碍航驱动机制上:松滋口分流区段(枝城-昌门溪)的4.5 m水槽贯通但宽度不足200 m,汊道分流关系不稳定及洲滩萎缩制约航道条件稳定,枯水位下降及“坡陡流急”现象仍然严峻,不利于航道条件保障及船舶安全航行;太平口分流区段水位下降、洲滩萎缩、汊道交替发展使得枯水航路不稳定或水深不足4.5 m;藕池口分流区域的洲滩崩退、洲滩冲刷引起的向下游泥沙输移,碍航表现为航宽不足200 m或出现水深不足4.5 m的浅滩;洞庭湖入汇影响区段(熊家洲-城陵矶)受弯道冲淤特性、湖泊出流顶托作用等影响,航槽内出现水深不足4.5 m的浅滩,但顶托作用存在趋缓趋势,有利于航道条件的改善。;A systematic analysis on the effects of the changed interactive relationships between rivers and lakes on fairway depth can help to support the sustainable development of the “Gold waterway” of the Yangtze River. In this study, we investigate the variation characteristics of water and sediment, and the relationships between the evolution of shoals and channels and the fairway depth, in the Lake Dongting-Jingjiang Reach confluence of the Yangtze River. The results indicate that since the operation of the Three Gorges Project, the split ratio of watersplit and the sediment distribution of the three outlets of Lake Dongting continued to decrease. Along with the decreasing flow of Lake Dongting, the relative increase in the runoff of the Jingjiang Reach increases the riverbed erosion scouring intensity, which keep the split ratio of water split and the sediment distribution of the three outlets at a lower value. The riverbed erosion scouring of the Jingjiang Reach has created a promising foundation for an increase in the waterway dimensions. However, the length of the reach fail to meet the requirement of 4.5 m× 200 m (water depth × width) is 12.6 km in the river-lake confluence, accounting for 68.35% of the total length of obstructed navigation. The mechanisms of the obstructed navigation are as follow: at the reach affected by the first outlet (Songzikou), a 4.5 m flume has been formed, but its width is less than 200 m. The instability of water split and the shrinkage of bottomland restrict the stability of channel conditions. Moreover, the reductions of low-flow water level and the phenomenon of “steep slope and rapid current” affect the ships' safety. At the reach (Taipingkou) affected by the second outlet, the shrinkage of the bottomland, and the intersecting braided river channels make the low-flow routes unstable, the water depth being less than 4.5 m. At the reach (Ouchikou) affected by the third outlet, the shoreline of the reach has collapsed and retreated significantly. Additionally, point bar beach erosion scouring has led to the downstream movement of sediment, which have caused the downstream waterway width at the reach to be reduced to less than 200 m or formed shoals with a water depth of less than 4.5 m. Owing to the characteristics of scouring and deposition in the bends reaches and the jacking effect of the lake's outflow backwater, the water depth of some shoals are less than 4.5 m in the navigation waterway. However, the jacking effect of the lake's outflow backwater is weakening, which benefits the improvement of channel conditions.

Patterns of genetic diversity: Stepping‐stone dispersal of an invasive fish introduced by an inter‐basin water transfer project

Abstract Knowledge of large‐scale dispersal processes and their associated genetic diversity patterns is significant for managing invasive species. These patterns remain unclear for invasive species that colonise new habitats through inter‐basin water transfer projects. A worm goby Taenioides cirratus colonised several large inland lakes in China, thereby expanding its distribution through the East Route of the South‐to‐North Water Transfer Project (ESNT). Here, we used nuclear microsatellites to analyse patterns of genetic diversity, population structure, gene flow and, invasion history across its native and invasive ranges in China. Our objectives were to: (a) reveal contemporary genetic diversity patterns of T. cirratus in two native and four invasive populations in China; and (b) identify its main model of dispersal in the ESNT. Invasive populations experienced significant loss of genetic diversity because of bottlenecks and low levels of gene flow, and strong population genetic structure due to founder effects that occurred during expansions. Significant genetic isolation‐by‐distance patterns were largely influenced by landscape connectivity and the reduced active dispersal ability of T. cirratus. The one‐way stepping‐stone dispersal model was identified as being most likely for T. cirratus in the ESNT, influenced by the long distance of upstream dispersal and downstream water transfer events. Our results highlight that T. cirratus could overcome low genetic diversity in founding populations, allowing for invasive spread. For management, we suggest physical removal of invasive populations and installation of deterrents near the pump intake area and the lake outflow to reduce further spread in the system. We provided a general paradigm for understanding dispersal patterns involved in inter‐basin water transfers.

Transcriptional Responses of Stress-Related Genes in Pale Chub (Zacco platypus) Inhabiting Different Aquatic Environments: Application for Biomonitoring Aquatic Ecosystems.

Pale chub (Zacco platypus) is a dominant species in urban rivers and reservoirs, and it is used as an indicator to monitor the effects of environmental contaminants. Gene responses at the molecular level can reflect the health of fish challenged with environmental stressors. The objective of this study was to identify correlations between water quality factors and the expression of stress-related genes in Z. platypus from different lake environments (Singal and Juam Lakes). To do so, transcriptional responses of genes involving cellular homeostasis (heat-shock protein 70, HSP70; heat-shock protein 90, HSP90), metal detoxification (metallothionein, MT), and antioxidation (superoxide dismutase, SOD; catalase, CAT) were analyzed in the gill and liver tissues of Z. platypus. HSP70, HSP90, and MT genes were overall upregulated in Z. platypus from Singal Lake, which suffered from poorer water quality than Juam Lake. In addition, gene responses were significantly higher in Singal Lake outflow. Upregulation of HSP70, HSP90, and MT was significantly higher in Z. platypus gills than in the liver tissue. In addition, integrated biomarker response and heatmap analysis determined correlations between expression of biomarker genes or water quality factors and sampling sites of both lakes. These results suggest that stress-related genes used as multiple biomarkers may reflect spatial characteristics and water quality of different lake environments, and they can be used for biomonitoring and ecological risk assessment.

Macroinvertebrate assemblages in acidified mountain lake inflows differs from lake outflows: the influence of lakes

Macroinvertebrate assemblages in acidified mountain lake inflows differs from lake outflows: the influence of lakes

Assessment of asbestos fiber contamination in lake sediment cores of the Thetford Mines region, southern Quebec (Canada)

• Lake sediments of the Thetford Mines region contain high asbestos concentrations. • Stratigraphic asbestos concentrations vary in accordance with mining history. • Asbestos mining waste erosion still today releases high amounts of asbestos fibers. • Asbestos fibers are transported over at least 25 km by the Bécancour River. The Bécancour River in southern Quebec (Canada) has evolved under the influence of the long asbestos mining history of the Thetford Mines region (1877–2011 Common Era; CE). This river originates at the outflow of Lake Bécancour, flows through the old mining sites, and then widens to successively form Stater Pond, Trout Lake, Lake William, and Lake Joseph (25 km from the closest mine). Our objective was to assess asbestos fiber contamination in dated sediment cores retrieved at these sites by means of transmission electron microscopy. Chrysotile was the most abundant form of asbestos identified in the samples. Asbestiform actinolite and tremolite were also found. Our results revealed that precolonial sediments deposited at 4 out of the 5 study sites naturally contained asbestos fibers (up to 3.5 wt%), with mass accumulation rates historically reaching up to 1.0 mg cm −2 yr −1 . However, fiber concentrations were greatly enhanced in the anthropogenic horizon of all sediment cores. The highest concentrations (up to 6.9 wt%) were found in layers from Stater Pond and Trout Lake deposited during the maximum of regional mining activities (1945‒1980 CE). Likewise, reconstructed mass accumulation rates indicated that asbestos fiber influx peaked (up to 38.6 mg cm −2 yr −1 ) during this epoch in all water bodies located downstream from Thetford Mines at magnitudes consistent with their proximity to the mining sites. They decreased after 1980 EC (≤ 24.4 mg cm −2 yr −1 ) following the decline of regional mining activities and improvement of air quality. Modern sediments of all study sites (including upstream Lake Bécancour) were nevertheless still highly enriched in asbestos fibers (up to 4.4 wt%), which can be attributed to intensive mining waste erosion in the lake catchments. Our study highlights that asbestos fiber contamination is important and widespread in the Bécancour River basin, which raises human and ecosystem health concerns.

Hydrological and Isotopic Variability of Perched Wetlands on North Stradbroke Island (Minjerribah), Australia: Implications for Understanding the Effects of Past and Future Climate Change

Over the first two decades of the 21st century, many wetlands in eastern Australia exhibited declining water levels, causing concern for communities and environmental managers and raising questions about the roles of climate change and other human activity in these water level declines. In this context we examine the causes of water level variability in four wetlands on North Stradbroke Island (Minjerribah), in the humid subtropics of south-eastern Queensland, Australia, using a combination of hydrological and water isotope monitoring and modelling. North Stradbroke Island has a high concentration of wetlands perched above the regional water table, with cultural and ecological significance, and value for palaeoclimate research. From 2015 to 2019, wetland water depths decreased markedly at all sites, coinciding with increases in oxygen isotope ratios in surface waters. The data indicate that climate, specifically a decrease in precipitation relative to evaporation, was responsible for those declining water levels, and that groundwater extraction did not play a critical role. At two of the sites—both palustrine wetlands—declining surface water levels led to intermittent connectivity with the local perched aquifers. At the other two sites, which are both shallow lakes, the surface waters were constantly fed by perched groundwater. The hydrology of the two lakes was modelled using simple mass balance. However, in order to accurately model lake level change, it was necessary to vary catchment runoff and lake outflow via groundwater through time, highlighting complexity in projecting future hydrological change in these lakes. The long term resilience of these lakes depends on a combination of rainfall regime and the balance between catchment runoff and groundwater throughflow, the future of which is highly uncertain. As a consequence, continued efforts to project future hydroclimate and to model the complex hydrology of subtropical wetlands are essential.

A field study of near-junction-apex flow at a large river confluence and its response to the effects of floodplain flow

• A field survey on flow at a large confluence of a compound tributary was conducted. • Kinetic energy and momentum correction coefficients at confluence were identified. • Floodplain flow decreased the size of stagnation zone and impacted its location. • Floodplain flow shifted the location of the mixing interface towards the centerline. • Floodplain flow was found to impact on the characteristics of the secondary flow. A field investigation was carried out in March 2019 at the large river confluence between the Yangtze River and the outflow channel with a large inner-side floodplain of Poyang Lake using acoustic Doppler current profiling (ADCP) and water quality sampling. The study was intended to complement previous field studies carried out under different flow conditions in August and December 2018 and to investigate the effects of the alternate flow mechanism (compound channel vs single channel) of the Poyang Lake outflow channel on confluence dynamics. The main hydrodynamics and morphodynamics features were identified and comparatively discussed with those from the previous field studies. The kinetic energy and momentum correction coefficients were found to peak near the confluence apex and decay farther downstream reflecting the gradual flow recovering process in the post-confluence channel. The alternate flow mechanism (compound channel vs single channel) of the Poyang Lake outflow channel and the change in momentum flux ratio M R were found to significantly impact on the characteristics of the stagnation zone, mixing interface and secondary flow at the confluence. The smallest and the largest stagnation zone were observed at the lowest M R and as the outflow channel acted as single channel, respectively, while also the location of the mixing interface was impacted. Furthermore, secondary flow consisting of dual counter-rotating helical cells and channel scale circulation were observed as the Poyang Lake outflow channel acted as compound channel and a single channel, respectively. These results provide novel information about confluence dynamics in large river systems where one tributary may have a floodplain flow in flooding conditions.

Pliocene-Pleistocene evolution of the lower Yellow River in eastern North China: Constraints on the age of the Sanmen Gorge connection

The Yellow River, the second longest river in China, originates in the northeastern Tibetan Plateau, flows in a contorted square-bend-shaped path through the Loess Plateau, then enters the Fenwei Graben before discharging through the Sanmen Gorge onto the North China Plain. Prior to its connection through the Sanmen Gorge, which cuts through the drainage divide of the Hua and Taihang mountain belt, the Yellow River had been partly impounded as a large lake filling the Fenwei Graben. To help decipher the Pliocene-Pleistocene history of the development of the lower reaches of the Yellow River, we analyzed the sedimentary histories of the Fenhe sub-basin in the northeast part of the Fenwei Graben and of fluvial-lacustrine sediments in the Huangdigou (HDG) section, which is now exposed at the upstream entrance to the current Sanmen Gorge. The Fenhe sub-basin changed from lacustrine/peat deposition to fluvial gravels at ~2.5 Ma. We suggest that this sedimentary facies change indicates the establishment of a significant outflow of the former Fenwei Graben Lake through the deepening Sanmen Gorge. The continuing lowering of base-level in the Fenwei Graben, which culminated in the direct flow of the Yellow River through the Sanmen Gorge and/or fluvial inland regressive erosion, also led to headward erosion of a new paleo-Fenhe River that cut through the Fenhe sub-basin. Tectonic activity and climate shifts may have contributed to the change to the modern path of the Yellow River. • The Fenhe sub-basin changed from lacustrine/peat deposits to fluvial gravels at ~2.5 Ma. • The detrital material deposited in the Sanmenxia isthmus was mainly from local sources at ~2.5 Ma. • Tectonic activity and climate shifts may have contributed to the change to the modern path of the Yellow River.

Simulation of Poyang Lake water levels and outflow under historical extreme hydrological scenarios

AbstractDue to an intensification of anthropogenic activities and climate change in recent decades, the hydrological connections and relationships between rivers and lakes have been significantly modified globally. Poyang Lake is one of the largest freshwater lakes globally and is one of the few that remain naturally connected to the Yangtze River. To investigate the full hydrological conditions (extreme high and low discharge) of Poyang Lake outflow under current bathymetric conditions, a large‐scale 1D‐ and 2D‐coupled high‐resolution hydrodynamic model of the Poyang Lake basin–Yangtze River system was developed. We simulated the outflow and water levels of Poyang Lake under nine different extreme hydrological scenarios with high precision and computational efficiency. We propose (1) a novel partition calibration method to characterize the roughness coefficient of large water bodies in complicated geographical terrain both for wet and dry seasons; (2) a new method for setting initial conditions for hydrodynamic simulation of large water bodies subject to strong hydrological regulation. Results indicated that (1) maximum outflow and water levels will reach 37,200 m3/s and 22.28 m when Poyang Lake basin floodwater coincides with flooding on the Yangtze River; (2) precipitation over the lake has increased outflow but this has had very limited influence on its changing hydrological pattern; (3) the effect of hydrological conditions within the system differs for both the lake outflow and water level. The research provides important reference conditions for the application of the InfoWorks ICM model in future applications and studies of large river–lake systems.

The development and impact of an ice‐contact proglacial lake during the Last Glacial Termination, Palaeolake Riada, central Ireland

ABSTRACTThe retreat of the last Irish Ice Sheet across the Irish midlands resulted in the formation of an ice‐contact lake, Palaeolake Riada, along its eastern margin during recession. However, the full extent and characteristics of the lake and its impact on ice dynamics are unknown, as glacigenic landforms and deposits relating to the lake have not been mapped in sufficient detail to allow reconstruction of ice‐marginal behaviour. This paper presents new mapping of subglacial, ice‐marginal and glaciolacustrine landforms and deposits in the Irish midlands. Glacial landforms and landform associations across this wide area have been identified using aerial photographs and high‐resolution digital elevation models derived from LiDAR and radar data. Reconstructions of successive ice‐margin retreat positions are combined with glacio‐isostatically adjusted digital elevation models to identify the probable position of lake outflows and reconstruct the development of the proglacial Palaeolake Riada through time. These reconstructions indicate that at its largest, Palaeolake Riada had an ice‐contact margin of 113 km, and covered an area of over 2300 km2. Initially, ice recession was concentrated around meltwater discharge outlets, but these became less important as the lake expanded and thermo‐mechanical erosion occurred along the entire subaqueous margin.