Variable Hydrological Regimes Research Articles

Learning Dilemmas in a Social-Ecological System: An Agent-Based Modeling Exploration

The process of learning in social-ecological systems is an emerging area of research, but little attention has been given to how social and ecological interactions motivate or inhibit learning. This is highly relevant to the South African water sector, where a major policy transition is occurring that provides local water users and managers with new opportunities to engage in adaptive learning about how to balance human and ecological needs for water. In this paper, an agent-based model is used to explore potential 'learning dilemmas', or barriers to learning in the South African water sector, whereby human perceptions combined with social-ecological conditions affect the capacity, understanding, and willingness required to learn. Agents manage water according to different management strategies and use various indicators to evaluate their success. The model shows that in areas with highly variable hydrological regimes, agents may be less able to learn because conditions are too unpredictable for them to benefit from past experience. Because of these changing conditions, however, agents are more likely to try new water management strategies, promoting a greater diversity of experience in the system for agents to learn from in the future. In water-stressed areas, where agents tend to have greater difficulty fulfilling demand for water than in areas with abundant water supplies, they are also more apt to try new strategies. When learning is restricted to small areas, agents may learn more quickly but based on a more narrow range of experience than in larger or more heterogeneous areas. These results suggest a need to enhance learning so that it accounts for interacting hydrological, ecological, and social dynamics. Although the model is a highly stylised version of reality, this preliminary exploration may eventually help to reverse the past trend of poor understanding of social-ecological dynamics as they relate to water management.

Dynamics of suspended sediment borne persistent organic pollutants in a large regulated Mediterranean river (Ebro, NE Spain)

Mediterranean rivers are characterized by highly variable hydrological regimes that are strongly dependent on the seasonal rainfall. Sediment transport is closely related to the occurrence of flash-floods capable to deliver enough kinetic energy to mobilize the bed and channel sediments. Contaminants accumulated in the sediments are likely to be mobilized as well during such events. However, whereas there are many studies characterizing contaminants in steady sediments, those devoted to the transport dynamics of suspended-sediment borne pollution are lacking. Here we examined the occurrence and transport of persistent organic microcontaminants present in the circulating suspended sediments during a controlled flushing flow in the low part of the River Ebro (NE Spain) 12km downstream of a well-known contaminated hot-spot associated to a nearby chloro-alkali industry. Polycyclic aromatic hydrocarbons (PAHs) and semi-volatile organochlorine pollutants (DDT and related compounds, DDX; polychlorinated byphenils, PCBs; and other organochlorine compound, OCs) were measured in the particulate material by GC–MS and GC–MS/MS, using previously developed analytical methods. The concentration levels observed were compared to previously reported values in steady sediments in the same river and discussed on a regulatory perspective. Hydrographs and sedigraphs recorded showed a peak-flow of 1300m3s−1 and a corresponding peak of suspended sediments of 315mgL−1. Combination of flow discharge, suspended sediments and pollutants' concentrations data allowed for quantifying the mass flows (mass per unit of time) and setting the load budgets (weight amount) of the different pollutants transported by the river during the monitored event. Mean mass-flows and total load values found were 20.2mgs−1 (400g) for PAHs, 38mgs−1 (940g) for DDX, 44mgs−1 (1038g) for PCBs and 8mgs−1 (200g) for OCs. The dynamic pattern behavior of PAHs differs substantially to that of organochlorine pollutants, thus reflecting different pollution origins.

Early successional vegetation assembly in a spatially variable hydrologic regime

Mitigation wetlands are intended to replace the ecological conditions of wetlands lost to development or drainage, resulting in a theoretical expectation that the replacement wetland will support a species composition and diversity comparable to a reference standard. I analyze a freshwater mitigation wetland in central Ohio, USA to evaluate factors that influence spatial patterns of plant community assembly in terms of floristic quality and diversity. Soil seed bank, bulk density, organic content, and pH are evaluated, along with average depth, exposure time, depth variance, and spatial location within the wetland basins, for their relationship with plant community characteristics in the first three years after wetland creation. The plant community was highly variable during the study period, with general trends toward greater diversity and more hydrophilic vegetation. The spatial distribution of nonnative cover, planted cover, mean coefficient of conservatism, and diversity in the third-year plant community were unrelated to soil and seed bank metrics; however, all were strongly related to hydrologic metrics. Shallow depth (<0.2 m) and moderate depth variance were found to facilitate plant community diversity, while floristic quality is more likely in deep (>0.2 m) and relatively stable water levels (<60% exposure time). Nonnative vegetation cover was particularly high at shallow locations with high depth variance and long exposure times. Geographic analysis of floristic quality and diversity indicates a weak spatial correlation between the two, suggesting that these two measures of the plant community are unlikely to coincide in early succession. The implications are that site design and hydrologic manipulation can drive the early successional vegetation characteristics, and that site design, compliance criteria, and monitoring regimes should seek to concertedly facilitate species composition, community diversity, or both.

Comparability of ecological quality boundaries in the Mediterranean basin using freshwater benthic invertebrates. Statistical options and implications

Within the Mediterranean region each country has its own assessment method based on aquatic macroinvertebrates. However, independently of the classification system, quality assessments should be comparable across members of the European Commission, which means, among others, that the boundaries between classes should not deviate significantly. Here we check for comparability between High–Good and Good–Moderate classifications, through the use of a common metric. Additionally, we discuss the influence of the conceptual and statistical approaches used to calculate a common boundary within the Mediterranean countries participating in the Intercalibration Exercise (e.g., using individual national type-boundaries, one value for each common type or an average boundary by country; weighted average, median) in the overall outcome. All methods, except for the IBMWP (the Iberian BMWP) when applied to temporary rivers, were highly correlated (0.82<R<0.98) with the common metric STAR-ICMi. This index was shown to respond well to a variety of pressures (especially local habitat alterations, riparian vegetation degradation, nutrient enrichment and chemical contamination, land use and global degradation; p<0.00001) affecting Mediterranean streams. However, depending on the statistical approach used, a different number of class boundaries failed the acceptable deviation from the common boundary (a quarter of class width). That leads to a different percentage of national sites failing to achieve good status and ultimately to a different technical and economic effort in the recovery of streams to that condition. We argue that low/high boundaries might have different meanings not restricted to the quality (good/bad) of reference conditions but also to the natural spatial and temporal variability contained in national and intercalibration types. The highly variable hydrological regimes of Mediterranean streams may influence the reference condition and require further investigation.

Impacts of global warming perturbation on water resources in arid zone: Case study of Kaidu River Basin in Northwest China

The main goal of this study was to assess the long-term impacts of global warming perturbation on water resources of the Kaidu River Basin in Northwest China. Temperature, precipitation and hydrology data during the past 29 years from 1979 to 2007 were collected and analyzed using parametric and non-parametric methods, the connection between temperature and precipitation by the combination of grey correlation analysis method and the hypothesis testing for trend of climate change. The results show a high increase in temperature in the study area as well as an extreme and highly variable hydrological regime in this region, where flash floods can exceed the total runoff from a sequence of years. These variations may be due to the geographical location of the Kaidu River Basin in arid zone. It also reveals that precipitation has a much greater impact on stream flow than that of temperature. The development of new approaches was proposed as responses to climate change in this arid region.

Differential responses of abandoned wet grassland plant communities to reinstated cutting management

The nature of ecological stability is still debated, and there is a need to establish which types of communities show resistance to environmental change and to explore community responses in relation to their environmental context. This study aims to investigate the effects of reinstating cutting management on abandoned wet grasslands by comparing responses in two different communities with contrasting environmental conditions, to elucidate the restoration potential of wet grasslands. Two coastal wet grassland plant communities in Estonia were monitored over 5 years: a species-poor lower shore grassland and a more diverse tall grassland. Piezometers and soil samples were used to characterise the hydrology, while cutting effects and ongoing abandonment were compared using replicate quadrats in both grasslands. Annual changes and significant differences in community composition were analysed using Detrended and Canonical Correspondence Analyses, diversity indices, and inferential statistics. The results showed that cutting produced greater changes in composition and species abundance in the lower shore community compared to the tall grassland, including a greater proportion of significant differences. The increased responsiveness of the lower shore community may be related to its variable hydrological regime, especially flooding, which creates a dynamic environment favouring adaptable species. In contrast, the tall grassland featured a more stable water regime and species that responded less to perturbation, and manifested resistance to cutting management. Thus, restoring abandoned wet grasslands through vegetation management may be a slow process, especially where there is residual diversity, and the importance of hydrological regime in determining wet grassland communities should be considered.

An interannual assessment of the relationship between the stable carbon isotopic composition of ecosystem respiration and climate in a high-elevation subalpine forest

[1] We measured the carbon isotopic composition (δ13C) of ecosystem respiration (δ13CR) in a subalpine forest across four growing seasons to examine whether patterns in δ13CR were consistent with those expected based on leaf-level gas-exchange theory, and in agreement with past studies of the relation between δ13CR and climate conducted across broad geographic regions. Conventional trends (i.e., less negative δ13CR with increased vapor pressure deficit (VPD) and air temperature (TAIR), and decreased soil moisture (θ)) were observed when we focused on the driest portions of average-wetness years and when δ13CR was positively correlated with nighttime ecosystem respiration (RE). Nonconventional trends (i.e., more negative δ13CR with decreased θ, and increased VPD and TAIR) were observed under specific climatic conditions (e.g., late snowmelt; extreme TAIR late in the growing season), and when δ13CR was negatively correlated with RE. These nonconventional trends were independently corroborated using δ13C of extracted sugars from needles of dominant tree species at the site. Our results clearly demonstrate that the commonly reported relations between δ13CR and climate may break down depending on the interactions among environmental conditions. Efforts to model and predict the variability of δ13CR under changing climatic variables must characterize and parameterize the effects of unique combinations of weather conditions and variable hydrologic regimes, in combination with the susceptibility of photosynthetic isotope discrimination to extreme air temperatures.

Vadose zone controls on weathering intensity and depth: Observations from grussic saprolites

Abstract An investigation of vadose zone weathering processes has been undertaken on grussic saprolites developed on Californian granitoids. Preliminary results indicate strong climatic control, through infiltration, on the depth and intensity of weathering. At sites with higher infiltration, the vadose zone is comprehensively altered to grussic saprolite and saprock. Conversely, lower infiltration sites display only thin grussic saprolites, strongly influenced by rock texture. Both vadose zone and weathering depth appear to be governed by local base level, and vadose zone hydrology exerts a fundamental control on the effective operation and relative dominance of the key weathering reactions. In zones of matrix permeability, oxidation of biotite comprehensively disaggregates the rock but results in little mass loss and clay mineral formation. Conversely, the higher transient flow rates that characterize zones of fracture permeability result in plagioclase hydrolysis, significant mass losses and accompanying clay mineral formation. A variable hydrological regime may also contribute to high partial pressures of O 2 in vadose zone pore waters and pore spaces, thereby enhancing the oxidative environment and further predisposing grussic saprolite formation.

Hydrological and biogeochemical controls on plant species distribution within calcareous fens

AbstractThis study determines spatial patterns of plant species richness and diversity in three calcareous fens and quantifies individual species response and range/tolerance to hydrological, peat physical, and porewater nutrient gradients using nonlinear quantile regression. Species richness and diversity were highly variable across the sites, with no single species occupying more than 25% of the area surveyed. Variable hydrological regime was related to distance from the principal source area of water to the fens. Surface peat organic matter and calcium carbonate content ranged from 11 to 91% and 2 to 83%, respectively. Dissolved inorganic nitrogen (DIN) and potassium concentrations from the shallow porewater ranged from 0 to 4 mg l−1, whereas available phosphorus was &lt; 100 µg l−1. Species exhibited various response shapes to the environmental gradients, which produced segregated realized ranges. The duration of initial saturation of the peat from the start of the growing season and the peat organic matter content produced the greatest number of complete species responses and ranges. Aggregation of species across growth types based on environmental responses mirrored visually described communities, demonstrating the applicability of the method of analysis. The generation of species response curves for a suite of readily measured environmental variables can be a useful tool for assessing conceptual models of ecosystem functioning and wetland restoration/creation success. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Are alien fish disadvantaged by extremely variable flow regimes in arid-zone rivers?

The proliferation of alien fish in dryland rivers potentially obstructs the maintenance of river health. Modified flow regimes are hypothesised to facilitate invasions by alien fish but in unregulated dryland rivers, large floods provide a recruitment advantage for native over alien species whereas droughts favour alien species. We tested these hypotheses by using data from a 3-year study (2000–2003) of fish populations in the unmodified rivers of the Lake Eyre Basin (LEB) of Australia. Results from a lower reach of Cooper Creek were compared with those of an earlier study (1986–1992). During both periods, large floods occurred, with return periods ranging from &gt;1 in 5 to &gt;1 in 25 years. In the lower Cooper, decreases in the abundance of alien species relative to native species, and dramatic increases in recruitment of native species, were observed during a 1–3-year period following large floods. In two other rivers in 2000–2003, there was no statistically significant change in the already low abundances of alien species. We suggest that the naturally variable hydrological regimes and native-dominant fish assemblages of the unregulated LEB rivers afford some resistance to the establishment and proliferation of alien fish through flood and drought conditions.

Implications of biological and physical diversity for resilience and resistance patterns within Highly Dynamic River Systems

The structure and function of alluvial Highly Dynamic River Systems (HDRS) are driven by highly variable hydrological disturbance regimes, and alternate between resistant, metastable states and resilient, transitional states. These are in turn subject to influences of feedback loops within hydrogeomorphic and biological processes. Here we consider how resistance and resilience largely determine HDRS ecosystem trajectories and how these characteristics can be modified by natural and anthropogenic processes. We review the mechanisms by which biodiversity can affect both resistance and resilience and introduce a conceptual framework that incorporates some unique HDRS characteristics. We suggest that resilient and resistant patterns frequently coexist in the active tract of these river systems, and that this coexistance promotes the return of metastable states after major disturbances. In contrast, highly resistant and poorly resilient patterns dominate at their external boundaries. The loss of these natural dynamics resulting from direct and indirect human impacts causes deviations to resistance and resilience patterns and therefore to HDRS trajectory. We propose that understanding the role of interactions between biological and physical processes that control resistance and resilience is crucial for system restoration and management.

Xylem potential- and water content-breakpoints in two wetland forbs: indicators of drought resistance in emergent hydrophytes

Freshwater wetlands commonly occur along the boundaries between terrestrial upland and deep open-water systems, and are consequently prone to dynamic and often highly variable hydrological regimes. Physiological adaptations to drought would be advantageous in emergent vegetation, as wetlands are often susceptible to short periods of water scarcity through seasonal declines in precipitation and/or surface water runoff. This study used 15 l microcosms to examine xylem water potential (ψxylem)- and tissue water content (θ)-breakpoints in 2 wetland forbs (Justicia americana and Saururus cernuus) to determine how these plants respond to short-term (2 wk) water deficits. Overall, treatment responses by these forbs included reductions in both θ and ψxylem during the second week of drought, followed by rapid recoveries of θ and ψxylem within a few days of water repletion. Rapid tissue-water recovery, following extreme water deficits (soil moisture levels <3%), is beneficial to plants residing in areas prone to intense water fluctuations. Furthermore, while θ and ψxylem breakpoints in S. cernuus occurred as soil moisture levels fell below 12% (ca. -800 kPa soil water potential), breakpoints for J. americana occurred at considerably lower soil moisture levels (ca. 7 and 10% soil water content for θ and ψxylem, respectively; or less than -1500 kPa soil water potential). The lower breakpoints observed in J. americana were comparable to values reported for terrestrial grasses, including species adapted to dryer, xeric conditions. Therefore, the results from this study suggest that some wetland forbs are physiologically adapted to environments that undergo rapid changes in water hydrology, including the ability to tolerate short-term water scarcity com- parable to dryer upland systems.

Evolution and maintenance of divergent lineages in an endangered freshwater fish, Macquaria australasica

Variable hydrological regimes and habitat availability are factors that affect the distribution of freshwater dependent species and are expected to influence their levels of genetic diversity. Although geologically relatively stable, the south eastern region of Australia has experienced significant changes in hydrological conditions during the Quaternary. This area has also been recently affected by anthropogenic activities, resulting in dramatic population declines of Macquarie Perch (Macquaria australasica). We conducted a range-wide phylogeographic study of this endangered fish to assess the relationship between landscape and freshwater fish evolution in south eastern Australia and infer levels of genetic diversity and population structure. Surprisingly, we detected high genetic diversity, with 46 mtDNA control region haplotypes found across 37 sampling locations. Some lineages were remarkably divergent; one represents a putative undescribed species that probably went extinct during the period of this study. Our reconstruction of population history using a combination of coalescent and phylogenetic methods indicates that the species originated on the coast, east of the Great Dividing Range (GDR), with subsequent colonisation of the Murray-Darling basin, west of the GDR. Nested clade and IM analyses inferred a series of range expansions and fragmentations across the species range consistent with the history of climatic oscillations in south eastern Australia during the Pleistocene. We conclude that the unexpected high levels of diversity and divergence observed in M. australasica may be due to specific habitat requirements, localised recruitment, and Pleistocene climate fluctuations. Under expectations of a drier climate and increased sea levels due to global warming, populations of this and other freshwater species may be expected to experience increased habitat fragmentation and loss of genetic diversity. Conservation management should focus on habitat protection, the maintenance of genetic diversity and taxonomic review.

Hydrologic Variability of the Cosumnes River Floodplain

Natural floodplain ecosystems are adapted to highly variable hydrologic regimes, which include periodic droughts, infrequent large floods, and relatively frequent periods of inundation. To more effectively manage water resources and maintain ecosystem services provided by floodplains – and associated aquatic, riparian, and wetland habitats – requires an understanding of seasonal and inter-annual hydrologic variability of floodplains. The Cosumnes River, the largest river on the west-slope Sierra Nevada mountains without a major dam, provides a pertinent test case to develop a systematic classification of hydrologic variability. By examining the dynamics of its relatively natural flow regime, and a 98-year streamflow record (1908 – 2005), we identified 12 potential flood types. We identified four duration thresholds, defined as short (S), medium (M), long (L), and very long (V). We then intersected the flood duration division by three magnitude classes, defined as small-medium (1), large (2), and very large (3). Of the 12 possible flood types created by this classification matrix, the Cosumnes River streamflow record populated 10 such classes. To assess the robustness of our classification, we employed discriminant analysis to test class fidelity based on independent measures of flood capability, such as start date. Lastly, we used hierarchical divisive clustering to classify water years by flood type composition resulting in 8 water year types. The results of this work highlight the significant seasonal and inter-annual variability in natural flood regimes in Central Valley rivers. The construction of water impoundment and flood control structures has significantly altered all aspects of the flood pulse. Restoring floodplain ecosystem services will require re-establishing key elements of these historic flood regimes in order to achieve regional restoration goals and objectives.

Effective discharge analysis of ecological processes in streams

Discharge is a master variable that controls many processes in stream ecosystems. However, there is uncertainty of which discharges are most important for driving particular ecological processes and thus how flow regime may influence entire stream ecosystems. Here the analytical method of effective discharge from fluvial geomorphology is used to analyze the interaction between frequency and magnitude of discharge events that drive organic matter transport, algal growth, nutrient retention, macroinvertebrate disturbance, and habitat availability. We quantify the ecological effective discharge using a synthesis of previously published studies and modeling from a range of study sites. An analytical expression is then developed for a particular case of ecological effective discharge and is used to explore how effective discharge varies within variable hydrologic regimes. Our results suggest that a range of discharges is important for different ecological processes in an individual stream. Discharges are not equally important; instead, effective discharge values exist that correspond to near modal flows and moderate floods for the variable sets examined. We suggest four types of ecological response to discharge variability: discharge as a transport mechanism, regulator of habitat, process modulator, and disturbance. Effective discharge analysis will perform well when there is a unique, essentially instantaneous relationship between discharge and an ecological process and poorly when effects of discharge are delayed or confounded by legacy effects. Despite some limitations the conceptual and analytical utility of the effective discharge analysis allows exploring general questions about how hydrologic variability influences various ecological processes in streams.

Influence of hydrology on heavy metal speciation and mobility in a Pb–Zn mine tailing

Among the inorganic toxicants of greatest concern in mine tailings, Pb, Zn, Cu, Cd and As figure prominently due to their abundance and potential toxicity. Here we report on their biolability and solid-phase speciation in two sediment cores subject to variable hydrological regimes at an abandoned pyritic mine tailing. The oxic conditions of well-drained sediments induced pyrite oxidation and the subsequent liberation of H +, SO 4 2− and considerable quantities of Fe(III), which precipitated as goethite. Solubility of Pb, Zn, Cu and Cd was closely coupled to pH and goethite presence. Metal lability was particularly low in zones of neutralization, formed by the accumulation of calcite, first carried then deposited by percolating waters in both saturated and unsaturated cores. We conclude that differential hydrology induces variable heavy metal speciation and biolability in Pb–Zn mine tailings, and suggest that site-specific risk assessments must account for past and present hydrological regimes.

Modelling stream flow for use in ecological studies in a large, arid zone river, central Australia

AbstractAustralian arid zone ephemeral rivers are typically unregulated and maintain a high level of biodiversity and ecological health. Understanding the ecosystem functions of these rivers requires an understanding of their hydrology. These rivers are typified by highly variable hydrological regimes and a paucity, often a complete absence, of hydrological data to describe these flow regimes. A daily time‐step, grid‐based, conceptual rainfall–runoff model was developed for the previously uninstrumented Neales River in the arid zone of northern South Australia. Hourly, logged stage data provided a record of stream‐flow events in the river system. In conjunction with opportunistic gaugings of stream‐flow events, these data were used in the calibration of the model. The poorly constrained spatial variability of rainfall distribution and catchment characteristics (e.g. storage depths) limited the accuracy of the model in replicating the absolute magnitudes and volumes of stream‐flow events. In particular, small but ecologically important flow events were poorly modelled. Model performance was improved by the application of catchment‐wide processes replicating quick runoff from high intensity rainfall and improving the area inundated versus discharge relationship in the channel sections of the model. Representing areas of high and low soil moisture storage depths in the hillslope areas of the catchment also improved the model performance. The need for some explicit representation of the spatial variability of catchment characteristics (e.g. channel/floodplain, low storage hillslope and high storage hillslope) to effectively model the range of stream‐flow events makes the development of relatively complex rainfall–runoff models necessary for multisite ecological studies in large, ungauged arid zone catchments. Grid‐based conceptual models provide a good balance between providing the capacity to easily define land types with differing rainfall–runoff responses, flexibility in defining data output points and a parsimonious water‐balance–routing model. Copyright © 2004 John Wiley &amp; Sons, Ltd.

SISM: A Multiscale Model for Cypress Swamp Regeneration

A multiscale model is presented to simulate the impact of hydrologic changes on the regeneration of a cypress swamp in southern Illinois. This model, SISM (Southern Illinois Swamp Model), captures three processes that operate at different spatial and temporal scales: cypress seed dispersal and germination, seedling growth and mortality, and the succession of saplings and mature trees. In SISM, the life history of cypress is represented in a coherent manner, spatio-temporal scales are related to different life phases, processes are consistently integrated, and the interactions amongst the multiple processes are effectively simulated. Three scenarios reflecting water management alternatives were tested and discussed. The results show that (1) the modeling output under current water variability is comparable with various field studies, (2) a stable hydrologic scenario restricts the spatial distribution of cypress, and (3) a more variable hydrologic regime does not necessarily result in a wide regeneration of cypress.

Water resources management of the southern rivers of Russia with reference to fisheries requirements

Water consumption from Russia's southern rivers has increased substantially in recent years as a consequence of intensified economic activity. The volume of irreversible river‐flow withdrawal from the Azov Sea basin varies between 11 and 13 km3 year−1. The majority of the abstracted water is used for irrigation. By 1990, the decrease in discharge into the Caspian Sea was 41 km3 year−1. Withdrawals of water from the Kura, Terek, Ural and other rivers have so far exceeded the ecologically admissible limits and become critical. The paper discusses methodological approaches for the elaboration of standards for water withdrawal and ecologically safe discharges into the southern seas. New data are presented on the dynamics of fish abundance as a function of the hydrological characteristics of rivers and water salinity. The results of the system analysis and regressive modelling formed the basis of a programme to develop policies for water management for fisheries in the lower Don. The principles and criteria for water withdrawal under variable hydrological regimes for the rivers entering the Azov and the Caspian Seas should be tested for their applicability for other water bodies (e.g. rivers, water reservoirs and lakes).

Longitudinal variations in exposed riverine sediments: a context for the ecology of the Fiume Tagliamento, Italy

1. A key component of physical habitat along braided river systems is the exposed riverine sediment within the active zone. The relatively unmanaged, gravel-bed Fiume Tagliamento, Italy, provides the focus for exploring two ecologically important properties of exposed riverine sediments: their within-patch and between-patch variability in calibre. 2. To characterize between-patch variation in exposed riverine sediments, replicate (within-patch) samples were obtained from three geomorphologically distinct locations along 130 km of the river: bar heads along the margin of the low-flow channel, the heads of major bars across the exposed surface of the active zone, and floodplain surfaces. A photographic technique enabled rapid and consistent field sampling of the coarse sediments at bar heads along the low-flow channel margin and on major bars across the dry bed. 3. A downstream decrease in particle size and an increase in within-patch heterogeneity in sediment size were observed within bar head sediments along the margin of the low-flow channel. Comparisons between major bar and low-flow channel samples revealed greatest within-patch variability in individual sediment size indices (D50, A- and B-axes of the larger particles) at headwater sites, greatest between-patch variability in the three measured indices in the central reaches, and lowest between-patch variability at downstream sites. However, there was a distinct increase in the overall heterogeneity in particle size, which was sustained across all patches, in a downstream direction. 4. There was a clear downstream decrease in the size of floodplain sediments in the headwaters, but thereafter there was no distinct downstream trend in any of the calculated particle size indices. 5. The geomorphological controls on the observed patterns and the potential ecological significance of the patterns, particularly for plant establishment, are discussed in relation to the relative relief of the active zone, and the highly variable hydrological and climatic regime along the river.