Changes In River Stage Research Articles

Indicators for the Assessment of the Impact of Hydropeaking on Aquifers

AbstractThe operation of hydropower plants leads to sudden changes in river stage and to a flow regime known as hydropeaking. Hydropeaking alters the morphology of the riverbed and water quality, and ultimately poses a risk to riverine ecosystems. While many indicators are available to quantitatively assess this problem in rivers, the impact of hydropeaking on aquifers is largely unknown and lacks of quantitative indicators. We analyze with wavelet techniques the spatial and temporal dynamics of surface water‐groundwater interaction in an aquifer impacted by two differently regulated rivers. We propose four indicators to study the aquifer stress produced by hydropeaking and classify the observed groundwater head time series into weakly, moderately and highly impacted. This study opens the possibility for a quantitative assessment of the impact of hydropeaking on the groundwater ecosystem.

Transport and removal of spores of Bacillus subtilis in an alluvial gravel aquifer at varying flow rates and implications for setback distances

To guarantee proper protection from fecally transmitted pathogen infections, drinking water wells should have a sufficiently large setback distance from potential sources of contamination, e.g. a nearby river. The aim of this study was to provide insight in regards to microbial contamination of groundwater under different flow velocities, which can vary over time due to changes in river stage, season or pumping rate. The effects of these changes, and how they affect removal parameters, are not completely understood. In this study, field tracer tests were carried out in a sandy gravel aquifer near Vienna, Austria to evaluate the ability of subsurface media to attenuate Bacillus subtilis spores, used as a surrogate for Cryptosporidium and Campylobacter. The hydraulic gradient between injection and extraction was controlled by changing the pumping rate (1, 10 l/s) of a pumping well at the test site, building upon previously published work in which tracer tests with a 5 l/s pumping rate were carried out. Attachment and detachment rate coefficients were determined using a HYDRUS-3D model and ranged from 0.12 to 0.76 and 0–0.0013 h−1, respectively. Setback distances were calculated based on the 60-day travel time, as well as a quantitative microbial risk assessment (QMRA) approach, which showed similar results at this site; around 700 m at the highest pumping rate. Removal rates (λ) in the field tests ranged from 0.2 to 0.3 log/m, with lower pumping rates leading to higher removal. It was shown that scale must be taken into consideration when determining λ for the calculation of safe setback distances.

Coupled modeling of bed deformation and bank erosion in the Jingjiang Reach of the middle Yangtze River

Recent significant channel evolution in the Jingjiang Reach has raised much attention, particularly the remarkable bank erosion. A coupled model for simulating bed deformation and bank erosion has been proposed in this study, which focuses on the erosion of the bank with a composite structure in the Lower Jingjiang Reach. In order to cover three contributing processes that may interact with each other, the proposed model integrates a one-dimensional morphodynamic module with a two-dimensional module of ground water flow and a bank erosion module for the cantilever failure of a composite riverbank. Model performance was evaluated through a detailed simulation of channel evolution along a 150.8-km subreach in the Jingjiang Reach over the 2005 hydrological year. Satisfying results were obtained from the simulation, showing relatively close agreement between the calculations and measurements in terms of hydrological data at the outlet section, bank erosion sites, longitudinal channel profile and typical cross-sectional profiles. In addition, investigations into temporal changes in bank soil properties and critical overhanging width at cantilever failure demonstrate that there was a seasonal variation in the volumetric water content of bank soil, which increased during the rising and flood periods and then decreased during the recession period, showing an impact on the occurrence timing of cantilever failures. The tensile strength and critical overhanging width had an inverse relationship with the water content, whereas the critical width sharply increased and then decreased during high flows affected by a rapid change in river stage. The temporal distribution of cantilever failure events indicates that cantilever failure primarily occurred in the flood and recession periods. The effects of bed roughness, water content variation and secondary flow on bank erosion were also discussed.

Recent changes in channel morphology of a highly engineered alluvial river – the Lower Mississippi River

Changes in channel morphology provide relevant insights into sediment transport and deposition in alluvial river systems. This study assessed three to four decades of morphological changes at seven locations along a 327-km reach of the Lower Mississippi River (LMR) to better understand channel adjustment processes of this large alluvial river. The assessment included analysis of three cross-sectional areas at each location during the period 1992–2013, as well as analysis of the changes in river stage and maximum surface slopes under four flow conditions over the last three to four decades . We found that the first 20–25 km LMR reach below its diversion to the Atchafalaya River and the reach from 80 to 140 km experienced significant riverbed aggradation, while the reach in between (i.e. from 20 to 80 km) experienced riverbed degradation. The lower 187-km reach (i.e. from 140 to 327 km) showed negligible sediment trapping. These findings may have relevant implications for management of river sediment diversions along the LMR and other large alluvial rivers in the world.

Observed response of a shield-driven tunnel to fluctuations in river stage

One issue of particular concern in construction of shield-driven tunnels beneath the Qiantang River is the evaluation of impacts of changes in river stage on serviceability and structural soundness of the tunnels in the long run. During construction of the Qiantang River Tunnel, four typical cross-sections of the tunnel linings were instrumented with total earth pressure cells and reinforcement strain gauges, embedded during precasting of the concrete segments, to measure the tunnel responses to changes in river stage during operation. This paper analyzes the field-measured total earth pressures acting on the tunnel linings and strains of the steel bars embedded within the segments and explores their correlations with river stage of the Qiantang River. Based on the field observations, a calculation model taking variations of river level with time into account is proposed to predict the total earth pressures acting on the tunnel lining situated beneath the Qiantang River.

Bank storage in karst aquifers: The impact of temporary intrusion of river water on carbonate dissolution and trace metal mobility

Storms can trigger changes in river stage that alter the hydraulic gradients between rivers and adjacent aquifers. In eogenetic carbonate karst aquifer systems, storms enable river water to intrude >1km into the adjacent aquifer systems for days to weeks. This process is similar to bank storage of streams in siliciclastic sediments but can have longer temporal and larger spatial scales. River intrusion triggers changes in mineral saturation states and redox conditions in the aquifer due to the input of low pH, low specific conductivity (SpC), and high dissolved organic carbon (DOC) flood water. To assess the effects of river intrusion into karst aquifers, we measured SpC, temperature, pH, redox state, and concentrations of dissolved major and trace elements through an intrusion event at Madison Blue Spring in northern Florida, USA. River water displaced groundwater in the conduit at least 1km into the aquifer and flowed into the pores of the unconfined aquifer matrix. Distinct Cl− concentrations between river water and groundwater provide estimates for mixing fractions. The location and magnitude of oxidation of organic matter in the subsurface controlled trace metal concentration, redox state and saturation state of the water with respect to calcite (SIcal). Organic matter oxidation in the phreatic conduits was limited by the terminal electron acceptors (TEAs) present in the conduit water. Calcite dissolution and trace metal sorption were limited by the lower surface area to porosity ratios in the conduits than the matrix. Organic matter oxidation was enhanced in the matrix by mixing with matrix waters with available DO and NO3−, resulting in greater CO2 production and calcite dissolution than in the conduit. After the intruded river water discharged, conditions remained more reducing in the aquifer than baseflow conditions due to the reduction of DO and NO3− in the matrix water. The organic matter transported into the aquifer during river intrusion drives carbonate dissolution, alters redox state, and impacts trace metal mobility, impacting groundwater and surface water quality.

Study of impact on surface water and groundwater around flow fields due to changes in river stage using groundwater modeling system

Excessive ground water withdrawal can lead to ground water depletion, and this may have serious social and economic consequences. Attempts to limit groundwater pumping have been commonly based on the concept of safe yield. In this study, in order to evaluate safe yield, short- and long-duration pumping tests have been performed at four different sites (Purulia District, India) on Damodar River bed. The lithological survey was carried out along the river bed at regular intervals from 500 m to 10.5 km, covering a total stretch of 10-km up-stream and 0.5-km down-stream, to determine the soil profile and other soil characteristics. The points, where the maximum depth of porous medium was obtained, were selected for carrying out the pumping yield tests. The appropriate hydrogeologic approaches were employed to estimate the values of aquifer parameters, viz., specific yield (S y), transmissivity(T), and hydraulic conductivity (K) for the aforesaid sites under study, which were found to be in the range of 0.043–0.175, 2082–3983, and 219–603 m/day, respectively. A hypothetical aquifer-stream water interaction system has been developed using GMS software (groundwater modeling system) not only to represent the hydraulic head fluctuations in the groundwater system around well fields but also to quantify the flow rate due to change in river stage, recharge due to rainfall, and also to determine how the surface water could affect groundwater due to pumping.

River-aquifer exchange fluxes under monsoonal climate conditions

An important prerequisite to better understand the transport of nutrients and contaminants across the river-aquifer interface and possible implications for biogeochemical transformations is to accurately characterize and asses the exchange fluxes. In this study we investigate how monsoonal precipitation events and the resulting variability in river discharge affect the dynamics of river-aquifer exchange and the corresponding flux rates. We evaluate potential impacts of the investigated exchange fluxes on local water quality. Hydraulic gradients along a piezometer transect were monitored at a river reach in a small catchment in South Korea, where the hydrologic dynamics are driven by the East-Asian Monsoon. We used heat as a tracer to constrain river-aquifer exchange fluxes in a two-dimensional flow and heat transport model implemented in the numerical code HydroGeoSphere, which was calibrated to the measured temperature and total head data. To elucidate potential effects of river-aquifer exchange dynamics on biogeochemical transformations at the river-aquifer interface, river water and groundwater samples were collected and analyzed for dissolved organic carbon (DOC), nitrate (NO3) and dissolved oxygen saturation (DOsat). Our results illustrate highly variable hydrologic conditions during the monsoon season characterized by temporal and spatial variability in river-aquifer exchange fluxes with frequent flow reversals (changes between gaining and losing conditions). Intense monsoonal precipitation events and the associated rapid changes in river stage are the dominant driver for the observed riverbed flow reversals. The chemical data suggest that the flow reversals, when river water high in DOC is pushed into the nitrate-rich groundwater below the stream and subsequently returns to the stream may facilitate and enhance the natural attenuation of nitrate in the shallow groundwater.

River-Induced Flow Dynamics in Long-Screen Wells and Impact on Aqueous Samples

Previously published field investigations and modeling studies have demonstrated the potential for sample bias associated with vertical wellbore flow in conventional monitoring wells constructed with long-screened intervals. This article builds on the existing body of literature by (1) demonstrating the utility of continuous (i.e., hourly measurements for ∼1 month) ambient wellbore flow monitoring and (2) presenting results from a field experiment where relatively large wellbore flows (up to 4 L/min) were induced by aquifer hydrodynamics associated with a fluctuating river boundary located approximately 250 m from the test well. The observed vertical wellbore flows were strongly correlated with fluctuations in river stage, alternating between upward and downward flow throughout the monitoring period in response to changes in river stage. Continuous monitoring of ambient wellbore flows using an electromagnetic borehole flowmeter allowed these effects to be evaluated in concert with continuously monitored river-stage elevations (hourly) and aqueous uranium concentrations (daily) in a long-screen well and an adjacent multilevel well cluster. This study demonstrates that when contaminant concentrations within the aquifer vary significantly over the depth interval interrogated, river-induced vertical wellbore flow can result in variations in measured concentration that nearly encompass the full range of variation in aquifer contaminant concentration with depth.

Variations in saturated and unsaturated water movement through an upland floodplain wetland, mid-Wales, UK

Summary Variations in inferred water movement are examined during a two-year study of an upland floodplain wetland at Afon Llwyd, in mid-Wales, UK in 2001 and 2002. Soil–water pressures ( ψ ) were monitored by six tensiometer nests, each comprising two sensors at depths of 30 and 60 cm below the surface. Detailed sedimentary sections were produced and changes in saturated and unsaturated hydraulic conductivity (and their relationship to ψ ) were estimated. Seasonal trends in ψ were identified and vertical hydraulic gradients within different tensiometer nests were derived and interpreted in the context of the local channel and floodplain sedimentology, channel bedform morphology and hydro-climatic conditions. Although there is considerable variation in ψ , the results display characteristic trends at several levels: (1) in relationship to precipitation; (2) reflecting changes in river stage (with downstream stage providing base-level control through a riffle-pool couplet); and (3) marked spatial variations in the direction and rate of water movement. These controls influence water redistribution through the wetland, which reflects the relative position of distinct sedimentary units within the floodplain. The results demonstrate that water movement is episodic and characterised by relatively rapid horizontal water fluxes during and immediately following individual rain events, followed by residual seepage to the river and water movement under a local soil moisture gradient. The implications of these results include elucidating the role of upland floodplains in buffering sub-surface drainage, and attenuating patterns of river flow under base flow conditions. This has the potential to advance the basis for future upland floodplain wetland and channel restoration schemes.

Solute dynamics during bank storage flows and implications for chemical base flow separation

Chemical base flow separation is a widely applied technique in which contributions of groundwater and surface runoff to streamflow are estimated based on the chemical composition of stream water and the two end‐members. This method relies on the assumption that the groundwater end‐member can be accurately defined and remains constant. We simulate solute transport within the aquifer during and after single and multiple river flow events, to show that (1) water adjacent to the river will have a concentration intermediate between that of the river and that of regional groundwater and (2) the concentration of groundwater discharge will approach that of regional groundwater after a flow event but may take many months or years before it reaches it. In applying chemical base flow separation, if the concentration in the river prior to a flow event is used to represent the pre‐event or groundwater end‐member, then the groundwater contribution to streamflow will be overestimated. Alternatively, if the concentration of regional groundwater a sufficient distance from the river is used, then the pre‐event contribution to streamflow will be underestimated. Changes in concentration of groundwater discharge following changes in river stage predicted by a simple model of stream‐aquifer flows show remarkable similarity to changes in river chemistry measured over a 9 month period in the Cockburn River, southeast Australia. If the regional groundwater value was used as the groundwater end‐member, chemical base flow separation techniques would attribute 8% of streamflow to groundwater, as opposed to 25% if the maximum stream flow value was used.

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.

Effects of fluctuating river flow on groundwater/surface water mixing in the hyporheic zone of a regulated, large cobble bed river

Physicochemical relationships in the boundary zone between groundwater and surface water (i.e. the hyporheic zone) are controlled by surface water hydrology and the hydrogeologic properties of the riverbed. We studied how sediment permeability and river discharge altered the vertical hydraulic gradient (VHG) and water quality of the hyporheic zone within the Hanford Reach of the Columbia River. The Columbia River at Hanford is a large, cobble-bed river where water level fluctuates up to 2 m daily because of hydropower generation. Concomitant with river stage recordings, continuous readings were made of water temperature, specific conductance, dissolved oxygen and water level of the hyporheic zone. The water level data were used to calculate VHG between the river and hyporheic zone. Sediment permeability was estimated using slug tests conducted in piezometers installed into the river bed. The response of water quality measurements and VHG to surface water fluctuations varied widely among study sites, ranging from no apparent response to covariance with river discharge. At some sites, a hysteretic relationship between river discharge and VHG was indicated by a time lag in the response of VHG to changes in river stage. The magnitude, rate of change and hysteresis of the VHG response varied the most at the least permeable location (hydraulic conductivity (K) = 2.9 × 10−4 cms−1) and the least at the most permeable location (K = 8.0 × 10−3 cms−1). Our study provides empirical evidence that sediment properties and river discharge both control the water quality of the hyporheic zone. Regulated rivers, like the Columbia River at Hanford, that undergo large, frequent discharge fluctuations represent an ideal environment in which to study hydrogeologic processes over relatively short time periods (i.e. days to weeks) that would require much longer periods (i.e. months to years) to evaluate in unregulated systems. Copyright © 2006 John Wiley & Sons Ltd.

Riverbank Filtration: Induced Infiltration and Groundwater Quality

Riverbank filtration relies on the streambed and aquifer matrixes to reduce pathogens under varying conditions of induced infiltration resulting from changes in river stage and flow velocity. The Greater Cincinnati (Ohio) Water Works monitored hydrologic parameters and water quality at its Charles M. Bolton well field during a comprehensive flowpath study. The study determined the frequency of occurrence of high river–stage events from historical data and monitored hydrologic parameters to estimate the potential unit infiltration rate. Giardia, Cryptosporidium, algae, spores, particle counts, and turbidity were also monitored. The project investigated potential pathogen/surrogate breakthrough during several high river–stage and infiltration events. High‐stage events occurred less than 4% of the time. Giardia and Cryptosporidium were not detected in any groundwater samples. Increases in surrogate concentrations were minimal and maintained >3.5‐log reduction. The streambed and aquifer have the ability to buffer water quality effects from major increases in the infiltration rate.

Development and application of an analytical model of stream/aquifer interaction

A mathematical model to simulate stream/aquifer interactions in an unconfined aquifer subjected to time varying river stage was developed from the linearized Boussinesq equation using the principle of superposition and the concept of semigroups. The mathematical model requires an estimate of three parameters to simulate ground-water elevations; transmissivity, specific yield, and recharge. The solution has physical significance and includes terms for the steady-state water level, the steady-state water level as influenced by a change in river stage, a transient redistribution of water levels in the aquifer from the previous day, and a transient change in water level caused by a change in river stage. The mathematical model was tested using observed water table elevations at three locations across a 2-km-wide alluvial valley aquifer. The average absolute deviation between observed and simulated daily water levels was 0.09 m. The difference in river stage over the test year was 4.9 m.

Assessment of River-Floodplain Aquifer Interactions

Research Article| January 01, 1997 Assessment of River-Floodplain Aquifer Interactions ANDREW S. ALDEN; ANDREW S. ALDEN Project Engineer K. W. Brown Environmental Services, College Station, TX 77845 Search for other works by this author on: GSW Google Scholar CLYDE L. MUNSTER CLYDE L. MUNSTER Assistant Professor Agricultural Engineering Department, Texas A&M University, College Station, TX 77843-2117 Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (1997) III (4): 537–548. https://doi.org/10.2113/gseegeosci.III.4.537 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation ANDREW S. ALDEN, CLYDE L. MUNSTER; Assessment of River-Floodplain Aquifer Interactions. Environmental & Engineering Geoscience 1997;; III (4): 537–548. doi: https://doi.org/10.2113/gseegeosci.III.4.537 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract The interaction between the Brazos River and the adjacent floodplain aquifer was studied for 200 days in 1995 at a ground-water research site near College Station, Texas. Two In Situ Permeable Flow Sensors (ISPFS) and a grid of well nests were used to correlate river stage to the magnitude and direction of ground-water flow at depths of 13.7 m and 18.3 m in the unconfined alluvial aquifer. Linear relationships between ground-water flow and river stage were determined at each depth. The floodplain aquifer responded differently to changes in river stage at the 13.7 m and 18.3 m depths. The horizontal velocity, parallel and perpendicular to the river, decreased with increasing river stage and increased with decreasing river stage, at both depths. However, the rates of change varied between the two depths. This caused the magnitude and direction of ground-water flow to be different at the two depths. The upward vertical velocity increased with increasing river stage at the 13.7 m depth and decreased with increasing river stage at the 18.3 m depth. At the 13.7 m depth, vertical ground-water flow gradually changed from upward to downward flow with long term river stage decline. Downward groundwater flow was not observed at the 18.3 m depth. Assessment of river-aquifer interactions indicates that a direct and measurable relationship exists between river stage and ground-water flow components at the site. The magnitude and direction of ground-water flow in the alluvial floodplain aquifer may be predicted if river stage is known. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Hydrologic and Geochemical Effects on Oxygen Uptake in Bottom Sediments of an Effluent‐Dominated River

More than 95% of the water in the South Platte River downstream from the largest wastewater treatment plant serving the metropolitan Denver, Colorado, area consists of treated effluent during some periods of low flow. Fluctuations in effluent‐discharge rates caused daily changes in river stage that promoted exchange of water between the river and bottom sediments. Groundwater discharge measurements indicated fluxes of water across the sediment‐water interface as high as 18 m3 s−1 km−1. Laboratory experiments indicated that downward movement of surface water through bottom sediments at velocities comparable to those measured in the field (median rate ≈0.005 cm s−1) substantially increased dissolved oxygen uptake rates in bottom sediments (maximum rate 212 ± 10 μmol O2 L−1 h−1) compared with rates obtained when no vertical advective flux was generated (maximum rate 25 ± 8.8 μmol O2 L−1 h−1). Additions of dissolved ammonium to surface waters generally increased dissolved oxygen uptake rates relative to rates measured in experiments without ammonium. However, the magnitude of the advective flux through bottom sediments had a greater effect on dissolved oxygen uptake rates than did the availability of ammonium. Results from this study indicated that efforts to improve dissolved oxygen dynamics in effluent‐dominated rivers might include stabilizing daily fluctuations in river stage.

Determination of inundation area in the Amazon River floodplain using the SMMR 37 GHz polarization difference

The 37 GHz polarization difference observed by the Scanning Multichannel Microwave Radiometer (SMMR), which was operated on board the Nimbus-7 satellite, provides a sensitive indicator of surface water. These data can provide information on seasonal inundation patterns in large tropical wetlands such as the Amazon River floodplain. Although the SMMR data are of low resolution, we were able to estimate the area inundated within a group of pixels by using linear mixing models which incorporate the major end-members of the observed microwave signatures. The models were then used to estimate seasonal changes in inundation area over a 7-year period for a 34,550 km 2 area along the Amazon River near Manaus. The seasonal changes in inundation area determined using mixing models correlate well with changes in river stage.

Paddlefish Movements and Habitat Use in Pool 13 of the Upper Mississippi River during Abnormally Low River Stages and Discharges

Abstract We determined the movements and habitat use by adult paddlefish Polyodon spathula during unusually low water levels in Pool 13 of the upper Mississippi River. Thirty-two large fish (6.3–25.4-kg) implanted with radio transmitters were located an aggregate of 812 times during March-August 1988, and spring 1989. No relation could be discovered between changes in river stage or discharge and direction of movement. No tagged paddlefish moved upstream from Pool 13, but during 1988 six fish moved downstream into Pool 14. Rates of movement were not significantly different between sexes, but the linear range for females was twice that of males. The greatest linear distance a paddlefish moved was 92 km downstream, and the greatest cumulative movement – entirely within Pool 13 – was 435 km; both records were set by females. Nearly three-fourths of all contacts with paddlefish occurred in about 5% of available habitat in Pool 13. Paddlefish were located most frequently at the head of Pool 13 in the tailwater...

Benthic Macroinvertebrates of Selected Habitats of the Lower Mississippi River

ABSTRACT We investigated the distribution of benthic macroinvertebrates in four aquatic habitat types in the lower Mississippi River: dike fields, a natural bank, a secondary channel, and an abandoned channel over a high flow (flood stage), two moderate, and two low flow periods. The biotas present in the natural bank, the secondary channel, and the abandoned channel showed only minor changes in composition over the various flow regimes. The natural bank was consistently dominated by the burrowing mayflies Tortopus incertus and Pentagenia vittigera and hydropsychid caddisflies; the consistently most common taxa in the secondary channel were the sand-dwelling chironomids Robackia claviger and Chernovskiia orbicus; phantom midges, tubificid oligochaetes, and fingernail clams were always the most abundant macroinvertebrates in the abandoned channel. The dike fields, however, showed large changes in biotic composition over the different flow regimes. These compositional changes correlated with changes in river stage and resultant alterations in current and substrate. This study indicates that the distribution of macroinvertebrates in the lower Mississippi River is a function of the physical characteristics of the system, notably current velocity and substrate composition.