High River Stage Research Articles

Effects of a large-scale dam structure on upstream and downstream lateral hyporheic exchange and residence time distributions – The Xinglong Water Conservancy Dam, China

Anthropogenic in-stream structures such as large dams can alter stream hydrodynamics and cause river stage fluctuations that have the potential to affect hyporheic exchange flow (HEF) patterns between rivers and groundwater. However, the development of HEF patterns and residence time distribution (RTD) in dam-regulated riverbank aquifers has mostly been studied focusing on areas downstream of large dam structures while upstream areas have often been neglected. In order to investigate the HEF and RTD patterns under the influence of a large dam in upstream and downstream areas simultaneously, groundwater level data was collected from thirteen monitoring wells distributed over three monitoring profiles both upstream and downstream of XingLong water conservancy dam (XLD), in the Han river, China. Based on field observations, a two-dimensional, horizontal numerical model was built to assess the spatiotemporal evolution of lateral hyporheic exchange flow, the spatial extent of the hyporheic zone (HZ) and groundwater RTD under the influence of the XLD. Our results demonstrate that the impact of the XLD is different in upstream and downstream aquifer regions. The storage effect induced by the XLD caused significant groundwater recharge upstream of the dam over long period of model, while significant groundwater discharge occurs downstream of the dam. As the stage of the regulated Han River varies, the HZ along the upstream river boundary significantly enhanced by the dam whereas substantial HEF downstream was observed in response to rising or high river stage. Furthermore, the operation of the XLD resulted in some substantial flow around the dam characterized by short flow path lengths and high flow velocity, which consequently resulted in the development of a significant HZ in the perimeter surrounding the XLD. Residence time distributions showed that the water in the HZ downstream of the XLD is dominated by rejuvenated (younger) groundwater, whereas the HZ upstream of the XLD comprises a mix of older and younger waters. HEF in the immediate vicinity of the dam comprises both aged and rejuvenated discharged groundwater. Our findings emphasize the need to more holistically consider river stage dynamics in river corridors where large dams are operated to better assess and understand the consequences on riverbank storage as well as flow, transport and attenuation patterns in connected floodplain aquifers and their impact on the aquatic environment.

Benthic macroinvertebrate assemblages in large river secondary channels: Contemporaneous and legacy effects of flow connectivity

AbstractIn large rivers, secondary channels occur where the main channel is divided by an instream island, forming one or multiple smaller channels outside the main channel. Secondary channels are highly variable in morphometry, flow characteristics, and degree of connectivity to the main channel. Engineered closing structures at the upstream end of most secondary channels restrict inflow from the main channel, resulting in gradients of flow connectivity among secondary channels that vary with river stage. We investigated the relationship of flow connectivity to benthic macroinvertebrate assemblage richness and structure among a series of secondary channels of the Lower Mississippi River. Samples were collected over 2 years at times of high and low river stages. We discovered (1) macroinvertebrate assemblage structure and taxonomic richness varied along the flow connectivity gradient, and (2) there was a legacy effect of prior connection on assemblage structure that lasted up to a year. We contend that for management and restoration planning aimed at conservation of large river biological diversity, an important consideration are the life history requirements of animals utilizing secondary channel habitats.

Factors Affecting the Rates and Modes of Landslide Colluvium Removal in River Channels of Podhale (Western Carpathians)

This paper presented some hydrological factors affecting the course and rate of fluvial erosion of landslide colluvium at its contact with river flow. Volumes of colluvium eroded by rivers in the period 2013–2019 were measured at Podhale (a part of Polish West Carpathians) on four landslides representing various geological settings. At each landslide, changes in shape and position of the contact zone between colluvium and river water were registered after episodes of high river stage. The obtained data on changes in relief of the landslide fronts and adjacent river channels were used to calculate volumes of colluvium removed during each episode. The course of erosion and volumes of colluvium eroded were compared with the water stage records for the studied period of time. Intensity of colluvium erosion was found to be strongly dependent on the water levels and cohesion of colluvium. Volumes of removed colluvium were the greatest during short-lived (1–2 days) and prolonged (7–10 days) periods of high river stages. The rate of erosional removal was the highest for colluvium consisting of poorly consolidated Quaternary matrix-supported massive gravel and overlying fine deposits stored within river terraces. Colluvium composed of Neogene mudstones and sandstones was removed at a lower rate and the rate of removal was lowest for large blocks and slices composed of solid layers of alternating sandstone and shales belonging to the Podhale Flysch series. Erosion of the landslide toes was more intense at those sites where the river flow approached the landslide front at a wider angle.

Assessment of hydrological, geohydraulic and operational conditions at a riverbank filtration site at Embaba, Cairo using flow and transport modeling

Study regionThis study focuses on the simulation of flow and transport of potential contaminants at the riverbank filtration (RBF) site at Embaba, Cairo. RBF has a high potential along the Nile in Egypt as a pre-treatment step for drinking water production. As the site in Cairo can be affected by low water quality and spills of contaminants, the aim was to assess the RBF efficiency under various hydrological, geohydraulic and operational conditions. Study focusMODFLOW, MT3D and MODPATH were used in this study. Three cases were considered to identify the share of bank filtrate in pumped water and the travel time. The first case focused on the river hydrograph and the permeability of the riverbed; the second on clay cap thickness, aquifer and riverbed hydraulic conductivities, and groundwater abstraction downside of the RBF scheme; and the third on different RBF well abstraction rates, depths, screen lengths and distances from the river bank. New hydrological insights for the regionThe results indicate that favorable conditions for RBF sites. Good conditions for RBF include high river stage and riverbed permeability, high aquifer hydraulic conductivity and abstraction rates resulting in high portions of bank filtrate. Design and operation analysis indicate that the abstraction wells should be shallow and located close to the river bank to achieve shorter travel times, thus increasing productivity and preventing high manganese concentrations in the pumped bank filtrate. Due to the natural flow gradient from the Nile towards the RBF wells, changing conditions have impact on the portions of bank filtrate, making the site special.

The Influence of Grain Size Distribution on the Hydraulic Gradient for Initiating Backward Erosion

Backward erosion by piping is one of the processes that threaten the stability of river embankments in the Netherlands. During high river stages, groundwater flow velocities underneath the embankment increase as a result of the steepened hydraulic gradient. If a single outflow point exists or forms, the concentrated flow can entrain soil particles, leading to the formation of a subsurface pipe. The processes controlling this phenomenon are still relatively unknown due to their limited occurrence and because piping is a subsurface phenomenon. To study the initiation of piping, we performed laboratory experiments in which we induced water flow through a porous medium with a vertically orientated outflow point. In these experiments, we explicitly considered grain size variations, thus adding to the existing database of experiments. Our experiments showed that the vertical velocity needed for the initiation of particle transport can be described well by Stokes’ law using the median grain size. We combine this with a novel method to relate bulk hydraulic conductivity to the grain size distribution. This shows that knowledge of the grain size distribution and the location of the outflow point are sufficient to estimate the hydraulic gradient needed to initiate pipe formation in the experiment box.

The spatial extent and timescales of bank infiltration and return flows in an upland river system: Implications for water quality and volumes

The temporary storage of river water in riverbanks and its subsequent return to the river is an important part of the hydrological cycle. Detailed multi-year head and electrical conductivity (EC) variations document the spatial and temporal scales of bank infiltration and return flows in the Ovens catchment, southeast Australia. The study represents one of only a few that has integrated geochemical data and hydraulic heads over several hydrological cycles in a catchment unimpacted by groundwater extraction. The river water infiltrates only a few metres to tens-of-metres into the riverbanks and dilution of regional groundwater in the banks is commonly observed for several weeks to months following high river stages. Two-dimensional numerical modelling shows that the extent and timescales of bank infiltration and return flows are governed mainly by the regional head gradients, hydraulic conductivities, and the height of the stream pulse. Variable topography in river catchments will result in significant variations in the volumes of bank storage and the timescales that these bank waters are retained. The combination of field observations and numerical modelling illustrates that parts of the riverbanks may store water for several months to years and reconnection of rivers to the regional groundwater will occur in different parts of the catchment at different times. In much of the catchment the bank storage zones act as a buffer against contaminated regional groundwater and bank return flows will sustain streamflow and riverine ecosystems during dry periods.

Hot Spots and Hot Moments of Nitrogen in a Riparian Corridor

AbstractWe use 3‐D high‐resolution reactive transport modeling to investigate whether the spatial distribution of organic‐carbon‐rich and chemically reduced sediments located in the riparian zone and temporal variability in groundwater flow direction impact the formation and distribution of nitrogen hot spots (regions that exhibit higher reaction rates when compared to other locations nearby) and hot moments (times that exhibit high reaction rates as compared to longer intervening time periods) within the Rifle floodplain in Colorado. Groundwater flows primarily toward the Colorado River from the floodplain but changes direction at times of high river stage. The result is that oxic river water infiltrates the Rifle floodplain during these relatively short‐term events. Simulation results indicate that episodic rainfall in the summer season leads to the formation of nitrogen hot moments associated with Colorado River rise and resulting river infiltration into the floodplain. The results further demonstrate that the naturally reduced zones (NRZs) present in sediments of the Rifle floodplain have a higher potential for nitrate removal, approximately 70% greater than non‐NRZs for typical hydrological conditions. During river water infiltration, nitrate reduction capacity remains the same within the NRZs, however, these conditions impact non‐NRZs to a greater extent (approximately 95% less nitrate removal). Model simulations indicate chemolithoautotrophs are primarily responsible for the removal of nitrate in the Rifle floodplain. These nitrogen hot spots and hot moments are sustained by microbial respiration and the chemolithoautotrophic oxidation of reduced minerals in the riparian zone.

Seasonal changes in habitat suitability for adult shovelnose sturgeon in the lower Mississippi River

Shovelnose sturgeon Scaphirhynchus platorynchus are a large-river fish distributed throughout the Mississippi River basin, including the lower 1,533 km of the Mississippi River where riverine habitat has been and continues to be modified for navigation and is a potential site for development of instream hydrokinetic electric power generation. Information about habitat use and preference is essential to future conservation efforts. Shovelnose sturgeon have previously been found to select particular habitat types, and these selected habitats vary seasonally; although these past analyses do not consider the selected habitats in a landscape context. We used ecological niche factor analysis (ENFA) that uses distributions of telemetry locations and environmental variables to model habitat suitability in a landscape context. We recorded 333 locations of shovelnose sturgeon during July–December 2013 that included periods of relatively high and low river stages. The ENFA analysis indicated high-suitability locations were in or near deep water during both high and low river stages. During high river stages, high-suitability locations were near island tip habitat, deep water, and steep bottom slope and far from main channel habitat. During low stages, high-suitability locations were in or near deep water and main channel habitat and far from secondary channel and wing dike habitats. This landscape-scale analysis supports seasonal shifts in habitat use and provides insights that can be used to inform habitat conservation and management to benefit shovelnose sturgeon in the lower Mississippi River and possibly other large rivers.

Estimating groundwater-ephemeral stream exchange in hyper-arid environments: Field experiments and numerical simulations

Surface water infiltration from ephemeral dryland streams is particularly important in hyporheic exchange and biogeochemical processes in arid and semi-arid regions. However, streamflow transmission losses can vary significantly, partly due to spatiotemporal variations in streambed permeability. To extend our understanding of changes in streambed hydraulic properties, field investigations of streambed hydraulic conductivity were conducted in an ephemeral dryland stream in north-western China during high and low streamflow periods. Additionally, streamflow transmission losses were numerically estimated using combined stream and groundwater hydraulic head data and stream and streambed temperature data. An analysis of slug test data at two different river flow stages (one test was performed at a low river stage with clean water and the other at a high river stage with muddy water) suggested that sedimentation from fine-grained particles, i.e., physical clogging processes, likely led to a reduction in streambed hydraulic properties. To account for the effects of streambed clogging on changes in hydraulic properties, an iteratively increasing total hydraulic resistance during the slug test was considered to correct the estimation of streambed hydraulic conductivity. The stream and streambed temperature can also greatly influence the hydraulic properties of the streambed. One-dimensional coupled water and heat flux modelling with HYDRUS-1D was used to quantify the effects of seasonal changes in stream and streambed temperature on streamflow losses. During the period from 6 August 2014 to 4 June 2015, the total infiltration estimated using temperature-dependent hydraulic conductivity accounted for approximately 88% of that using temperature-independent hydraulic conductivity. Streambed clogging processes associated with fine particle settling/wash up cycles during flow events, and seasonal changes in streamflow temperature are two considerable factors that affect water infiltration in ephemeral dryland streams. Our results show that time series measurements of stream and sediment temperature and surface and groundwater head can be used to effectively determine the seasonal dynamics of streambed water exchange. Such combined heat and head monitoring at field sites is useful for calibrating regional surface-groundwater models. The results of this study may provide insights into hyporheic exchange in ephemeral dryland streams.

Comparison of ArcToolbox and Terrain Tiles processing procedures for inundation mapping in mountainous terrain

Floodplain management consists of efforts to reduce flood damage to critical infrastructure and to protect the life and health of individuals from flooding. A major component of this effort is the monitoring of flood control structures such as dams because the potential failure of these structures may have catastrophic consequences. To prepare for these threats, engineers use inundation maps that illustrate the flood resulting from high river stages. To create the maps, the structure and river systems are modeled using engineering software programs, and hydrologic events are used to simulate the conditions leading to the failure of the structure. The output data are then exported to other software programs for the creation of inundation maps. Although the computer programs for this process have been established, the processing procedures vary and yield inconsistent results. Thus, these processing methods need to be examined to determine the functionality of each in floodplain management practices. The main goal of this article is to present the development of a more integrated, accurate, and precise graphical interface tool for interpretation by emergency managers and floodplain engineers. To accomplish this purpose, a potential dam failure was simulated and analyzed for a candidate river system using two processing methods: ArcToolbox and Terrain Tiles. The research involved performing a comparison of the outputs, which revealed that both procedures yielded similar inundations for single river reaches. However, the results indicated key differences when examining outputs for large river systems. On the basis of criteria involving the hydrologic accuracy and effects on infrastructure, the Terrain Tiles inundation surpassed the ArcToolbox inundation in terms of following topography and depicting flow rates and flood extents at confluences, bends, and tributary streams. Thus, the Terrain Tiles procedure is a more accurate representation of flood extents for use by floodplain engineers, hydrologists, geographers, and emergency managers.

Obrenovac municipality drinking water supply status and prospects after the 2014. flood

In 2014, major floods in Serbia caused enormous damage in Obrenovac Municipality. Apart from numerous businesses, the flood affected the water supply system. The water source “Vic Bare” in the village of Zabrežje on the Sava river was inundated and the water distribution network, parts of which were relatively old, sustained further harm. Over the past several decades, insufficient groundwater reserves have been one of the main water supply issues in Obrenovac. Recent investigations identified the reserves and the possibility of ensuring additional amounts of groundwater at the existing source was explored. The largest problem, however, is a high water demand due to inefficient water use and water losses in the distribution network. Given that the said water source relies on bank filtration, the flood was extremely important from a hydrogeological perspective. The paper assesses the data collected during the course of extremely high river stages and water tables and examines source discharge capacities to provide further insight into surface water and groundwater interactions and compare the pre- and post-flood status of a municipal water supply source.

Synoptic estimates of diffuse groundwater seepage to a spring-fed karst river at high spatial resolution using an automated radon measurement technique

Groundwater (GW) seepage can provide a major source of water, solutes, and contaminants to rivers, but identifying magnitudes, directions and locations of seepage is complicated by its diffuse and heterogeneous distributions. However, such information is necessary to develop programs and policies for protecting ecosystems and managing water resources. Here, we assess GW seepage to the Ichetucknee River, a spring-fed, low gradient, gaining stream in north-central Florida, through automated longitudinal surveys of radon (222Rn) activities at three different flow conditions. A 222Rn mass balance model, which integrates groundwater and spring water end member 222Rn activities and longitudinal 222Rn distributions in river water, shows that diffuse groundwater seepage represents about 16% of the total river baseflow, consistent with previous results obtained from ion (Ca2+, Cl−, SRP and Fe) mass balances and dye tracer methods. During high river stage, the contribution from seepage increases to 18–23% of the river flow. The spatial distribution of GW seepage is more variable in the upper 2.2-km reach of the river than the lower 2.8-km reach, regardless of river flow conditions. The upper reach has a narrower flood plain than the lower reach, which limits evapotranspiration and increases hydraulic gradients toward the river following storm events. Seepage in the lower reach is also limited by hydrologic damming by the receiving river, which inundates the floodplain during high flow conditions, and reduces the hydraulic head gradient. These results demonstrate the variable nature of seepage to a gaining river in both time and space and indicate that multiple synoptic analyses of GW seepage are required to assess seepage rates, determine time-averaged solute fluxes, and develop optimal management policies for riverine ecosystems.

A climate informed model for nonstationary flood risk prediction: Application to Negro River at Manaus, Amazonia

Historically, flood risk management and flood frequency modeling have been based on assumption of stationarity, i.e., flood probabilities are invariant across years. However, it is now recognized that in many places, extreme floods are associated with specific climate states which may recur with non-uniform probability across years. Conditional on knowledge of the operating climate regime, the probability of a flood of a certain magnitude can be higher or lower in a given year. Here we explore nonstationary flood risk for the streamflow series of the Negro River at the city of Manaus in Brazil by investigating climate teleconnections associated with the interannual variability of the peak flows. We evaluate attributes and the fit of a generalized extreme value (GEV) distribution with nonstationary parameters to the annual peak series of the Negro River stages. The annual peak flood occurs between May and July and its magnitude depends on the Negro River stage at the beginning of the year and on the previous December sea surface temperature (SST) of a region in the tropical Pacific Ocean. A statistically significant monotonic trend is also observed in the peak level series. The indexing of the parameters of a GEV distribution to the NINO3 index and to the observed river stage at the beginning of the year reveals a changing flood hazard for the city, with the joint occurrence of high values associated with La Nina conditions in the previous December and high river stages in January preceding the flood season. The proposed model is shown to be useful for quantifying the changing flood hazard several months in advance for Manaus, thus providing an early flood alert system for the city and may be an important tool for the dynamic flood risk management for the region.

Control of river stage on the reactive chemistry of the hyporheic zone

AbstractWe examined the influence of river stage on subsurface hydrology and pore water chemistry within the hyporheic zone of a groundwater‐fed river during the summer baseflow period of 2011. We found river stage and geomorphologic environment to control chemical patterns in the hyporheic zone. At a high river stage, the flux of upwelling water in the shallow sediments (>20 cm) decreased at sample sites in the upper section of our study reach and increased substantially at sites in the lower section. This differential response is attributed to the contrasting geomorphology of these subreaches that affects the rate of the rise and fall of a river stage relative to the subsurface head. At sites where streamward vertical flux decreased, concentration profiles of a conservative environmental tracer suggest surface water infiltration into the riverbed below depths recorded at a low river stage. An increase in vertical flux at sites in the lower subreach is attributed to the movement of lateral subsurface waters originating from the adjacent floodplain. This lateral‐moving water preserved or decreased the vertical extent of the hyporheic mixing zone observed at a low river stage. Downwelling surface water appeared to be responsible for elevated dissolved organic carbon (DOC) and manganese (Mn) concentrations in shallow sediments (0–20 cm); however, lateral subsurface flows were probably important for elevated concentrations of these solutes at deeper levels. Results suggest that DOC delivered to hyporheic sediments during a high river stage from surface water and lateral subsurface sources could enhance heterotrophic microbial activities. Copyright © 2013 John Wiley & Sons, Ltd.

Using temperature modeling to investigate the temporal variability of riverbed hydraulic conductivity during storm events

Summary Understanding the impact of storm events on riverbed hydraulic conductivity is crucial in assessing the efficacy of riverbank filtration as a water-treatment option. In this study, the variability of riverbed hydraulic conductivity and its correlation to river stage during storm events was investigated. Water levels and temperatures were continuously monitored in the river using creek piezometers screened beneath the riverbed, and monitoring wells located on the river bank. The range of values for water levels during the study period was from 161.3 to 163.7 m AMSL while temperatures ranged from 3.75 °C to 24 °C. During the duration of the study the Great Miami River was losing water to the underlying aquifer due to pumping in the adjacent municipal well field. Flow and heat transport were simulated in a groundwater heat and flow program VSH2D to determine the hydraulic conductivity of the riverbed. Hydraulic conductivity was estimated by using it as a calibration parameter to match simulated temperatures to observed temperatures in a monitoring well. Hydraulic heads in the aquifer responded to storm events at the same times but with dampened amplitudes compared to the river stage. The relative responses resulted in increased head gradients during the rising limb of the stage-hydrograph. Heat-flow modeling during five storm events demonstrated that a rise in head gradient alone was not sufficient to produce the temperature changes observed in the wells. Simulated temperatures were fitted to the observed data by varying both river stage (as measured in the field) and riverbed hydraulic conductivity. To produce the best fit temperatures, riverbed hydraulic conductivity consistently needed to be increased during the rising and peak stages of the storm events. The increased conductivity probably corresponds to a loss of fine sediments due to scour during high river stage. Hydraulic conductivity increases during storm events varied from a factor of two (0.0951–0.2195 m/d) to almost one order of magnitude (0.0007–0.00658 m/d). Despite these predicted changes the highest model-predicted hydraulic conductivity value was 0.66 m/d, which is still much lower than the infiltration rate used in sand filtration systems (3.59 m/d). These low values suggest that storm events do not pose a significant risk to the water quality at this well field. There was a direct correlation between the duration of rising limb, rate of change of stage and maximum river stage and the magnitude of change of riverbed hydraulic conductivity.

Mobilization of arsenic and iron from Red River floodplain sediments, Vietnam

Sediments from the Red River and from an adjacent floodplain aquifer were investigated with respect to the speciation of Fe and As in the solid phase, to trace the diagenetic changes in the river sediment upon burial into young aquifers, and the related mechanisms of arsenic release to the groundwater. Goethite with subordinate amounts of hematite were, using Mössbauer spectroscopy, identified as the iron oxide minerals present in both types of sediment. The release kinetics of Fe, As, Mn and PO 4 from the sediment were investigated in leaching experiments with HCl and 10 mM ascorbic acid, both at pH 3. From the river sediments, most of the Fe and As was mobilized by reductive dissolution with ascorbic acid while HCl released very little Fe and As. This suggests As to be associated with an Fe-oxide phase. For oxidized aquifer sediment most Fe was mobilized by ascorbic acid but here not much As was released. However, the reduced aquifer sediments contained a large pool of Fe(II) and As that is readily leached by HCl, probably derived from an unidentified authigenic Fe(II)-containing mineral which incorporates As as well. Extraction with ascorbic acid indicates that the river sediments contain both As(V) and As(III), while the reduced aquifer sediment almost exclusively releases As(III). The difference in the amount of Fe(II) leached from river and oxidized aquifer sediments by ascorbic acid and HCl, was attributed to reductive dissolution of Fe(III). The reactivity of this pool of Fe(III) was quantified by a rate law and compared to that of synthetic iron oxides. In the river mud, Fe(III) had a reactivity close to that of ferrihydrite, while the river sand and oxidized aquifer sediment exhibited a reactivity ranging from lepidocrocite or poorly crystalline goethite to hematite. Mineralogy by itself appears to be a poor predictor of the iron oxide reactivity in natural samples using the reactivity of synthetic Fe-oxides as a reference. Sediments were incubated, both unamended and with acetate added, and monitored for up to 2 months. The river mud showed the fastest release of both Fe and As, while the effect of acetate addition was minor. This suggests that the presence of reactive organic carbon is not rate limiting. In the case of the river and aquifer sediments, the release of Fe and As was always stimulated by acetate addition and here reactive organic carbon was clearly the rate limiting factor. The reduced aquifer sediment apparently can sustain slower but prolonged microbially-driven release of As. The highly reactive pools of Fe(III) and As in the river mud could be due to reoxidation of As and Fe contained in the reducing groundwater from the floodplain aquifers that are discharging into the river. Deposition of the suspended mud on the floodplain during high river stages is proposed to be a major flux of As onto the floodplain and into the underlying aquifers.

Geochemistry of Surficial Sediments from Lake Pontchartrain Resulting from the 1997 Opening of the Bonnet Carré Spillway

Abstract The Bonnet Carre Spillway is a flood-control structure that diverts Mississippi River water into Lake Pontchartrain during exceptionally high river stages. Because of elevated water levels in the Mississippi River in the spring of 1997, the Bonnet Carre Spillway was opened on March 17 and fully closed on April 18. The total volume of water discharged into Lake Pontchartrain was approximately 11.8 km3, or two times the volume of the lake, and the total mass of sediment discharged into the lake was approximately 7.1 × 108 kg (780,000 US tons). In 1996, 757 surface sediment samples were collected in Lake Pontchartrain and were analyzed by x-ray fluorescence spectroscopy for major cation constituents. These same sites were revisited following the 1997 Mississippi River discharge event. Analysis of the 1996 and 1997 lake-bed sediment samples was accomplished utilizing fundamental statistical and graphical methods. Element concentration contour maps and variograms for the major cations illustrate meani...

Implications of anthropogenic river stage fluctuations on mass transport in a valley fill aquifer

In humid regions a strong coupling between surface water features and groundwater systems may exist. In these environments the exchange of water and solute depends primarily on the hydraulic gradient between the reservoirs. We hypothesize that daily changes in river stage associated with anthropogenic water releases (such as those from a hydroelectric dam) cause anomalous mixing in the near‐stream environment by creating large hydraulic head gradients between the stream and adjacent aquifer. We present field observations of hydraulic gradient reversals in a shallow aquifer. Important physical processes observed in the field are explicitly reproduced in a physically based two‐dimensional numerical model of groundwater flow coupled to a simplistic surface water boundary condition. Mass transport simulations of a conservative solute introduced into the surface water are performed and examined relative to a stream condition without stage fluctuations. Simulations of 20 d for both fluctuating river stage and fixed high river stage show that more mass is introduced into the aquifer from the stream in the oscillating case even though the net water flux is zero. Enhanced transport by mechanical dispersion leads to mass being driven away from the hydraulic zone of influence of the river. The modification of local hydraulic gradients is likely to be important for understanding dissolved mass transport in near‐stream aquifer environments and can influence exchange zone processes under conditions of high‐frequency stream stage changes.

Biogeochemical response of organic-rich freshwater marshes in the Louisiana delta plain to chronic river water influx

To help evaluate effects of Mississippi River inputs to sustainability of coastal Louisiana ecosystems, we compared porewater and substrate quality of organic-rich Panicum hemitomon freshwater marshes inundated by river water annually for more than 30 years (Penchant basin, PB) or not during the same time (Barataria basin, BB). In the marshes receiving river water the soil environment was more reduced, the organic substrate was more decomposed and accumulated more sulfur. The porewater dissolved ammonium and orthophosphate concentrations were an order of magnitude higher and sulfide and alkalinity concentrations were more than twice as high in PB compared with BB marshes. The pH was higher and dissolved iron concentrations were more than an order of magnitude lower in PB marshes than in BB marshes. The influx of nutrient-rich river water did not enhance end-of-year above-ground standing biomass or vertical accretion rates of the shallow substrate. The differences in porewater chemistry and substrate quality are reasonably linked to the long-term influx of river water through biogeochemical processes and transformations involving alkalinity, nitrate and sulfate. The key factor is the continual replenishment of alkalinity, nitrate and sulfate via overland flow during high river stage each year for several weeks to more than 6 months. This leads to a reducing soil environment, pooling of the phytotoxin sulfide and inorganic nutrients in porewater, and internally generated alkalinity. Organic matter decomposition is enhanced under these conditions and root mats degraded. The more decomposed root mat makes these marshes more susceptible to erosion during infrequent high-energy events (for example hurricanes) and regular low-energy events, such as tides and the passage of weather fronts. Our findings were unexpected and, if generally applicable, suggest that river diversions may not be the beneficial mitigating agent of wetland restoration and conservation that they are anticipated to be.

Nitrogen retention in the hyporheic zone of a glacial river in interior Alaska

We examined the hydrologic controls on nitrogen biogeochemistry in the hyporheic zone of the Tanana River, a glacially-fed river, in interior Alaska. We measured hyporheic solute concentrations, gas partial pressures, water table height, and flow rates along subsurface flowpaths on two islands for three summers. Denitrification was quantified using an in situ 15NO 3 − push–pull technique. Hyporheic water level responded rapidly to change in river stage, with the sites flooding periodically in mid−July to early−August. Nitrate concentration was nearly 3-fold greater in river (ca. 100 μg NO 3 − –N l−1) than hyporheic water (ca. 38 μg NO 3 − –N l−1), but approximately 60–80% of river nitrate was removed during the first 50 m of hyporheic flowpath. Denitrification during high river stage ranged from 1.9 to 29.4 mg N kg sediment−1 day−1. Hotspots of methane partial pressure, averaging 50,000 ppmv, occurred in densely vegetated sites in conjunction with mean oxygen concentration below 0.5 mgO2 l−1. Hyporheic flow was an important mechanism of nitrogen supply to microbes and plant roots, transporting on average 0.41 gNO 3 − –N m−2 day−1, 0.22 g NH 4 + –N m−2 day−1, and 3.6 g DON m−2 day−1 through surface sediment (top 2 m). Our results suggest that denitrification can be a major sink for river nitrate in boreal forest floodplain soils, particularly at the river-sediment interface. The stability of the river hydrograph and the resulting duration of soil saturation are key factors regulating the redox environment and anaerobic metabolism in the hyporheic zone.