Hydraulic Habitat Research Articles

Sampling sufficiency for determining hydraulic habitat diversity

The geometry and hydrodynamics of river reaches are key ecohydraulic descriptors. Statistics of water depth and velocity measurements are usually taken as proxies for habitat suitability in rivers. However, little is known about the sufficiency of data to produce effective and representative results. In this research, 19 reaches with differences in terms of discharge, river width, substrate, reach length, cross-section spacing and geomorphology are investigated. Measurements of flow depth and velocity were taken at multiple, equally spaced cross-sections along each reach. Data were sub-sampled using different methodologies and analysed each time. The sets of sub-sampled data were then compared with those calculated with the full data set from a reach. The focus was put towards the hydro-morphological index of diversity (HMID), a combination of the classical ecohydraulic variables flow depth and velocity. It represents the spatial variability of hydraulic habitats in a reach. The results point out that, with a well-defined sampling strategy, 100 measurement points lead to a good estimation of the HMID value in a reach, if more than eight measurement points are taken per cross-section. For geomorphologies with small complexity or when the analysis only includes the estimation of mean flow depth or mean flow velocity, this number can be decreased according to the results presented here. These findings help both, aquatic ecologists and engineers to estimate their data reliability for hydraulic field measurements in a river reach and are herein discussed taking into account the different studied morphologies.

A modeling approach to establish environmental flow threshold in ungauged semidiurnal tidal river

Due to shortage of flow monitoring data in ungauged semidiurnal river, ‘environmental flow’ (EF) determination based on its key component ‘minimum low flow’ is always difficult. For EF assessment this study selected a reach immediately after the Halda-Karnafuli confluence, a unique breeding ground for Indian Carp fishes of Bangladesh. As part of an ungauged tidal river, EF threshold establishment faces challenges in changing ecological paradigms with periodic change of tides and hydrologic alterations. This study describes a novel approach through modeling framework comprising hydrological, hydrodynamic and habitat simulation model. The EF establishment was conceptualized according to the hydrologic process of an ungauged semi-diurnal tidal regime in four steps. Initially, a hydrologic model coupled with a hydrodynamic model to simulate flow considering land use changes effect on streamflow, seepage loss of channel, friction dominated tidal decay as well as lack of long term flow characteristics. Secondly, to define hydraulic habitat feature, a statistical analysis on derived flow data was performed to identify ‘habitat suitability’. Thirdly, to observe the ecological habitat behavior based on the identified hydrologic alteration, hydraulic habitat features were investigated. Finally, based on the combined habitat suitability index flow alteration and ecological response relationship was established. Then, the obtained EF provides a set of low flow indices of desired regime and thus the obtained discharge against maximum Weighted Usable Area (WUA) was defined as EF threshold for the selected reach. A suitable EF regime condition was obtained within flow range 25–30.1 m3/s i.e., around 10–12% of the mean annual runoff of 245 m3/s and these findings are within researchers’ recommendation of minimum flow requirement. Additionally it was observed that tidal characteristics are dominant process in semi-diurnal regime. However, during the study period (2010–2015) the validated model with those reported observations can provide guidance for the decision support system (DSS) to maintain EF range in an ungauged tidal river.

Coupling Unmanned Aerial Vehicle (UAV) and hydraulic surveys to study the geometry and spatial distribution of aquatic macrophytes

Aquatic macrophytes are a key component of river systems around the world. Surveys of macrophyte surface cover, cross-sectional blockage and plant/patch sizes provide data for river managers to assess in-stream habitat, hydraulic resistance, and the success of stream restorations. Manual surveying techniques are labour intensive, provide low spatial detail and are predominantly applied at the cross-section scale, resulting in a lack of published data on macrophyte size distributions. In this study, 1099 Ranunculus penicillatus patches were surveyed using a UAV-mounted digital camera. Geometric properties such as patch area, length, aspect ratio and orientation were determined from the aerial orthophotos. These data were then coupled with hydraulic measurements. Macrophyte abundance corresponded to specific ranges of velocity, Froude number and stream power, indicating clear patterns of hydraulic habitat use (and preferential modification) by Ranunculus. At the reach scale, flow redirection around dense vegetation clusters was observed, with implications for localised sedimentation and bank erosion. The reported data can improve the design of laboratory experiments to represent Ranunculus characteristics in the field. The aerial surveying techniques can be used to efficiently estimate vegetation abundance, surface area blockage factor and also to visualise flow through patch mosaics, enabling targeted management of aquatic vegetation.

Comment 2: Don't Throw Out the Baby (PHABSIM) with the Bathwater: Bringing Scientific Credibility to Use of Hydraulic Habitat Models, Specifically PHABSIM

Railsback (2016) raises valid points directed toward inadequate use of habitat selection models (HSMs) and Physical Habitat Simulation System (PHABSIM) specifically. Although Railsback (2016) notes...

Spatio-temporal effects of river regulation on habitat quality for Atlantic salmon fry

Many upland rivers in the Northern Hemisphere contain important habitat for Atlantic salmon (Salmo salar L.). Owing to their sensitivity to environmental change, salmon are often used as bio-indicators. In Scotland, rivers containing potentially suitable habitat for salmon fry are often also regulated for hydropower. Regulated flow regimes can differ substantially spatially and temporally. Thus, where river management may be needed to maintain, restore, and protect their ecological functioning, this needs to be based on evidence of such spatio-temporal effects. This study investigated the effects of different types of river regulation on the hydraulic characteristics of downstream river reaches and the inferred consequences for salmon fry using hydraulic habitat quality models. The study focussed on the River Lyon (390km2), a tributary of the Tay (4587km2), Scotland, UK. Hydraulic habitat variability was assessed for three reach-scale sites with contrasting flow regimes characterised by (a) releases from hydropower generation, (b) compensation flow and (c) partly re-naturalised flow conditions. For each site, high resolution Digital Terrain Models (DTMs) were developed from bathymetric surveys and 2D hydraulic models were used to assess hydraulic characteristics. Discharge time series were used to simulate hydraulic conditions for regulated and simulated natural flows. Depth and velocity data were extracted from the hydraulic models and used to infer habitat quality using a habitat model developed for Atlantic salmon fry in similar-sized Scottish rivers. Results showed the effects of regulation can vary substantially within reaches and between seasons. Comparison to natural flow regimes suggested that flow alteration has a variable influence on habitat quality depending on the type of regulation and time of year. This work has improved understanding of the effects of regulation on biophysical processes and may also be useful for managing trade-offs between management, restoration, and societal benefits.

Habfuzz: A tool to calculate the instream hydraulic habitat suitability using fuzzy logic and fuzzy Bayesian inference

Theodoropoulos et al, (2016), Habfuzz: A tool to calculate the instream hydraulic habitat suitability using fuzzy logic and fuzzy Bayesian inference, Journal of Open Source Software, 1(6), 82, doi:10.21105/joss.00082

Generalized models of riverine fish hydraulic habitat

ABSTRACTPhysical habitat models are used to relate the availability of suitable habitat to river flows by describing the combined availability of depths, velocities and substrates deemed suitable for a species. Field data are often used to calibrate hydraulic models and therefore calculate availability of suitable physical habitat over a range of flows at a site. This paper describes and tests a parameterize-then-cluster-then-predict strategy that extends physical habitat models across New Zealand. Mixed-effects models were used to represent habitat–flow relationships at 274 sites for each of 17 sets of habitat suitability criteria. Parameter values for each set of habitat suitability criteria and for each formulation were clustered into groups. Cluster membership was related to catchment characteristics. Cross-validated generalized habitat predictions were tested for their ability to predict: (1) availability of suitable physical habitat across a range of flows; and (2) loss of suitable physical habitat between two reference flows. Of the 17 sets of habitat suitability criteria, 12 had predicted habitat–flow patterns that exceeded a set of a priori defined performance criteria, providing evidence that predictions were able to distinguish between-site patterns in relationships between availability of suitable physical habitat and flow at unvisited sites.

Modelling storage‐driven connectivity between landscapes and riverscapes: towards a simple framework for long‐term ecohydrological assessment

AbstractThe importance of conceptualizing the dynamics of storage‐driven saturation area connectivity in runoff generation has been central to the development of TOPMODEL and similar low parameterized rainfall–runoff models. In this contribution, we show how we developed a 40‐year hydrometric data base to simulate storage–discharge relationships in the Girnock catchment in the Scottish Highlands using a simple conceptual model. The catchment is a unique fisheries reference site where Atlantic salmon populations have been monitored since 1966. The modelling allowed us to track storage dynamics in hillslopes, the riparian zone and groundwater, and explicitly link non‐linear changes of streamflows to landscape storage and connectivity dynamics. This provides a fundamental basis for understanding how the landscape and riverscape are hydrologically connected and how this regulates in‐stream hydraulic conditions that directly influence salmonids. We use the model to simulate storage and discharge dynamics over the 40‐year period of fisheries records. The modelled storage‐driven connectivity provides an ecohydological context for understanding the dynamics in stream flow generation which determine habitat hydraulics for different life stages of salmon population. This new, long‐term modelling now sets this variability in the riverscape in a more fundamental context of the inter‐relationships between storage in the landscape and stream flow generation. This provides a simple, robust framework for future ecohydrological modelling at this site, which is an alternative to more increasingly popular but highly parameterized and uncertain commercial ecohydrological models. It also provides a wider, novel context that is a prerequisite for any model‐based scenario assessment of likely impacts resulting from climate or land use change. Copyright © 2016 The Authors Hydrological Processes Published by John Wiley & Sons Ltd. Copyright © 2016 The Authors Hydrological Processes Published by John Wiley & Sons Ltd.

Management of the Spring Snowmelt Recession in Regulated Systems

AbstractIn an effort to restore predictable ecologically relevant spring snowmelt recession flow patterns in rivers regulated by dams, this study defined a methodology by which spring flow regimes can be modeled in regulated systems from the quantifiable characteristics of spring snowmelt recessions in unregulated rivers. An analysis of eight unregulated rivers across the Sierra Nevada mountain range in California found that unregulated systems behaved similarly with respect to seasonal spring patterns and recession limb curvature, and thus prescribed flows could be designed in a manner that mimics those predictable characteristics. Using the methodology to quantify spring recession flows in terms of a daily percent decrease in flow, a series of flow recession scenarios were created for application in an existing hydrodynamic model for the regulated Rubicon River. The modeling results showed that flow recessions with slow ramping rates similar to those observed in unregulated rivers (less than 10% per day) were likely to be protective of native aquatic species, such as the Foothill yellow‐legged frog, while flows that receded at greater rates would likely result in desiccation of egg masses and potential stranding of tadpoles and fry. Furthermore, recession rates of less than 10% per day provided the most spatially diverse hydraulic habitat in the modeled domain for an appropriate duration in spring to support all native species guilds and maximize aquatic biodiversity.

Assessing the transferability of hydraulic habitat models for juvenile Atlantic salmon

Hydraulic habitat models are logistically and technically challenging and expensive to produce. They are therefore frequently transferred between rivers, often with different environmental conditions, without validation. Although studies have recognised problems with model transfer, few have assessed the consequences for model predictions. This study investigated the local (within sub-catchment) transfer of hydraulic habitat models developed for Atlantic salmon (Salmo salar) fry. Two adjacent reaches were chosen for the study, each containing pool, riffle, glide and run habitats. Detailed topographic surveys were used to develop hydraulic models for the study reaches. Substrate and cover were characterised using transects. Seasonal fish habitat use was characterised by electrofishing. Uniform saturation stocking was employed to ensure standard starting densities across all habitats. Generalised additive models were fitted to fry abundance data. Depth, velocity, dominant substrate and cover were used as predictor variables. The distribution of depth, velocity, dominant substrate and cover differed between the two reaches, but was consistent within reaches across seasons. Substrate exhibited the greatest inter-reach difference. Velocity and depth were the most important individual predictors of fry abundance, with the highest densities observed at moderate velocities (∼0.4ms−1) and low depths (∼0.1m) across all seasons. When models were transferred locally between reaches, those that were adjusted for inter-reach differences in mean fry abundance (i.e. those predicting changes in relative abundance) performed better than those transferred without adjustment. Complex models that included substrate and cover typically explained some of the variation in abundance in the transferred reach but performed less well than models containing hydraulic parameters alone. This potentially reflected inter-reach differences in the distribution of substrate and cover. This study suggests that (1) uniform stocking is useful for examining habitat-abundance relationships free from the influence of patchy egg deposition; (2) habitat models should be developed at sites offering maximum environmental complexity at a local level; (3) scientists and managers should avoid transferring models between locations with different environmental characteristics, especially in the absence of model validation; (4) complex models should be avoided, and those containing hydraulic variables alone should be considered, if predictions of habitat quality are to be made at new sites.

Assessment of Chinese sturgeon habitat suitability in the Yangtze River (China): Comparison of generalized additive model, data-driven fuzzy logic model, and preference curve model

Summary To date, a wide range of models have been applied to evaluate aquatic habitat suitability. In this study, three models, including the expert knowledge-based preference curve model (PCM), data-driven fuzzy logic model (DDFL), and generalized additive model (GAM), are used on a common data set to compare their effectiveness and accuracy. The true skill statistic (TSS) and the area under the receiver operating characteristics curve (AUC) are used to evaluate the accuracy of the three models. The results indicate that the two data-based methods (DDFL and GAM) yield better accuracy than the expert knowledge-based PCM, and the GAM yields the best accuracy. There are minor differences in the suitable ranges of the physical habitat variables obtained from the three models. The hydraulic habitat suitability index (HHSI) calculated by the PCM is the largest, followed by the DDFL and then the GAM. The results illustrate that data-based models can describe habitat suitability more objectively and accurately when there are sufficient data. When field data are lacking, combining expertise with data-based models is recommended. When field data are difficult to obtain, an expert knowledge-based model can be used as a replacement for the data-based methods.

Assessing minimum environmental flows in nonpermanent rivers: The choice of thresholds

The criteria used in the computation of the minimum environmental flow regime and flow cessation periods in nonpermanent rivers are often left to open criteria. This study proposes a stochastic approach for evaluating the choice of local thresholds in the characterization of minimum environmental flows through both the Monte Carlo technique and local hydrological relationships. This approach is applied to four regimes obtained by hydrologic and hydraulic habitat modeling in a Mediterranean watershed. The operationality, defined as the probability of the calculated environmental regime being satisfied by the natural regime over 25 years, was assessed for eight different scenarios. Two monthly minimum environmental flow regimes were then generated, with 90 and 95% operationality levels. This analysis allows the generation of minimum flow regime prescriptions from a strictly hydrologic point of view. The methodology proposed constitutes a useful tool for the implementation of uncertainty analysis of environmental flows in water resource management.

Long-reach Biotope Mapping: Deriving Low Flow Hydraulic Habitat from Aerial Imagery

Understanding of the type and distribution of hydraulic habitat along watercourses is valuable from an ecological and a morphological perspective. The data quantify system state and may be used against benchmark criteria to define system status level and degradation. Current mapping techniques are subjective, time consuming and expensive when carried out over long reaches often requiring specialist field skills. This paper proposes a novel approach to hydraulic habitat mapping using readily available aerial imagery (GoogleEarth and Bing maps) to generate long-reach digital elevation models, which are subsequently used in a 2D modelling domain (JFlow+) to predict hydraulic habitat in the form of biotope types and distribution from Froude number classification. The approach is tested on a 1-km reach of the river Wharfe, England, a morphologically and hydraulically varied watercourse. Biotope mapping of the study reach recorded a distribution of 49% pools, 33% glides and 17% riffles, compared with an observed 54% pools, 32% glides, 13% riffles and 1% broken standing waves/chutes, suggesting that gross biotope distribution may be reliably mapped using the technique when compared with field mapping but that depth estimation error leads to classification issues around transition zones. The improved spatial detail and objective mapping achieved by the technique also provide valuable sub-feature detail on hydraulic habitat variation not picked up by conventional survey. The ease of digital elevation model construction allows for rapid assessment of extended reaches offering an efficient mechanism for whole river ecological assessment, flagging critical sites that would benefit from more detailed field assessment. Copyright © 2016 John Wiley & Sons, Ltd.

Numerical study of an innovative fish ladder design for perched culverts

A fish ladder designed to facilitate fish passage at the outlet end of perched culverts is investigated with a 3D computational fluid dynamics model. The fish ladder consists of a series of alternating arch baffles with geometries providing options for fish passage over varying flow and debris conditions within the ladder. At high flows, the baffle’s protruding center arch increases pool depth, reducing the volumetric bulk turbulence of the pools and improving fish habitat. The arch baffle is compared to a standard baffle design currently in use and demonstrates potential advantages for fish passage. A recirculation zone of low velocity occupies a large volume of the pool believed to provide appropriate hydraulic habitat for resting and staging jump attempts upstream. This numerical study provides an acceptable design for future physical prototype testing in the laboratory or field to verify hydraulics and evaluate fish passage effectiveness.

A habitat suitability model for Chinese sturgeon determined using the generalized additive method

Summary The Chinese sturgeon is a type of large anadromous fish that migrates between the ocean and rivers. Because of the construction of dams, this sturgeon’s migration path has been cut off, and this species currently is on the verge of extinction. Simulating suitable environmental conditions for spawning followed by repairing or rebuilding its spawning grounds are effective ways to protect this species. Various habitat suitability models based on expert knowledge have been used to evaluate the suitability of spawning habitat. In this study, a two-dimensional hydraulic simulation is used to inform a habitat suitability model based on the generalized additive method (GAM). The GAM is based on real data. The values of water depth and velocity are calculated first via the hydrodynamic model and later applied in the GAM. The final habitat suitability model is validated using the catch per unit effort (CPUE d ) data of 1999 and 2003. The model results show that a velocity of 1.06–1.56 m/s and a depth of 13.33–20.33 m are highly suitable ranges for the Chinese sturgeon to spawn. The hydraulic habitat suitability indexes (HHSI) for seven discharges (4000; 9000; 12,000; 16,000; 20,000; 30,000; and 40,000 m 3 /s) are calculated to evaluate integrated habitat suitability. The results show that the integrated habitat suitability reaches its highest value at a discharge of 16,000 m 3 /s. This study is the first to apply a GAM to evaluate the suitability of spawning grounds for the Chinese sturgeon. The study provides a reference for the identification of potential spawning grounds in the entire basin.

A hydrologically sensitive invertebrate community index for New Zealand rivers

Identifying ecological response variables sensitive to hydrological change is a key step in determining the impacts of river flow alterations on aquatic ecosystems and in setting environmental flows that sustain certain ecological values. Building on the successful use of flow regime sensitive aquatic invertebrate indices in other countries, particularly the UK based Lotic Index for Flow Evaluation (LIFE), we provide two variants of a similar index for use in New Zealand (LIFENZ and a weighted variant: LIFENZ_W). As in the original LIFE, the New Zealand indices were based on water velocity preference categories assigned to aquatic invertebrate taxa using professional judgement. To calculate the indices a lookup table is used to assign a score to each taxon based on their velocity category and abundance. For the LIFENZ_W variant an additional step down weighted the scores if the taxon has a general compared to a more specific velocity preference. The two index variants were correlated with each other and to similar environmental parameters. Across a total of 74 sites, both indices were correlated with depth-averaged water velocity. Changes in index values, both between sites and temporally within sites, were predominantly associated with changes in hydrological parameters, such as the magnitude and length of time since a recent high flow, and to a lesser degree with other physico-chemical parameters. Commonly used indices in New Zealand designed to respond to nutrient enrichment (MCI and variants) were not correlated with local water velocity, but were correlated with antecedent flow conditions and were likely influenced by effects of flow stability on algal growth. Further testing of LIFENZ and LIFENZ_W in combination with MCI is recommended, particularly in rivers subject to more extreme hydrological and water quality stresses and with regard to other physical parameters such as hydraulic habitat. However, the LIFENZ and its weighted variant (LIFENZ_W) appear to be useful tools for understanding and managing the effects of hydrological alteration on aquatic invertebrate communities in New Zealand. As LIFENZ and LIFENZ_W were strongly correlated and only showed a relatively small deviation from a slope of 1 we recommend the use of the more straightforward LIFENZ in almost all circumstances.

Aquatic vegetation – a primer for hydrodynamic specialists

ABSTRACTThis paper is about aquatic vegetation, written by a biologist, and is aimed at those working in hydraulics, hydrology, fluvial geomorphology and related fields. It provides information on the basic biology of riverine plants, their evolution, their basic requirements and their distribution patterns in space and time. Key ecological processes, such as succession, are described. It discusses how these plants can be grouped according to their shapes and traits and how these are associated with specific types of hydraulic habitat. The realistic parameterization of empirical experiments is considered. The application of fundamental hydraulic research to environmental management is discussed, focusing on eutrophication, channel conveyance and the downstream impacts of dams.

Satellite-based remote sensing of running water habitats at large riverscape scales: Tools to analyze habitat heterogeneity for river ecosystem management

We illustrate an approach to quantify patterns in hydraulic habitat composition and local heterogeneity applicable at low cost over very large river extents, with selectable reach window scales. Ongoing developments in remote sensing and geographical information science massively improve efficiencies in analyzing earth surface features. With the development of new satellite sensors and drone platforms and with the lowered cost of high resolution multispectral imagery, fluvial geomorphology is experiencing a revolution in mapping streams at high resolution. Exploiting the power of aerial or satellite imagery is particularly useful in a riverscape research framework (Fausch et al., 2002), where high resolution sampling of fluvial features and very large coverage extents are needed. This study presents a satellite remote sensing method that requires very limited field calibration data to estimate over various scales ranging from 1m to many tens or river kilometers (i) spatial composition metrics for key hydraulic mesohabitat types and (ii) reach-scale wetted habitat heterogeneity indices such as the hydromorphological index of diversity (HMID). When the purpose is hydraulic habitat characterization applied over long river networks, the proposed method (although less accurate) is much less computationally expensive and less data demanding than two dimensional computational fluid dynamics (CFD). Here, we illustrate the tools based on a Worldview 2 satellite image of the Kiamika River, near Mont Laurier, Quebec, Canada, specifically over a 17-km river reach below the Kiamika dam. In the first step, a high resolution water depth (D) map is produced from a spectral band ratio (calculated from the multispectral image), calibrated with limited field measurements. Next, based only on known river discharge and estimated cross section depths at time of image capture, empirical-based pseudo-2D hydraulic rules are used to rapidly generate a two-dimensional map of flow velocity (V) over the 17-km Kiamika reach. The joint distribution of D and V variables over wetted zones then is used to reveal structural patterns in hydraulic habitat availability at patch, reach, and segment scales. Here we analyze 156 bivariate (D, V) density function plots estimated over moving reach windows along the satellite scene extent to extract 14 physical habitat metrics (such as river width, mean and modal depths and velocity, variances and covariance in D and V over 1-m pixels, HMID, entropy). A principal component analysis on the set of metrics is then used to cluster river reaches in regard to similarity in their hydraulic habitat composition and heterogeneity. Applications of this approach can include (i) specific fish habitat detection at riverscape scales (e.g., large areas of riffle spawning beds, deeper pools) for regional management, (ii) studying how river habitat heterogeneity is correlated to fish distribution and (iii) guidance for site location for restoration of key habitats or for post regulation monitoring of representative reaches of various types.

Characteristics of the Cross-Sectional Vorticity of the Natural Spawning Grounds of Schizothorax prenanti and a Vague-Set Similarity Model for Ecological Restoration.

Schizothorax prenanti is an endemic fish in the mountain rivers of southwestern China with unique protection value. To further explore the vortex motion of hydraulic habitats, which is closely related to the fish breeding process, the cross-sectional vorticity was used to evaluate the hydraulic conditions of the natural spawning habitat of S. prenanti. A coupled level-set and volume-of-fluid (CLSVOF) three-dimensional (3D) model was applied to simulate the hydraulic habitat of the Weimen reach, a typical natural spawning ground for S. prenanti in the upper Yangtze River. The model was used in conjunction with the Wilcoxon rank sum test to distinguish the distributions of vertical vorticity in spawning and non-spawning reaches. Statistical analysis revealed that the cross-sectional vorticity in spawning reaches was significantly greater than in non-spawning reaches, with likely biological significance in the spawning process. The range of cross-sectional mean values of vorticity was 0.17 s-1–0.35 s-1 in areas with concentrated fish sperm and eggs; the minimum value was 0.17 s-1, and the majority of values were greater than 0.26 s-1. Based on this study, a vague-set similarity model was used to assess the effectiveness of ecological restoration by evaluating the similarity of the cross-sectional vorticity of the natural spawning reach and rehabilitated spawning reach after implementing ecological restoration measures. The outcome might provide a theoretical basis for the recovery of damaged S. prenanti spawning grounds and act as an important complement for the assessment of recovery effectiveness and as a useful reference for the coordination of ecological water use with the demands of hydraulic and hydropower engineering.

Ecological significance of hydrological connectivity for wetland plant communities on a dryland floodplain river, MacIntyre River, Australia

Hydrological connections between river channels and their adjacent floodplains facilitate the flux of organisms and nutrients and access to increased habitat and new resources. Hydrological connections also deliver water subsidy and potentially disturb (through hydraulic forces) floodplain ecosystems. This study investigates the role of hydrological connectivity as a driver of patterns in wetland plant assemblages in billabongs on the floodplain of an Australian dryland river, exploring indirectly the relative importance of the mechanisms of flux, subsidy and disturbance. Wetland plants were surveyed in billabongs across gradients of hydrological connectivity and depth. Surveys were accompanied by experiments examining germination from the soil seed banks of each site under submerged and waterlogged conditions. The patterns in extant and germinant plant communities in relation to connectivity and depth gradients were used to infer the relative importance of the connectivity-related mechanisms of flux, subsidy and hydraulic disturbance in structuring wetland plant communities. Depth influenced both extant and germinating plant communities. Shallow billabongs supported a greater diversity and abundance of plants, and greater numbers and diversity of germinable seeds in the seed bank. Germination of seeds was greater in waterlogged soils than submerged soils. Thus, the main controls of plant abundance in wetlands appear to be availability of waterlogged soil habitat for germination and absence of light limitation for growth. Hydrological connectivity did not influence the abundance of plants or germinable seeds, but did influence species presence-absence in growing vegetation; this effect did not extend to the germinating community. Thus, hydrological connection does not appear to influence wetland vegetation by facilitating the movement of propagules between habitats. Instead, the patterns observed are consistent with hydrological connection providing a cue for germination through the delivery of water, and by modifying hydraulic habitat.