Discharge Fluctuations Research Articles

Determining recharge groundwater using delineation of morphological surface in Ligarmukti karst area Bogor Regency

The Ligarmukti area is one part of the Klapanunggal karst hills in Bogor Regency. This area relies on springs as the fulfillment of raw water. It is feared that the limestone mining process in this area will damage groundwater recharge areas. This research was conducted to determine groundwater catchment areas in the Ligarmukti karst area. The method used is the morphological lineament delineation analysis of SRTM imagery compiled with the results of investigations in the field. Based on the results of delineation, the SRTM image was able to detect 76 alignment patterns in the direction of N 28° - 235° E. The fracture location was mostly located at elevation 280 - 700 m. The difference in direction of the alignment pattern is not too far away, so it is interpreted that the groundwater recharge pattern in the secondary porosity is derived from an aquifer system. The main catchment zone is at an altitude of 440-700 m above sea level in the hills with limestone constituent Klapanunggal Formation. The infiltration system in this area is an autogenic recharge, ie the recharge occurs in areas with relatively homogeneous rocks in the form of karst rocks and rainwater which is directly infiltrated in the area. This condition can be seen from the fluctuation of discharge during the rainy and dry seasons. Watershed conservation must be carried out to maintain the availability of springs by minimizing the limestone mining process in hilly areas.

Impact of human intervention on channel shrinkage and restoration in the Huanghe Estuary

Human intervention has become a globally dominant factor in the morphological evolution of estuary channels in recent decades. This paper analyzes seasonal morphological changes in the Huanghe estuarine channel for a 37-year period from 1976 to 2012 as a measure of estuarine shrinkage and restoration. The results show that the channel underwent four stages of development: enlargement (1976–1985), a gradual decreased area (1986–1994), shrinkage (1995–2001), and restoration (2002–2012). Generally, changes in the channel transect geometry during the first stage are regulated by natural fluctuations in water and sediment discharges, but the other three stages are impacted mainly by human intervention. The decreased runoff and sediment discharge caused by dam construction and water and soil conservation are the main reasons for the decrease and shrinkage of the channel transect. During the fourth stage, the increased ratio between water and sediment due to new reservoir construction and the water–sediment regulation scheme (WSRS) progressively affects runoff discharge, producing remarkable impacts on channel morphology during restoration. Human intervention has varying influences on different channel transects. The upstream channel section is more stable than the downstream channel, and human intervention in the downstream unstable channel has a 50% greater effect than that in the upstream stable channel. The WSRS has greater effects on the downstream unstable channel than on the upstream stable channel; however, a higher water–sediment ratio during the restoration stage results in greater impacts on the upstream stable channel than on the downstream unstable channel. Seasonal changes in the channel geometry indicate that seasonal adjustment is important for channel self-adjustment and that human intervention enhances variations in seasonal transect areas and impacts channel morphology.

Estuary-shelf interactions off the Changjiang Delta during a dry-wet seasonal transition

Estuary-shelf interaction is very complex and vital for maintaining healthy ecosystem in the estuaries and adjacent seas. The topic has been intensively studied through cruise observations and numerical models, but less by long-term observations at a fixed station. We present here for the first time roughly 2-months ADCP and CTD continuous data from the Xiaoqushan pilot station of seafloor observatory, aiming to deliberate river plume frontal processes off the Changjiang Estuary in response to river discharge transition from the dry to wet season in 2009. Multiple lines of evidence attest that inner river plume is always in a bottom-trapped form, but isohalines and isothermals are very sensitive to seasonal variations in freshwater input, wind climate and surface heating. Semidiurnal oscillations of salinity and temperature are good tracers for seasonal variations in plume frontal processes. In the dry season, both salinity and temperature have weak semidiurnal oscillations due to minimal cross-shore straining under strong mixing conditions near the inner plume front where isohaline and isothermal are nearly vertical with small horizontal salinity and temperature gradient. They are suddenly interrupted by a sharp increase of semidiurnal salinity oscillations owing to the arrival of newly established plume front with a less steep isocline slope but a maximum horizontal salinity gradient of 0.8 psu/km. The response time of inner plume front to the first discharge increasing pulse is roughly two weeks, but should change a lot when examined at different plume locations and under varied river-discharge increasing magnitudes and rates. The frontal processes mirror well the river discharge fluctuations in the transitional period which lasts for a few weeks before entering a typical wet season. The latter features strongest semidiurnal temperature oscillations linked with summer surface heating, and highly depressed semidiurnal salinity oscillations because the main plume path shifts from southward to northeastward due to alternating southerly to northerly prevailing winds. The three stages of inner plume frontal processes can be more clearly demarcated by the wavelet power spectral and the amplitudes of semidiurnal/diurnal oscillations of salinity and temperature through wavelet transform and the associated band-pass filtering methods. The seasonal plume frontal transition has been vividly portrayed for the first time through secular observation at the fixed station in the river plume region, significantly improving our understanding their controlling factors and mechanisms.

Controls Over Particle Motion and Resting Times of Coarse Bed Load Transport in a Glacier‐Fed Mountain Stream

AbstractCoarse bed load transport is a crucial process in river morphodynamics but is difficult to monitor in mountain streams. Here we present a new sediment transport data set obtained from 2 years of field‐based monitoring (2014–2015) at the Estero Morales, a high‐gradient stream in the central Chilean Andes. This stream features step‐pool bed geometry and a glacier‐fed hydrologic regime characterized by abrupt daily fluctuations in discharge. Bed load was monitored directly using Bunte samplers and by surveying the mobility of passive integrated transponder (PIT) tags. We used the competence method to quantify the effective slope, which is the fraction of the topographical slope responsible for bed load transport. This accounts for only 10% of the topographical slope, confirming that most of the energy is dissipated on macroroughness elements. We used the displacement lengths of PIT tags to analyze displacement lengths and virtual velocity of a wide range of tracer sizes (38–415 mm). Bed load transport in the Estero Morales shown to be size‐selective, and the distance between steps influences the displacement lengths of PIT tags. Displacement lengths were also used to derive the statistics of flight distances and resting times. Our results show that the average length of flight scales inversely to grain size. This contradicts Einstein's conjecture about the linear relationship between grain size and intervals between resting periods in a steep step‐pool stream in ordinary flood conditions.

Anchor ice effects on river hydraulics

Anchor ice can cause significant water level and discharge fluctuations in rivers. This study presents an analysis of anchor ice effects on water level and discharge in the Peace River. Field data collected in two winters from the Peace River are analyzed using a river ice model that include the anchor ice process and effects. The anchor processes considered include the bed elevation change due to anchor ice, the change in discharge due to the mass flux of anchor ice growth and release, and the change in ice-bed roughness due to anchor ice growth and release. The analysis showed that the dominating contributing factor to the water level and discharge fluctuations, i.e. anchor ice waves, by anchor ice is the changing ice-bed roughness. This suggests the need for further studies on developing a formulation on ice-bed roughness. The field data also showed that the anchor ice waves occurred over a longer period towards downstream as the water temperature decreases towards downstream during the freezing period, and the anchor ice wave decays while travelling downstream due to the energy loss, but enhanced by anchor ice waves generated along the way.

Relative contribution of evapotranspiration and soil compaction to the fluctuation of catchment discharge: case study from a plantation landscape

ABSTRACT Over the last decade, monoculture plantations have rapidly developed in Jambi Province on Sumatra, Indonesia. Meanwhile, there has been intensification of discharge fluctuation in the study area. We examined the relative contribution of changes in evapotranspiration and soil compaction to the catchment discharge by using the Soil Water Assessment Tool model. Evapotranspiration values based on the catchment water balance analysis in intensively cultivated oil palm plantations, smallholder oil palm plantations, rubber plantations, and the secondary forest are 5.03 ± 0.30, 4.11 ± 0.38, 3.36 ± 0.32, and 4.50 ± 0.18 mm d−1, respectively. Infiltration rate in active interrows of oil palm, rubber plantations, agroforest, oil palm frond pile is 2.6 ± 1.7, 16.3 ± 6.8, 28.0 ± 3.9, 58.2 ± 21.8 cm h−1, respectively. We found that increased evapotranspiration and soil compaction increased the frequency of low discharge by 30%, with increased evapotranspiration contributing 10% and increased soil compaction contributing 20%.

Impact of Combined Sewer Systems on the Quality of Urban Streams: Frequency and Duration of Elevated Micropollutant Concentrations

Water quality in urban streams is highly influenced by emissions from WWTP and from sewer systems particularly by overflows from combined systems. During storm events, this causes random fluctuations in discharge and pollutant concentrations over a wide range. The aim of this study is an appraisal of the environmental impact of micropollutant loads emitted from combined sewer systems. For this purpose, high-resolution time series of river concentrations were generated by combining a detailed calibrated model of a sewer system with measured discharge of a small natural river to a virtual urban catchment. This river base flow represents the remains of the natural hydrological system in the urban catchment. River concentrations downstream of the outlets are simulated based on mixing ratios of base flow, WWTP effluent, and CSO discharge. The results show that the standard method of time proportional sampling of rivers does not capture the risk of critical stress on aquatic organisms. The ratio between average and peak concentrations and the duration of elevated concentrations strongly depends on the source and the properties of the particular substance. The design of sampling campaigns and evaluation of data should consider these characteristics and account for their effects.

Land use assessment of Jeneberang watershed using hydrology and water availability analysis

Jeneberang watershed management plays an important role to the function of Bili-Bili Dam as a multipurpose dam in maintaining year-round water availability (quantity and distribution), agricultural irrigation, water supply for the local freshwater provider (PDAM), demand on drinking water, and hydroelectric power plants in Gowa, Takalar, and Makassar. The study tried to answer the emerging problem of how to predict the hydrological state and water availability in the Jeneberang watershed throughout the year. This study applied the SWAT (Soil and Water Assessment Tool) model, a distributed model connected to the Geographic Information System (GIS) developed by Jeff Arnold for the USDA ARS (US Department of Agriculture - Agriculture Research Service). The hydrological model would be applied to evaluate a scenario of the climate change effects on the hydrology state. Understanding the watershed hydrological response to the changes in physics (land use) and climate (rainfall and air temperature) is an important component of water resource planning and management (Vorosmarty et al., 2000). One of the performance indicators for the watershed water balance is river discharge fluctuation (Surahman, 2016). The river regime coefficient demonstrates fluctuation in river discharge, a number showing the ratio between maximum discharge (Qmax) and minimum discharge (Qmin). SWAT model analysis showed the highest discharge (Qmax) of 30,805 m³/sec and lowest discharge (Qmin) of 994 m³/sec. The analysis employed the data of 2006 and 2009, resulted in the Nash-Sutcliffe (NS) Efficiency values of 0.65 (satisfied) and R2 of 0.88 that in line with a study of Yusuf (2010) at the Cirasea watershed (0.74) and Junaidi (2011) at the Cisadane watershed (0.70). This value indicates that the SWAT method can predict the hydrological state of watersheds in Indonesia, including the Jeneberang watershed.

Spatio-statistical analysis of rainfall fluctuation, anomaly and trend in the Hindu Kush region using ARIMA approach

This paper focuses on the spatio-statistical analysis of rainfall fluctuation, anomaly and trend in the Hindu Kush region using auto-regressive integrated moving averages (ARIMA) approach. In the study area, trend in rainfall has significant impact on fluctuations in river discharge, which ultimately led to floods and hydrological drought. In this study, rainfall has been used as a climatic parameter. For this study, average annual and mean monthly rainfall data for Dir, Timergara, Saidu, Chitral, Drosh, Malam Jabba and Kalam meteorological stations located in the study region were gathered from Regional Meteorological Center Peshawar. In the study area, the rainfall is mostly received during two prominent periods, i.e., summer rainfall from monsoon, whereas winter and spring rainfall from western depressions. In the study area, Malam Jabba has recorded the heavy mean annual rainfall (1647 mm) and is considered as the humid station followed by met station Dir with a 1362 mm mean annual rainfall. Similarly, Saidu met station received 1050 mm mean annual rainfall and Kalam 1038 mm, whereas Timergara, Drosh and Chitral recorded 796 mm, 568 mm and 458 mm, respectively. The temporal data regarding rainfall were calculated and simulated in Addinsoft Excel state 2014 by applying ARIMA statistical model for trend prediction, fluctuations and anomaly. The analysis indicates that in terms of rainfall, an increasing trend has been detected at Dir, Chitral, Saidu and Kalam meteorological stations, whereas a declining trend has been recorded at Timergara, Drosh and Malam Jabba meteorological stations. In terms of rainfall anomaly, the met station Dir has indicated comparatively high positive anomaly. Contrary to this, the met stations of Saidu and Drosh have experienced negative rainfall anomaly.

Experimental verification of fine particle emission reduction based on acoustic agglomeration for steel sintering flue gas

The sintering flue gas in steel production has the characteristics of large discharge capacity, fine size, high temperature, corrosion and fluctuation of working conditions, and is the primary control object of PM2.5 emission source. Traditional dust removal technologies and equipment are difficult to achieve efficient collection of fine particles. In order to quantitatively study the effect and regularity of high intensity sound waves on fine particles, the online acoustic agglomeration test system was built by using high-power pneumatic source and wave tube with abrupt section. The high intensity standing wave field of 140 to 150 dB was generated in the wide frequency range between 50 Hz and 2 kHz by the acoustic resonance synthesis, and interference from compressed airflow was avoided by the acoustic-flow separation module. Variation of the sintered flue gas PSD was obtained under different sound intensities and frequencies. Data analysis shows that the high concentration condition can greatly reduce the operation time and energy consumption. Under the action of high intensity acoustic waves with a fundamental sound pressure level of 87.3-161.8dB and duration of 3s, the measured PM2.5 removal efficiency is 16%-92%, and the optimal frequency is near 800Hz. This paper verifies the feasibility of acoustic agglomeration (AA) in the treatment of sintering flue gas, and provides a theoretical reference and technical basis for the development of PM2.5 real-time emission control device for steel production.

Planform and stratigraphic signature of proximal braided streams: remote-sensing and ground-penetrating-radar analysis of the Kicking Horse River, Canadian Rocky Mountains

ABSTRACTBraided rivers have accumulated a dominant fraction of the terrestrial sedimentary record, and yet their morphodynamics in proximal intermountain reaches are still not fully documented—a shortcoming that hampers a full understanding of sediment fluxes and stratigraphic preservation in proximal-basin tracts. Located in the eastern Canadian Cordillera near the continental divide, the Kicking Horse River is an iconic stream that has served as a model for proximal-braided rivers since the 1970s. Legacy work on the river was based solely on ground observations of small, in-channel bars; here we integrate field data at the scale of individual bars to the entire channel belt with time-lapse remote sensing and ground-penetrating-radar (GPR) imaging, in order to produce a more sophisticated morphodynamic model for the river.Cyclical discharge fluctuations related to both diurnal and seasonal variations in melt-water influx control the planform evolution and corresponding stratigraphic signature of trunk channels, intermittently active anabranch channels, and both bank-attached and mid-channel bars. Three-dimensional GPR fence diagrams of compound-bar complexes are built based on the identification of distinct radar facies related to: i) accretion and migration of unit bars, ii) both downstream and lateral outbuilding of bar-slip foresets; iii) buildup of bedload sheets, iv) channel avulsion, and v) accretion of mounded bars around logs or outsized clasts. Trends observed downstream-ward include decreases in gradient and grain size decreases, trunk-channel shrinkage, intensified avulsion (with increase in abundance for anabranch channels), and a shift from high-relief to low-relief bar topography. The integration of ground sedimentology, time-lapse remote sensing, and GPR imaging demonstrates that proximal-braided streams such as the Kicking Horse River can be critically compared to larger systems located farther away from their source uplands despite obvious scale differences.

Temporal variation of water discharges in the lower course of the Danube River across the area from Reni to Izmail under the influence of natural and anthropogenic factors

The Danube River mouth is a main source for fresh water used for water supply and irrigation purposes in the arid southern part of Ukraine. In addition, the water of the mouth is used for filling the Danube Lakes. Climate change and numerous social and economic factors reduce the area and the water level of the Danube Lakes and increase their salinity. Under these circumstances the water exchange between the Danube River and the lakes is a very important process which allows maintenance of the water–salt balance of the latter. Such water exchange massively depends on the Danube River regime. The paper presents research of temporal variation of typical water discharges in the lower course of the Danube River associated with assessment of natural and anthropogenic factors affecting river runoff fluctuations. Time series of annual average water discharge at the water gauging stations of such cities as Reni and Izmail were considered uniform for the selected periods. They cover the period of conventionally natural river runoff regime (1840–1920), the period of the least altered river runoff regime (1921–1960) and the most altered river runoff regime under a heavy anthropogenic influence (1961–1989). The analysis of the impact of climate changes on the river runoff allowed introduction of a new, modern period of the Danube River hydrological regime (1990–2015). It is established that the series of maximum and minimum water discharges are characterized by the lack of their uniformity from the beginning of the period of initial influence of hydraulic engineering structures on the hydrological regime (since 1961). The analysis of the aggregate multi-year series (1840–2015) of average annual, maximum and minimum water discharges of the Danube River across its length from Reni to Izmail showed the presence of positive trends. At the same time the period of climatic changes (after 1989) is characterized by a less intensive growth of maximum water discharges. The research resulted in establishing the periodicity and synchronism of water discharge fluctuations in the lower course of the Danube River from Reni to Izmail within the territory of Ukraine. In particular, the study showed for the first time that from the 2000s onwards the redistribution of river runoff between the Danube River branches led to unsynchronization of minimum river runoff in different parts of the Danube Delta. The research shows that comprehensive analysis of the Danube River hydrological regime under modern climatic conditions is required to ensure efficient engineering regulation of the Danube Lakes filling pattern. Such regulation, in its turn, will ensure that the water supply and irrigation purposes are achieved.

Annual suspended sediment load of the Yenisei river

Field studies, including high-frequency sampling for suspended sediment flux estimation, were conducted in 20142016 in the city of Igarka, at the outlet of the Yenisei River. During spring freshet of each year, multiple suspended sediment concentration (SSC) peaks were observed, irrelated to water discharge fluctuations. The form of hysteresis loops evidences the importance of input from local in-channel sediment sources, i.e. banks and bedforms, and scouring of bed material, deposited during winter, in observed sharp SSC peaks. On the falling stage of the freshet, longer peaks are related to sediment waves from major tributaries, notably the Nizhnyaya Tunguska River. Annual sediment load was calculated based on the daily water discharge and observed SSC data, using sediment rating curves and LOADEST models as two reference methods. Mean annual suspended sediment load of the Yenisei River in Igarka is estimated at 8.1 0.5 mln t., which significantly exceeds previously published values for the 19702001 period, from 4.6 to 5.9 mln t. Cumulative sediment load for 20142016 totals 24.2 2.1 mln t.

Diel discharge variations in dormant and growing seasons in a headwater catchment suggest potential sources of an evapotranspiration signal

AbstractDiel fluctuations in discharge not associated with precipitation have been identified in streams around the world and attributed to evapotranspiration of both hillslope and near‐stream vegetation. Several mechanisms have been hypothesized to explain the generation of these signals, but a consensus has not been reached. In order to investigate the origin of these daily discharge fluctuations, we measured discharge and groundwater levels in a 43‐ha headwater catchment near Cullowhee, NC, USA. The catchment was logged at least twice in the past 120 years, allowing the evergreen invasive Chinese privet (Ligustrum sinense) to become established in near‐stream areas. Stream discharge was measured at the outlet, and stream stage and near‐stream groundwater level were measured at a second location higher in the catchment. Daily fluctuations in discharge were quantified and found to increase in magnitude with daily mean discharge. Interestingly, daily discharge fluctuations were observed in the dormant season, and our analysis suggests that their magnitude may increase more with discharge than during the growing season. Two previously hypothesized processes may explain our observations. A decrease in signal dampening due to faster transmission of an evapotranspiration signal at elevated flows may explain the increased diel fluctuation magnitude at higher flows. Additionally, because the primary active vegetation in our catchment in the dormant season is in near‐stream areas, we suggest that this indicates most of the evapotranspiration signal in stream discharge is derived from near‐stream vegetation, not hillslopes.

Satellite remote sensing reveals impacts from dam‐associated hydrological changes on chlorophyll‐a in the world's largest desert lake

AbstractWe present an approach that uses satellite products to derive models for predicting lake chlorophyll from environmental variables, and for investigating impacts of changing environmental flows. Lake Turkana, Kenya, is the world's largest desert lake, and environmental flows from the Omo River have been modified since 2015 by the Gibe III dam in Ethiopia. Using satellite remote sensing, we have evaluated the influence of these altered hydrological patterns on large‐scale lake phytoplankton concentrations for the first time. Prior to dam completion, strong seasonal cycles and large spatial gradients in chlorophyll have been observed, related to natural fluctuations in the Omo River's seasonal discharge. During this period, mean lake chlorophyll showed a strong relationship with both river inflows and lake levels. Empirical models were derived which considered multiple hydro‐climatic drivers, but the best model for predicting chlorophyll‐a was a simple model based on Omo River discharge. Application of this model to data for 2015–2016 estimated that during the filling of Gibe III annual mean Lake Turkana chlorophyll declined by 30%. Future water management scenarios based on Gibe III operations predict reduced seasonal chlorophyll‐a variability, while irrigation scenarios showed marked declines in chlorophyll‐a depending on the level of abstraction. These changes demonstrate how infrastructure developments such as dams can significantly alter lake primary production. Our remote sensing approach is easy to adapt to other lakes to understand how their phytoplankton dynamics may be affected by water management scenarios.

Climatic elements, discharge and groundwater trends over time using Mann-Kendall Test in the Mighan Sub-basin of Arak

This study was an analytical research across 84 meteorological stations, performed in the Mighan sub-basin over a fifty-year study period (1961–2011).This research seeks to answer the basic question of how declining streamflow, increasing temperatures, and fluctuation in precipitation have impacted water resource allocation in the Mighan sub-basin. The research method is analytical based on Mann-Kendall method. Analysis of independent flow measures (discharge and groundwater levels) using the Mann-Kendall trend test suggests evidence for climate change trends for many of the 84 stations. The results highlighted a mix of positive (increasing) and negative (decreasing) trends (monthly, seasonal, and annual) in the Mighan sub-basin. The results showed that during the 10-year period (1961-70), the minimum temperature occurrence in the first decade at -33.5 ° C (1973) and the maximum temperature event in the same decade at 57 ° C (1974) made this decade one of the most volatile decades in a fifty year period. Despite the inter-annual climatic fluctuations, results revealed that the Mighan Sub-basin has encountered many severe fluctuations of precipitation and average annual discharge during the five decades.

A New Technical Concept for Water Management and Possible Uses in Future Water Systems

A new degree of freedom in water management is presented here. This is obtained by displacing water, and in this paper is conceptually explained by two methods: using an excavated cavern as a container for compressed air to displace water, and using inflatable balloons. The concepts might have a large impact on a variety of water management applications, ranging from mitigating discharge fluctuation in rivers to flood control, energy storage applications and disease-reduction measures. Currently at a low technological readiness level, the concepts require further research and development, but the authors see no technical challenges related to these concepts. The reader is encouraged to use the ideas within this paper to find new applications and to continue the out-of-the-box thinking initiated by the ideas presented in this paper.

Spatial and seasonal variability in grain size, magnetic susceptibility, and organic elemental geochemistry of channel-bed sediments from the Mekong Delta, Vietnam: Implications for hydro-sedimentary dynamic processes

Understanding the relationship between depositional processes and their products in the fluvial–marine transition zone in tide-dominated depositional systems is fundamental to improving environmental and stratigraphic interpretations of long-term sedimentary records. The distributary channels of the Mekong Delta represent a typical example of a complex sedimentary environment characterized by strong fluvial and tidal interactions, and seasonal variability in hydro-sedimentary dynamic processes, particularly the migration of a salt wedge front linked to seasonal fluctuations in freshwater discharge. For this study, we analyzed grain size, magnetic susceptibility, and organic elemental geochemistry of channel-bed surficial sediments collected in 2015 from all distributary channels during the dry season and from the Mekong–Co Chien River during the flood season. This paper reports spatial and seasonal differences in the characteristics of channel-bed surficial sediments, which are closely linked to sediment sources and depositional processes during the dry and flood seasons.Distributary channels are characterized by coarse-grained sediments in the upper reaches and fine-grained sediments in the lower reaches, in both dry and flood seasons, reflecting the difference between the upstream fluvial-dominated environment and the downstream tide-dominated environment. Flood season samples from the Co Chien River show higher magnetic mineral contents and a terrestrial source of organic carbon compared with dry season samples, indicating sourcing from the drainage basin and trapping of suspended materials in the distributary channel due to the barrier effect of the saltwater wedge at the river mouth. Compared with the flood season, during the dry season the mud content was lower in the downstream reach close to the river mouth and higher in the upstream reach of the Co Chien River, and the content of magnetic minerals was lower in both reaches, indicating dissolution of magnetic minerals during early diagenesis. Furthermore, the lower C/N ratios (<10) and slightly enriched δ13C values of the dry season samples from the Co Chien River imply an additional contribution of organic carbon from a marine source. These various features imply a landward import of mud into the Co Chien River induced by stronger salinity intrusion during the dry season. Along the downstream tract of the Bassac River, two mud-dominated reaches were identified during the dry season. The magnetic mineral contents, C/N ratios and δ13C values indicate that the mud-dominated reach near the Bassac River mouth is also supplied from resuspended sediments by estuarine circulation and tidal pumping, whereas the mud in the reach that is transitional from fluvial to tide dominance has a riverine supply. The results suggest that the Mekong distributary channels are important sinks for suspended sediments during both the flood and dry seasons, controlled by a sediment trapping mechanism provided by either estuarine processes or the morphological configuration of the channels, both of which are controlled by fluvial–tidal dynamics. These depositional processes may be compensating for the incision of channels induced by sand mining in recent decades.

Foraminiferal Changes and Response to the Channel Diversion and Discharge Fluctuation of the Yellow River in the North Yellow Sea During the Past Century

Diversion of the Yellow River is a unique geological event in offshore China, causing changes of the sedimentary environment in eastern China Seas. The last diversion took place in AD 1855, with the estuary diverted from the Yellow Sea into the Bohai Sea. The identification of the river diversion events in the shelf sediments would not only provide the definite ages for the sediments, but also give a clue for better understanding of the sedimentation in that area. In this study, 210Pb, grain size, geochemical element, and foraminiferal data in core H205 from the north Yellow Sea were systematically investigated. A high-resolution sedimentary record was established, which was coupled with the Yellow River diversion and runoff changes. The results show that the foraminiferal composition and foraminiferal abundance of the sediments from the north Yellow Sea had good response to the Yellow River diversion in 1855. Before the change, shallow water assemblages dominated the foraminifera, and the abundance of each foraminiferal species was very low. After the diversion event, the abundance of most foraminifera increased sharply, with a maximum increase of 16 times, and the assemblage was still dominated by shallow water species. Furthermore, the changes in foraminiferal abundance in the core sediments corresponded well with the discharge fluctuation of the Yellow River since 1855. When the Yellow River began entering the Bohai Sea, the Yellow River water, which is rich in nutrients, along with the coastal currents affected the north Yellow Sea, increased the primary productivity in the north Yellow Sea, which is the main reason for the abrupt increase and fluctuation of foraminiferal abundance in this area. At the meantime, the East Asian winter monsoon could also promote the development of nearshore foraminiferal species by enhancing the coastal currents.

2D HYDRODYNAMIC MODELLING FOR IDENTIFICATION OF DEWATERED OR FLOODED STREAM CHANNEL AREAS DOWNSTREAM LARGE HYDROPOWER PLANT

Hydropower is oldest available source for renewable energy generation in Lithuania and the world. However, because of unequal demand of electricity during the day large hydropower plants are adapted to work in hydropeaking regime, which causes rapid water level and discharge fluctuations and has impact on the environment. To assess the extent of this impact in this study 2D numerical hydrodynamic modelling was carried out to reveal the dewatered or flooded stream channel areas at the Nemunas River downstream Kaunas Hydropower Plant. Such estimation of dewatered or flooded areas, considering different operating modes of hydropower plant, was carried out for the first time in Lithuania. It was revealed that largest areas are flooded when Kaunas hydropower plant starts its operation with two and four turbines and accordingly are dewatered when the plant stops such operations. During this study the impact of water level and discharge fluctuations on river ecosystems was not analysed, however the obtained results will be the initial data for more detailed assessment of fish habitats quality under impact of hydropeaking.