Gravel-bed Streams Research Articles

Start and end of bedload transport in gravel-bed streams

[1] The threshold of incipient bedload motion, expressed either as a critical force or as a critical water discharge, is a key parameter in bedload transport prediction. Measuring the threshold of motion is difficult, and reliable data from natural streams are rare. By recording the vibrations triggered by bedload particles when moving over a steel plate mounted in the channel bed, we determined the time at start and end of bedload transport in four streams, where discharge is continuously monitored. The threshold discharge scatters over approximately one order of magnitude for each stream, reinforcing previous observations that critical discharge is characterized by a distribution of values. We interpret a strong correlation between the discharge at the start of transport and the discharge at the end of transport of the previous event to reflect temporal changes in bed structure and consequent effects on the driving and resisting forces acting on the bed.

갑천의 웅덩이-여울 연속구조에서 하상토의 입도 및 이동 특성

Natural streams meander, forming pools at the outer part of bend and riffles at the crossing. Pools are deep at a lower flow velocity, and riffles are shallow at a higher flow velocity. Attentions are being paid to pool-riffle sequences in meandering streams because pool-riffle sequences tend to increase biodiversity of the stream ecosystem. This study investigates the characteristics of distribution of bed sediment particles in the upstream reach of the Gap Stream, which is a tributary of the Geum River in Korea. The upstream part of the Gap Stream, the study reach, is a gravel-bed stream, showing a pool and three riffles due to meandering. The reach also includes pools at the upstream and downstream parts of the weir. The characteristics of bed sediment particles sampled at the wetland and in the side channel are studied, revealing that the median particle diameter in the riffle is about four times larger than that in the pool. In addition, flow simulations are carried out for ordinary discharge and design flood, and such parameters important to sediment transport as velocity, shear stress, dimensionless shear stress (or Shields number), and dimensionless shear velocity are provided to see the mobility of sediment particles in the pool-riffle sequence.

Coarse sediment movement in the vicinity of a logjam in a neotropical gravel-bed stream

Wood influences channel morphology by increasing channel roughness, altering flow hydraulics, and decreasing sediment transport in most fluvial systems where it is present. In many locations, the collection of wood into jams leads to even greater interruption of sediment transport by means of sediment impoundment and the creation of low-velocity backwater flow zones. We examined tracer clast movement near a wood jam in a neotropical gravel-bed stream in order to evaluate the effect of wood mobility and piece transience on the ability of the jam to impede sediment transport. We found that the jam had minimal influence on the passage of any individual clast, with tracer clast mobilization and travel distance being best predicted by flow magnitude and grain size rather than location relative to the jam. The rate of clast passage through the jam did not correlate with either the piece retention rate of the jam or with the density of the jam. The jam did, however, temporarily trap a sediment wedge during a period when fluvially imported wood pieces blocked flow under a key wood piece, diverting flow to a secondary flow path. These results suggest that there is an upper bound on the flows in which wood is able to disrupt sediment transport, and unlike for bedrock roughness elements, this bound is frequently crossed at our study site.

Bedload Transport and Morphological Effects of High-Magnitude Floods in Small Headwater Streams - Moravskoslezské Beskydy Mts. (Czech Republic)

Bedload transport observed during a flood in May 2010 gave rise to several forms of accumulations in small headwater basins located in the Western Flysch Carpathian Mountains, Czech Republic. We have investigated critical conditions of incipient motion of the largest boulders deposited during a c. Q100 flood event (flood competence method). We have tested several formulas designed for high gradient streams in two small basins in the conditions of local mid-mountain relief. The results show that a flood of such a magnitude is able to transport almost all surface bed material and that bedload transport in steep headwater streams (A ≤ 1 km 2 ) is probably less selective as for the grain size than that in lower gradient gravel-bed streams. The authors discuss the importance of local basin predispostion factors in order to determine critical conditions for the onset of bedload transport.

Sediment mobility in a forced riffle-pool

Forced riffle-pools occur in gravel-bed rivers where a large nonalluvial element leads to local scour and deposition. To manage rivers where this type of morphology is found and to specify restoration measures that mimic this process, more field data on sediment mobility in forced riffle-pools is needed. The objectives of this study are to describe the spatial variability of sediment mobility and deposition in a developing forced riffle-pool and to use high-resolution flow velocity measurements to explain the observed dynamics. The field site is a forced riffle-pool in Moras Creek, a 6-m-wide gravel-bed stream with a 1.2% bed slope in Quebec, Canada. Topography and the movement of sediment particles equipped with Passive Integrated Transponder (PIT) tags are surveyed through a series of competent floods. Bed shear stress is estimated from near-bed measurements of time-averaged velocity and turbulent kinetic energy. The sediment transport regime of the creek is characterized by partial mobility, exponential distributions of path lengths, and a negative relation between particle size and path length. Full mobility occurs in the center of the pool and over the exit slope where flow is accelerated from the constriction of flow during two events above the bankfull discharge. Partial mobility occurs during the same events over the entrance slope to the pool. Lateral gradients of deposition and mobility suggest that the majority of sediment in motion is routed over the side bar at the entrance to the pool. High levels of turbulence intensity that occur as a result of flow deceleration may explain the removal of finer sediments from the head of the pool.

Quasi‐equilibrium conditions of urban gravel‐bed stream channels in southern Ontario, Canada

ABSTRACTUrban gravel‐bed stream channels in southern Ontario, Canada identified to be in a state of quasi‐equilibrium have been studied over the past 15 years and compared against rural gravel bed stream channels of the same hydrophysiographic region. Bankfull width and depth versus bankfull discharge were not found to increase as a function of increasing urbanization as has been found in many other studies. The observed annual frequency of bankfull discharge was typically less than a 1‐year return period with many sites ranging between 2 and 18 bankfull events per year with higher intensity and shorter duration urban flood responses, which further identified significant limitations in using annual peak discharge methods for predicting morphological forming flows in urban watersheds.The cumulative volume of bankfull and larger flood events from the urban stream channels were very similar to the same annual event volumes in the rural comparison study reaches. Bed material supply was found to decrease with increasing urbanization and the reduction in bed material supply appears to be off‐set by the smaller bankfull channel width, depth and access to floodplains during large flood events. Field evidence may also suggest a even greater reduction in channel width trajectory, relative to the rural setting, with expansive floodplains to maintain quasi‐equilibrium conditions as bed material supply continues to decrease with increased anthropogenic activity. Copyright © 2010 John Wiley & Sons, Ltd.

The effects of plant growth on stream invertebrate communities during low flow: a conceptual model

Few studies have examined the effects of low flows on stream biota—a paradoxical situation considering the mounting pressure on streams as a result of increased human abstraction. We present a conceptual model describing processes operating in rivers during low flows and argue that such processes reflect both direct effects of reduced flow on benthic invertebrates and indirect effects of enhanced plant growth occurring during such periods. Our model suggests that the longer the duration of low flow, the more the plant community will change and, in turn, the more habitat quality will change, with consequences to benthic invertebrate communities and higher trophic levels. A fundamental part of the model is the recognition of different stream types, based on a habitat-template matrix of resource supply, interflood velocity, and substrate stability during flood events. This habitat template results in 12 stream types, each of which supports specific plant communities that set the antecedent plant conditions prior to a low-flow period. During low flows, hydraulic factors interact with the antecedent plant communities, which undergo specific responses. For example, in low-nutrient streams with gravel substrates and dominated by diatoms, low-flow conditions will cause little or no change to the antecedent plant community because this plant community is structured by top-down grazing pressure. Consequently, benthic invertebrate composition will not change. In contrast, cover and biomass of filamentous green algae in higher-nutrient gravel-bed streams will increase during low-flow events, and this increase will affect habitat suitability for invertebrates over and above those caused solely by hydraulic changes. The greatest changes to invertebrate communities are expected in macrophyte-dominated streams because these plants change instream habitat conditions the most during extended periods of low flow. Therefore, current flow-management techniques must consider the type of plant communities that exist prior to low flows and be cognizant of that fact that these plant communities can have dramatic influences on benthic communities during low-flow periods.

Laboratory Measurements on Turbulent Pressure Fluctuations in and above Gravel Beds

The statistics of pressure fluctuations above and within three types of porous granular beds such as in gravel bed streams, rivers, and man-made canals are investigated by data gained via laboratory flume experiments. The flow conditions examined include a diversity of hydrodynamic loads that increase up to the point where single grains are moving from time to time, without causing severe modification to the bed texture and the related positions of the pressure sensors. Analysis is performed by means of histograms and spectral techniques and vertical intensity profiles. Two simplified equations are found that describe the vertical decrease for the standard deviation of the measured fluctuations indicating drag and lift, respectively, nondimensionalized by the mean bed shear stress. The former fluctuation is described by a crude linear fit, whereas the latter clearly shows that the lift intensity decreases exponentially in the porous bed with a decay distance of one to two times the equivalent grain roughness. Within the subsurface layer the standard deviation reaches a nonzero constant, mainly dominated by long-wave pressure fields that are convected in the outer flow. These findings can be used in future sediment transport models that use force balance approaches to determine incipient motion conditions.

Preferential Flow Effects on Subsurface Contaminant Transport in Alluvial Floodplains

For sorbing contaminants, transport from upland areas to surface water systems is typically considered to be due to surface runoff, with negligible input from subsurface transport assumed. However, certain conditions can lead to an environment where subsurface transport to streams may be significant. The Ozark region, including parts of Oklahoma, Arkansas, and Missouri, is one such environment, characterized by cherty, gravelly soils and gravel bed streams. Previous research identified a preferential flow path (PFP) at an Ozark floodplain along the Barren Fork Creek in northeastern Oklahoma and demonstrated that even a sorbing contaminant, i.e., phosphorus, can be transported in significant quantities through the subsurface. The objective of this research was to investigate the connectivity and floodplain-scale impact of subsurface physical heterogeneity (i.e., PFPs) on contaminant transport in alluvial floodplains in the Ozarks. This research also evaluated a hypothesis that alluvial groundwater acts as a transient storage zone, providing a contaminant sink during high stream flow and a contaminant source during stream baseflow. The floodplain and PFP were mapped with two electrical resistivity imaging techniques. Low-resistivity features (i.e., less than 200 O-m) corresponded to topographical depressions on the floodplain surface, which were hypothesized to be relict stream channels with fine sediment (i.e., sand, silt, and clay) and gravel deposits. The mapped PFP, approximately 2 m in depth and 5 to 10 m wide, was a buried gravel bar with electrical resistivity in the range of 1000 to 5000 O-m. To investigate the PFP, stream, and groundwater dynamics, a constant-head trench test was installed with a conservative tracer (Rhodamine WT) injected into the PFP at approximately 85 mg/L for 1.5 h. Observation wells were installed along the PFP and throughout the floodplain. Water table elevations were recorded real-time using water level loggers, and water samples were collected throughout the experiment. Results of the experiment demonstrated that stream/aquifer interaction was spatially non-uniform due to floodplain-scale heterogeneity. Transport mechanisms included preferential movement of Rhodamine WT along the PFP, infiltration of Rhodamine WT into the alluvial groundwater system, and then transport in the alluvial system as influenced by the floodplain-scale stream/aquifer dynamics. The electrical resistivity data assisted in predicting the movement of the tracer in the direction of the mapped preferential flow pathway. Spatially variable PFPs, even in the coarse gravel subsoils, affected water level gradients and the distribution of tracer into the shallow groundwater system.

河川流出と融雪流出過程の調査法

Streamflow can be only measured as a spatially integrated value among the basin-wide water balance components. As the measurement of high-water flow is important for the river planning and the flood control measures in large rivers in Japan, new studies are proceeding with the improving the accuracy of streamflow measurement such as ADCP and PIV techniques. Meanwhile, during the low-flow period or in the small river, conventional gauging methods are used, including the current meter method and the weir. Dilution gauging method is effective particularly in the mountainous gravel-bed streams. However, reported examples are not so much. Understanding to the role of snow within the snowmelt runoff process is essential in the snowy cold region. Snow lysimeter and snow pillow are useful in these studies. Preferential flow through the snowpack and the dependence of percolation speed on the snow properties are studied using those equipments. Most difficult problem is how to estimate the basin averaged snowmelt amount. Additional researches are needed which combines conventional snow melting factor method and satellite imagery data.

Operator bias characterizing stream substrates using Wolman pebble counts with a standard measurement template

This study characterizes the variability associated with using a standard measurement template to perform Wolman pebble counts of coarse stream substrates. Four 10-count and two 12-count replicate experiments were conducted by five operators at three sample sites within gravel-bed streams using a gravel template. Results show significant differences between different operators sampling the same location, as observed in previous studies conducted without a standard sampling template, as well as significant variability within each operator's replicate samples. Variability was highest in clast estimates larger than D50 suggesting that these metrics cannot be consistently derived from Wolman pebble count samples. Variability was also highest in the low order study site suggesting that the sedimentary characteristics of low order streams increase the chance for operator bias. In light of our findings, we suggest that traditional 100-particle Wolman counts be utilized with more caution than is the current norm.

The role of hydrologic processes and geomorphology on the morphology and evolution of sediment clusters in gravel-bed rivers

The effect of geomorphic features and hydraulic conditions on the formation, evolution, and morphology of sediment cluster microforms in an unregulated gravel-bed stream were investigated at field sites on the Entiat River, which drains the eastern slopes of the Cascade Mountains in central Washington state, USA. Sediment clusters were marked, described, and photographed before and after a series of moderate to high discharges over an 18-month period to quantify the evolution of the cluster morphologies. Individual sediment particles were tracked to calculate the range of flows and bed shear stresses that maintain and destroy clusters. Examination of geomorphic settings, channel morphology, and particle size distributions documented the conditions that favor cluster formation. The investigation tested the hypotheses that clusters in this environment delay sediment entrainment and that their morphology and evolution follow a predictable evolution similar to that found in laboratory studies. Clusters formed on gravel bars adjacent to riffles with slopes ≥ 1%, poor to moderate sorting, and mean and maximum particle size values 1.5 times greater than those associated with noncluster bars. Clusters were more stable under the bimodal sediment size distribution contributed by a tributary alluvial fan and were destroyed with and without anchor clast mobilization at sites where sediment size was more uniform. The six cluster morphologies identified in the field were similar to those in flume studies, but did not follow the same evolutionary cycle over multiple flow events. This contrast was attributed to the flow events resetting the cluster cycle, leading to a high percentage of upstream triangles. The dimensionless critical shear stress required to entrain the mean grain size ranged from 0.06 to 0.08. The hydraulic thresholds and geomorphic features that result in stable vs. mobilized cluster microforms in this setting can serve as a model for regulated rivers and restoration projects aimed at sustaining instream flows to maintain natural sediment transport conditions.

Special Issue on “Transport Phenomena at the Interface Between Fluid and Porous Domains”

Free flow above permeable domains is a common problem in both natural and industrial environments. The classical problem of turbulent flows above forests and urban areas, known as the canopy flow problem (Finnigan 2000), was addressed by hundreds of publications. A library search for the keywords ‘canopy and flow’ in the ISI database returned 1312 results in April 2007 and 1845 results in April 2009, about 40% increase in the last 2years alone. In fact, this search underestimates the actual number of publications since research of flow above permeable domains covers a much wider range of scenarios. Vegetated flows in rivers and wetlands, flow above gravel streambeds, and flow above coral reefs are a few environmental examples; industrial examples include flows in coating processes, chemical reactors, and electronic cooling; and examples of a much smaller length scale include flows above bio-films and inside bio-tissues. Studies of flows over permeable domains include both laminar and turbulent flow regimes. Where most of the porous media studies focus on laminar and creeping flows, terrestrial, ocean, and atmospheric flows are all turbulent. Combinations such as turbulent channel flows above laminar flow inside dense gravel beds are also common. Most of the past and present research activities are often classified by the following four disciplines: porous media flows (e.g., Whitaker 1999), atmospheric canopy flows (e.g., Finnigan 2000), vegetated flow (e.g., Nepf 1999), and fluvial flows. Studies of fluvial flows were recently addressed in a special issue on double averaging applications (Nikora and Rowinski 2008). The following special issue with its 13 publications covers both laminar and turbulent flow regimes while emphasizing on fundamental questions of formulating the transport conditions at the interface between the fluid and the porous domains. The most famous publication on laminar flows above porous interfaces is the study by Beavers and Joseph (1967) (cited 771 times, ISI, April 2009). By investigating the flow above porous domains, they developed a linear relationship between the velocity gradient above the interface and the interface slip velocity. However, numerous studies showed that the Beavers and Joseph condition is not general enough and a coupled solution of both the free flow and

Review of Sedimentation Engineering—Processes, Measurements, Modeling, and Practice (ASCE Manuals and Reports on Engineering Practice No. 110) by Marcelo H. GarciaSedimentation Engineering—Processes, Measurements, Modeling, and Practice (ASCE Manuals and Reports on Engineering Practice No. 110)ASCE9780784408148$138.75

Sedimentation engineering is a very important subject that deals with sedimentation processes such as erosion, entrainment, transport, deposition, and compaction of sediment induced by water, wind, gravity, and ice and affected by human activities. The original ASCE Manual 54 Sedimentation Engineering, edited by Vito A. Vanoni 1975 and the ASCE Task Committee for the Preparation of a Manual on Sedimentation assembled state-ofthe-art information and knowledge on sedimentation engineering available at that time. Since then, the scientific and engineering understanding and knowledge of sedimentation processes have been greatly advanced and expanded. The recently published Manual 110, edited by Marcelo H. Garcia and his task team, updates selected topics in the original manual and presents recent advances and new topics in the field of sedimentation science and engineering as a complement to the original Manual 54. This update is timely and makes significant contributions to the literature. This comprehensive volume consists of 23 chapters and 6 appendices. Among them, Chapters 2–4 broadly cover fundamentals of sediment transport mechanics, and a section of Chapter 19 briefly presents the mechanics of hyperconcentrated sediment flows. The original Manual 54 gives a good understanding of sedimentation processes and well defines the scope of sediment transport mechanics, whereas the new volume presents in considerable detail many recently developed theories and methods for non-cohesive sediment transport in sand-bedded streams Chapter 2 and gravel-bedded streams Chapter 3 , as well as for cohesive sediment transport Chapter 4 . Chapter 2 also introduces turbidity currents, while Chapter 3 also presents the transport of multiple-sized nonuniform sediment mixtures, which has been extensively studied in the last three decades. Chapter 4 explains the effects of waves on cohesive sediment transport, but such effects on noncohesive sediment transport are omitted in this manual. Chapter 5 reviews technologies for measuring bed-material properties and suspended and bed-load discharges, while Appendix D describes methods for estimating sediment discharges in streams using direct measurement data and empirical relations developed between hydraulic parameters and sediment transport potential. Several new types of samplers are introduced, as well as some traditional ones developed in the mid-twentieth century but modified recently. The measurement of bed load

Changes in hyporheic exchange flow following experimental wood removal in a small, low‐gradient stream

We investigated the response of hyporheic exchange flow (HEF) to wood removal in a small, low‐gradient, gravel bed stream in southeast Alaska using a series of groundwater models built to simulate HEF for the initial conditions immediately after wood removal and 1 month, 2 years, 4 years, and 16 years following wood removal. The models were based on topographic surveys of the stream channel and surrounding floodplain, and surveyed water surface elevations (WSEs) were used to assign stream boundary conditions. Using the groundwater flow model, MODFLOW, and the particle tracking model, MODPATH, we calculated hyporheic exchange fluxes, their residence time distributions, and both longitudinal and plan view spatial patterns of downwelling and upwelling zones. In the first few years, streambed scour and sediment deposition smoothed the streambed and WSE profile, reducing HEF. Also, large contiguous patches of downwelling or upwelling were fragmented, nearly doubling the total number of patches present on the streambed. As the stream continued to adjust to the loss of wood, those trends began to reverse. Accretion of sediment onto alternating bars resulted in better developed pool‐riffle morphology, enhanced HEF, and increased residence times and also resulted in downwelling and upwelling zones coalescing into elongated patches along bar margins. This study showed that the hyporheic zone is sensitive to changes in wood loading and that initial changes in HEF resulting from the direct effects of wood removal were contrary to longer‐term channel adjustments to changes in wood loading.

Evaluation of a gravel transport sensor for bed load measurements in natural flows

A recent acoustic instrument (Gravel Transport Sensor, GTS) was tested for predicting sediment transport rate (bed load rate) in gravel bed streams. The GTS operation is based on the particle collision theory of submerged obstacles in fluids. When particles collide with the GTS cylinder their momentum is recorded in the form of ping rates. The GTS is attractive for further consideration here because of its potential to provide continuous unattended local bed load measurements, especially in areas found in streams that access may be difficult under extreme conditions. Laboratory experiments coupled with numerical simulations for the same flow conditions were performed in order to determine the conditions under which particles of different size will hit the GTS cylinder and be able to register a ping rate. The GTS was able to detect the number of particles with diameter in the range of 15.9 to 25.4 mm, with reasonable accuracy, if the applied Shields effective stress τ *e = τ * − τ *cr was in the range of 0.006 to 0.015. A drawback of the tested prototype GTS, however, was that it exerted increased resistance on the incoming particles. The added drag effects increased the overall resistance that was exerted by the flow on particles and thus increased the likelihood that particles will rest in the ambient region of the cylinder instead of hitting it. Numerical simulation of the flow around the GTS cylinder revealed that changing the prototype geometry from cylindrical to ellipsoid or rhomboid will increase the likelihood of the particles hitting the instrument under the same flow conditions failed by the original tested GTS cylinder.

Channel stability in bed load–dominated streams with nonerodible banks: Inferences from experiments in a sinuous flume

Experimental and computational evidence shows that bed state adjustments can accommodate a twofold to fourfold change in sediment supply without otherwise affecting the morphology of a sinuous gravel bed stream channel with nonerodible banks. In contrast, previously published experiments of the same kind but with erodible channel banks suggest that changes in sediment supply are taken up almost exclusively by changes in channel sinuosity and hence the reach‐average water surface gradient. It appears that stream channel response to changes in sediment supply is strongly conditioned by the nature of the stream boundary, and that determines which of several potential adjustments are possible.

Assessing the effect of vegetation‐related bank strength on channel morphology and stability in gravel‐bed streams using numerical models

AbstractBank strength due to vegetation dominates the geometry of small stream channels, but has virtually no effect on the geometry of larger ones. The dependence of bank strength on channel scale affects the form of downstream hydraulic geometry relations and the meandering‐braiding threshold. It is also associated with a lateral migration threshold discharge, below which channels do not migrate appreciably across their floodplains. A rational regime model is used to explore these scale effects: it parameterizes vegetation‐related bank strength using a dimensionless effective cohesion, Cr*. The scale effects are explored primarily using an alluvial state space defined by the dimensionless formative discharge, Q*, and channel slope, S, which is analogous to the Q–S diagrams originally used to explore meandering‐braiding thresholds. The analyses show that the effect of vegetation on both downstream hydraulic geometry and the meandering‐braiding threshold is strongest for the smallest streams in a watershed, but that the effect disappears for Q* > 106. The analysis of the migration threshold suggests that the critical discharge ranges from about 5 m3/s to 50 m3/s, depending on the characteristic rooting depth for the vegetation. The analysis also suggests that, where fires frequently affect riparian forests, channels may alternate between laterally stable gravel plane‐bed channels and laterally active riffle‐pool channels. These channels likely do not exhibit the classic dynamic equilibrium associated with alluvial streams, but instead exhibit a cyclical morphologic evolution, oscillating between laterally stable and laterally unstable end‐members with a frequency determined by the forest fire recurrence interval. Copyright © 2008 John Wiley & Sons, Ltd.

Determination of Equivalent Roughness for Estimating Flow Resistance in Stabled Gravel-Bed River: I. Theory and Development of the Model

Flow resistance in a natural stream is caused by complex factors, such as the grains on the bed, vegetation, and bed-form, reach profile. Flow resistance in a generally stable gravel bed stream is due to protrudent grains from bed. Therefore, the flow resistance can be calculated by equivalent roughness in gravel bed stream, but estimation of equivalent roughness is difficult because nonuniform size and irregular arrangement of distributed grain on natural stream bed. In previous study, equivalent roughness is empirically estimated using characteristic grain size. However, application of empirical equation have uncertainty in stream that stream bed characteristic differs. In this study, we developed a model using an analytical method considering grain diameter distribution characteristics of grains on the bed and also taking into account flow resistance acting on each grain. Also, the model consider the protrusion height of grain.

The role of the hyporheic zone in the nitrogen dynamics of a semi‐arid gravel bed stream located downstream of a heavily polluted reservoir (Tafna wadi, Algeria)

AbstractNitrogen retention was measured along the Tafna wadi downstream of a heavily polluted reservoir in North‐West Algeria to understand the role of the hyporheic zone (HZ) in nitrogen dynamics. Nutrient concentrations were measured monthly for 2 years within the bed sediments of a 300 m reach located 20 km downstream from the dam. Due to strong hydrological fluctuations hyporheic water was analysed during natural low and high water (HW) periods, and during water reservoir releases. Nutrient concentrations in surface water (SW) increased during water releases and in the HZ during the low water (LW) periods. Surface/hyporheic water interactions were characterized by determining the vertical hydraulic gradient (VHG) and the chemical signature of the ground water (GW). The latter was obtained from regional GW monitoring. Hyporheic chemistry was strongly influenced by patterns of surface flow. Hyporheic and SWs had similar chloride concentrations during high flow when they were significantly lower than those of the regional GW. GW was generally richer in nitrates and nitrites, but was lower in ammonium concentrations than interstitial and river waters. Nitrates decreased significantly from upstream to downstream within the HZ throughout the hydrological period even though temporal fluctuations were high. Ammonium concentrations in interstitial water (IW) were significantly higher than in SW and generally increased from upstream to downstream. This study demonstrates the importance of the HZ in altering the dissolved inorganic nitrogen composition and concentrations of heavily polluted arid streams. The study is of interest because it documents a large ‘natural experiment’ that being the effect of periodic water release from a reservoir with serious water quality problems on the water quality dynamics (particularly nitrogen) of subsurface and SWs downstream. Copyright © 2008 John Wiley & Sons, Ltd.