Hydraulic Habitat Research Articles

Differential effect of wave stress on the physiology and behaviour of native versus non-native benthic invertebrates

In fresh waters, non-native invertebrate species preferentially spread via navigation waterways, where they often dominate assemblages. Littoral habitats in navigation waterways are regularly exposed to ship-induced waves. We conducted experiments to test the effects of artificial wave exposure on the relative performance of wide-spread native and non-native species. We compared physiological and behavioural response variables (i.e. growth rate, glycogen content, feeding and swimming activity) of two amphipods (native Gammarus roeselii and non-native Dikerogammarus villosus) and two gastropods (native Bithynia tentaculata and non-native Physella acuta) subject to wave and control (i.e. no wave) treatment flumes across a 6-week period. Growth, and in part glycogen content (as a measure of energy storage), were significantly reduced after exposure to waves in native invertebrates, but not in non-native invertebrates. The reduction in growth may be associated with the disturbance effects of waves, such as the higher swimming activity of G. roeselii and lower food uptake of B. tentaculata. In comparison, the effective hiding behaviour observed for D. villosus and good swimming ability of P. acuta, were identified as important traits facilitating the successful colonisation of the harsh habitat conditions of littoral waterways. Our study demonstrates that artificial wave regimes may contribute significant selective pressure, thus explaining the observed dominance of non-native species in navigational waterways. The success of non-native species under the harsh hydraulic habitat conditions of these socio-economically driven ecosystems may consequently be traced directly to behavioural and/or physiological traits.

Biomechanical properties of aquatic plants and their effects on plant–flow interactions in streams and rivers

We analysed the biomechanical properties of aquatic plant stems of four common submerged river macrophyte species with bending, tension and cyclic loading/unloading tests and related these properties to the hydraulic habitats of the plants. The studied species included Glyceria fluitans, Ranunculus penicillatus, Myriophyllum alterniflorum and Fontinalis antipyretica. Habitat assessment shows that these species occur in a range from low to high flow velocities, respectively. G. fluitans is a semi-aquatic species with stems of a high flexural rigidity and high breaking force and breaking stress that enable them to carry their own weight and balance gravity when growing upright in slow flowing rivers. G. fluitans may also grow horizontally often producing emerged terrestrial stems. In contrast, F. antipyretica grows in fierce water flow. Its stems have the highest flexibility, a significantly higher ‘tension’ Young’s modulus, breaking stress and work of fracture and a lower plastic deformation compared to M. alterniflorum and R. penicillatus. These traits enable F. antipyretica to survive even in swift flowing streams and constrict the growth of M. alterniflorum and R. penicillatus to the river reaches with moderate flow velocities. R. penicillatus has a weak bottom part with a low breaking force and breaking stress acting as a predetermined breaking point and enabling seasonal regrowth from root parts.

Downstream environmental effects of dam operations: Changes in habitat quality for native fish species

AbstractHydropeaking dam operation and water extractions for irrigation have been broadly stated as alterations to natural flow regimes, which have also been noticed in the Biobío Watershed, in Central Chile, since 1996. In the Biobío River, most of native fish species are endemic and very little is known about them. Their ecological and social values have never been estimated, and there is lack of information about their habitat preferences. Furthermore, changes on fish habitat availability due to natural and/or man‐made causes have not been evaluated. In this study, eight native fish species, in a representative reach of the Biobío River, were studied and their preferred habitats were surveyed and characterized. A hydrodynamic model was built and linked to the fish habitat simulation model CASiMiR. Fuzzy rules and fuzzy sets were developed for describing habitat preference of the native fish species. CASiMiR was then used to simulate how physical habitat conditions vary due to flow control (i.e. upstream dams). Results show how overall habitat quality, expressed as weighted usable area (WUA) and hydraulic habitat suitability (HHS), changes and fluctuates due to the dam operation and how the daily hydropeaking is influencing quantity, quality and location of different habitats. The study suggests that the analysed fish are highly susceptible to flow control, as dams are currently operated, and fish habitat improvement suggestions are proposed. Copyright © 2010 John Wiley & Sons, Ltd.

Linking a spatially explicit watershed model (SWAT) with an in‐stream fish habitat model (PHABSIM): A case study of setting minimum flows and levels in a low gradient, sub‐tropical river

AbstractAs changes in landuse and the demand for water accelerate, regulators and resource managers are increasingly asked to evaluate water allocation against the need for protection of in‐stream habitat. In the United States, only a small number of river basins have the long‐term hydrograph data needed to make these assessments. This paper presents an example of how to bridge the conceptual and physical divide between GIS‐based watershed modelling of basin‐discharge and in‐stream hydraulic habitat models. Specifically, we used a Soil and Water Assessment Tool (SWAT) model for the Hillsborough River to produce data for use in a Physical HABitat SIMulation (PHABSIM) model of the same river. This coupling of models allowed us to develop long‐term discharge data in ungauged river systems based on watershed characteristics and precipitation records. However this approach is not without important limitations. Results confirm that accuracy of the SWAT‐predicted hydrograph declines significantly when either the DEM resolution becomes too coarse or if DEM data are resampled to a coarser or finer resolution. This is due to both changes in the size and shape of the river basin with the varying DEMs and subsequent shifts in the proportions of land use, soils and elevation. Results show the use of 30 m DEMs produced hydrographic patterns amenable for using in‐stream habitat protocols like PHABSIM model, especially where little or no hydrographic and land use information exists. Copyright © 2010 John Wiley & Sons, Ltd.

Empirical modelling of large scale patterns in river bed surface grain size

The large-scale management of river resources requires an understanding of the spatial distribution of bed surface grain size. According to Sternberg's law, bed surface grain size at basin scales (i.e., 10 2–10 4 km), reduces in the downstream direction. We tested if such patterns were consistent across multiple large basins, and inferred their potential explanatory variables. Specifically , we fitted random forest regression models to mean grain size estimates (Φ units) derived from visual observations on several occasions in 677 stream and river reaches throughout France. Model predictors included the distance from the headwaters, variables describing rainfall, topography and geology of the upstream watershed and the slope of the valley segment on which the reach was located. We made three sets of independent predictions of mean grain size by 1) withholding each observation reach in turn during model-fitting and making a prediction for the withheld reach, 2) withholding reaches in six subsets defined by large drainage basins during model-fitting and making predictions for the withheld reaches in turn and 3) making predictions for 106 hydraulic habitat survey sites where grain size had been measured using protocols that differed from the fitting data. We also evaluated model consistency by fitting it to the six subsets defined by large drainage basins. Regression of the observed ( y) vs. predicted ( x) mean grain size generated from our three sets of independent predictions had r 2 of 52%, 47%, and 38%, respectively; and were unbiased. The characteristic model error, as defined by root mean squared deviation was 2.9 (Φ units), however, the tests indicated that the model uncertainties for the finer size classes were larger. Contrary to expectations, we found consistently weak relationships between mean grain size and distance from the headwaters. The most important contributors to modelled grain size were (consistently across the 6 large basins) watershed slope, segment slope and watershed variables describing rainfall and geological hardness. This suggests that grain size distribution at the scale of multiple basins is mainly governed by patterns associated with sediment sources and hydraulic sorting. Though uncertain at the reach scale, the high generality of our model supports the research of empirical or simplified physically based models of bed surface grain size distribution in multiple basins.

Mesohabitat use by brown trout (Salmo trutta) in a small groundwater‐dominated stream

ABSTRACTGroundwater‐dominated streams have particular flow regimes that commonly support populations of trout. Meso‐ and micro‐habitat surveys were carried out on a reach of the river Tern that drains a Triassic sandstone aquifer in the English West Midlands, to investigate brown trout (Salmo trutta) habitat use with varying flows. Mesohabitats were mapped over a range of summer and autumn flows and coupled with direct underwater observation (snorkelling) of fish locations together with point measurements of velocity and depth. The number of habitat types recorded was low and dominated by glides, runs, and backwaters. Brown trout showed a strong association with glides and runs with adults being more associated with runs and parr with glides. General habitat use curves showed brown trout to favour depths between 0.30 and 0.40 m and velocities below 0.40 m s−1. A clear preference was shown for sand and gravel bed materials. However, the differentiation of hydraulic habitats was weak and there was no trend in mesohabitats or change in trout use of mesohabitats with discharge. The study raises limitations of the mesohabitat survey approach when linking fish ecology, flow and physical habitat in small streams with low flow variability and low habitat diversity. In these situations, other factors (especially cover features) appear to strongly influence brown trout distribution. Copyright © 2010 John Wiley & Sons, Ltd.

Ecological effects of small dams on benthic macroinvertebrate communities of mountain streams (Córdoba, Argentina)

We assessed the effect of small dams on stream habitats and on the structure and function of benthic macroinvertebrate communities in mountain streams of Cordoba, Argentina. Two streams with small dams (impacted streams) and two streams without dams (control streams) from the Carcarana River upper sub-basin were selected. Dominant hydraulic units were identified and benthic samples were collected in each of these units during high and low water periods. According to this study small dams may have impact on the function and structure of the invertebrate community, though hydraulic habitats would not be affected. Fourteen hydraulic units were found and changes due to dams were not observed as assessed by the detrended correspondence analysis. The canonical correspondence analysis grouped samples mainly in relation to hydraulic habitats whereas TWINSPAN grouped samples according to the presence of dams. Nested ANOVAs showed changes in richness and diversity, in the abundance of filterers and shredders and in the abundance of certain species. Small dams generate quite small reservoirs and their permeability may be greater than those of large dams. The behaviour of macroinvertebrate populations in drift and colonization processes, the continuity of the riparian structure and the discharge of planktonic algae downstream the dams may be some factors determining the ecological changes produced by small impoundments.

Integrating hydrology, hydraulics and ecological response into a flexible approach to the determination of environmental water requirements for rivers

There are many different approaches to environmental water requirement (EWR) determinations that have been reported in the international literature. Many of these focus on different aspects of the problem, while few of them consider all of the issues associated with the eventual implementation of EWR as part of integrated water management. It is also necessary to recognize that there is a wide range of different types and resolution of information available in different parts of the world. This paper presents an integrated framework that has been in use in South Africa for several years and which is based on integrating the impacts of changing flow regimes on different ecosystem response components through indices of ‘stress’ measured on a common scale (0–10). Software to support its implementation has been included as part of an existing hydrological modelling framework package that includes a GIS interface and database management procedures. The framework is flexible enough to be used with different approaches to analyzing ecosystem responses, ranging from complex hydraulic habitat assessments to the interpretation of expert opinion and therefore should be widely applicable. The framework can also be used to design a modified flow regime for a given set of ecological objectives, or it can be applied to assess scenarios of flow regimes based on a range of possible future water management options. The paper explains the approach, provides some illustrations of its application and discusses some of the issues associated with its more widespread use.

Model application to assess effects of urbanisation and distributed flow controls on erosion potential and baseflow hydraulic habitat

The catchment model SWMM was modified to include on-site flow-control devices, and then linked with models of hydraulic habitat suitability and erosive potential for specific reaches in a study catchment. Urbanisation decreased the modelled baseflow by up to 33% and reduced the area of suitable hydraulic habitat by 0.5–13.3%, depending on the reach and species. Hypothetical infiltration devices increased the baseflow to pre-development levels, with an associated increase in habitat, but other measures were not effective. Urbanisation increased the erosion potential index by a factor of 1.58–9.32, depending on the reach. Erosion-control ponds decreased the erosion potential to pre-development level in some reaches. Detention tanks reduced the erosion potential significantly, but not to pre-development levels. The poor predictive ability of the models for baseflow compromised the utility of the hydraulic habitat assessment. Predictions of the effects of urbanisation on baseflow habitat should therefore be treated cautiously.

Hydrogeomorphic effects on bedload scour in bull char (Salvelinus confluentus) spawning habitat, western Washington, USA

We investigated the vulnerability of fall-spawned bull char ( Salvelinus confluentus ) embryos to redd scour during winter rain and rain-on-snow flood discharges in western Washington, USA. It was hypothesized that the magnitude of bedload scour at bull char redds is reduced by the provision and selection of stable refugia habitat controlled by local-, reach-, or subcatchment-scale variables such as hydraulic habitat unit and channel type. Bedload scour and channel change were measured using 96 scour monitors and 34 elevational transects in three catchments over 2 to 4 years. Scour to cited egg burial depths of bull char did not commence until discharge typically exceeded the 2-year recurrence interval. At a local scale, scour varied significantly among side channel, protected main channel, and unprotected main channel redd sites. Unprotected gravel patches in simplified channel types with moderate gradients were most susceptible to deep scour, especially if coupled with the transient supply and storage of sand and gravel from mass wasting. Partially transport-limited reaches had reduced scour due to lower stream power and armored gravel beds. Complex spawning habitat (i.e., with abundant large woody debris and side channels) was important in providing refuge from deep scour and in buffering embryos against inhospitable hydrologic or sediment regimes.

Experimental field assessment of suspended sediment pathways for characterizing hydraulic habitat

AbstractThe distribution of particulate matter within river channels, including sediments, nutrients and pollutants, is fundamental to the survival of aquatic organisms. However, the interactions between flow and sediment transport at the patch scale of river systems represents an under‐researched component of physical habitat studies, particularly those concerning the characterization of ‘physical biotopes’ (riffles, runs, pools, glides). This paper describes a field methodology for exploring the transfer of particulate matter at small scales within river channels, which may be used to aid hydraulic habitat characterization. The field protocol combines field measurement of high frequency flow properties, to characterize hydraulic habitat units, and deployment of spatial arrays of turbidity probes, to detect the passage of artificially‐induced sediment plumes through different biotope units. Sediment plumes recorded by the probes are analysed quantitatively in the manner of the flood hydrograph, and qualitative inferences are made on the dominant mixing processes operating within different parts of the channel. Relationships between the nature of spatio‐temporal hydraulic variations within glide, riffle and pool biotopes, and the character and mixing behaviour of sediment plumes within these habitat units are identified. Results from these preliminary experiments suggest that investigating and characterizing the transfer and storage of sediments, nutrients and pollutants within and between different biotopes is a viable avenue for further research, with potential to contribute to improved physical habitat characterization for river management and habitat restoration. The experiments are also an illustration of the value of neglected synergies between process geomorphology, ecology and river hydraulics. Copyright © 2010 John Wiley & Sons, Ltd.

Instream physical habitat modelling types: an analysis as stream hydromorphological modelling tools for EU water resource managers

The introduction of the EU Water Framework Directive (WFD) is providing member state water resource managers with significant challenges in relation to meeting the deadline for ‘Good Ecological Status’ by 2015. Overall, instream physical habitat modelling approaches have advantages and disadvantages as management tools for member states in relation to the requirements of the WFD, but due to their different model structures they are distinct in their data needs, transferability, user-friendliness and presentable outputs. Water resource managers need information on what approaches will best suit their situations. This paper analyses the potential of different methods available for water managers to assess hydrological and geomorphological impacts on the habitats of stream biota, as requested by the WFD. The review considers both conventional and new advanced research-based instream physical habitat models. In parametric and non-parametric regression models, model assumptions are often not satisfied and the models are difficult to transfer to other regions. Research-based methods such as the artificial neural networks and individual-based modelling have promising potential as water management tools, but require large amounts of data and the model structure is complex. It is concluded that the use of habitat suitability indices (HSIs) and fuzzy rules in hydraulic–habitat modelling are the most ready model types to satisfy WFD demands. These models are well documented, transferable, user-friendly and have flexible data needs. They can easily be implemented in new regions using expert information or different types of local data. Furthermore, they are easily presentable to stakeholders and have the potential to be applied over large spatial scales. Integral care must be taken in the use of appropriate HSIs as these are the most sensitive part of the modelling and inaccurate results will be gained if not correctly formulated. If representative HSIs are not available, fuzzy rule-based modelling is recommended, but care must also be taken in the designing of the rule sets. For larger-scale modelling or when only few field data are available, generalized habitat models hold promise for quantifying habitat suitability based on average stream characteristics.

Book Reviews

Book Reviews

The ecological limits of hydrologic alteration (ELOHA): a new framework for developing regional environmental flow standards

Summary1. The flow regime is a primary determinant of the structure and function of aquatic and riparian ecosystems for streams and rivers. Hydrologic alteration has impaired riverine ecosystems on a global scale, and the pace and intensity of human development greatly exceeds the ability of scientists to assess the effects on a river‐by‐river basis. Current scientific understanding of hydrologic controls on riverine ecosystems and experience gained from individual river studies support development of environmental flow standards at the regional scale.2. This paper presents a consensus view from a group of international scientists on a new framework for assessing environmental flow needs for many streams and rivers simultaneously to foster development and implementation of environmental flow standards at the regional scale. This framework, the ecological limits of hydrologic alteration (ELOHA), is a synthesis of a number of existing hydrologic techniques and environmental flow methods that are currently being used to various degrees and that can support comprehensive regional flow management. The flexible approach allows scientists, water‐resource managers and stakeholders to analyse and synthesise available scientific information into ecologically based and socially acceptable goals and standards for management of environmental flows.3. The ELOHA framework includes the synthesis of existing hydrologic and ecological databases from many rivers within a user‐defined region to develop scientifically defensible and empirically testable relationships between flow alteration and ecological responses. These relationships serve as the basis for the societally driven process of developing regional flow standards. This is to be achieved by first using hydrologic modelling to build a ‘hydrologic foundation’ of baseline and current hydrographs for stream and river segments throughout the region. Second, using a set of ecologically relevant flow variables, river segments within the region are classified into a few distinctive flow regime types that are expected to have different ecological characteristics. These river types can be further subclassified according to important geomorphic features that define hydraulic habitat features. Third, the deviation of current‐condition flows from baseline‐condition flow is determined. Fourth, flow alteration–ecological response relationships are developed for each river type, based on a combination of existing hydroecological literature, expert knowledge and field studies across gradients of hydrologic alteration.4. Scientific uncertainty will exist in the flow alteration–ecological response relationships, in part because of the confounding of hydrologic alteration with other important environmental determinants of river ecosystem condition (e.g. temperature). Application of the ELOHA framework should therefore occur in a consensus context where stakeholders and decision‐makers explicitly evaluate acceptable risk as a balance between the perceived value of the ecological goals, the economic costs involved and the scientific uncertainties in functional relationships between ecological responses and flow alteration.5. The ELOHA framework also should proceed in an adaptive management context, where collection of monitoring data or targeted field sampling data allows for testing of the proposed flow alteration–ecological response relationships. This empirical validation process allows for a fine‐tuning of environmental flow management targets. The ELOHA framework can be used both to guide basic research in hydroecology and to further implementation of more comprehensive environmental flow management of freshwater sustainability on a global scale.

Climatic changes and 13‐year trends in stream macroinvertebrate assemblages in New South Wales, Australia

AbstractOver the period from 1994 to 2007, air and water temperatures in the Australian state of New South Wales (NSW) increased while rainfall and river flows declined. Data on the occurrence of stream macroinvertebrate families in bioassessment samples collected in NSW during this period were examined to see whether a biological response to these climatic and hydrological trends could be discerned. Multiple logistic regression was used to test for long‐term trends in the probability of detection of individual macroinvertebrate families within the samples, taking account of the latitude, longitude, altitude, hydraulic habitat, time of year and subsampling method associated with each sample. Of the 124 families and family groups tested, 33 had statistically significant increasing trends, 37 had significantly declining trends and 54 had no significant trend; however, many of the last group were seldom collected and their trend estimates had wide confidence limits. Significant relationships were found between the thermophily and rheophily of the families and the estimated strength and direction of their long‐term trends, with families that favour colder waters and faster‐flowing habitats more likely to have declined. Although many families showed trends of increasing detection within samples, such trends do not necessarily equate to increasing prevalence in the environment because the extent of wetted habitat has probably declined, especially flowing habitat. In addition, because of likely intrafamilial trait diversity, increasing or apparently increasing families may include species in decline. Many freshwater macroinvertebrate species in NSW may be threatened by anthropogenic climate change, exacerbated by water withdrawals. The thermal tolerances, rheophily and other pertinent traits of individual species need to be determined to identify those most at risk.

Impact of groundwater abstraction on physical habitat of brown trout (Salmo trutta) in a small Danish stream

The purpose of this study was to assess the impact of groundwater abstraction on stream discharge and physical habitat conditions for brown trout (Salmo trutta) in a small Danish stream. Stream discharge was simulated using a lumped hydrological model (NAM) and a scenario was set up for stream discharge reference conditions. Stream physical habitat conditions (WUA) were simulated for four life stages of trout using a hydraulic habitat model (RHYHABSIM). The impact of groundwater abstraction on WUA for trout was assessed by combined simulations from the NAM model and the RHYHABSIM model. The model predicted that groundwater abstraction reduced median annual discharge by 37% and mean annual 90th percentile discharge by 82%. Summer discharge was relatively most affected by groundwater abstraction and WUA was therefore particularly affected by groundwater abstraction during summer. WUA for adult trout was mainly controlled by suitable water depths (>40 cm) even under conditions without abstraction. On an annual basis WUA for fry and juvenile trout was most affected by abstraction. Future modelling should consider improving simulation of low discharges and preferably not use general hydrological models.

Relationship of stream ecological conditions to simulated hydraulic metrics across a gradient of basin urbanization

The relationships among urbanization, stream hydraulics, and aquatic biology were investigated across a gradient of urbanization in 30 small basins in eastern Wisconsin, USA. Simulation of hydraulic metrics with 1-dimensional unsteady flow models was an effective means for mechanistically coupling the effects of urbanization with stream ecological conditions (i.e., algae, invertebrates, and fish). Urbanization, characterized by household, road, and urban land density, was positively correlated with the lowest shear stress for 2 adjacent transects in a reach for the low-flow summer (p < 0.001) and autumn (p < 0.01) periods. Urbanization also was positively correlated with Reynolds number and % exposed stream bed during months with moderate to low flows. Our study demonstrated the value of temporally and spatially explicit hydraulic models for providing mechanistic insight into the relationships between hydraulic variables and biological responses. For example, the positive correlation between filter-feeding invertebrate richness and minimum 2-transect shear stress observed in our study is consistent with a higher concentration of water-column particulates available for filtration. The strength of correlations between hydraulic and biological metrics is related to the time period (annual, seasonal, or monthly) considered. The hydraulic modeling approach, whether based on hourly or daily flow data, allowed documentation of the effects of a spatially variable response within a reach, and the results suggest that stream response to urbanization varies with hydraulic habitat type.

Transverse velocity distributions in channels with emergent bank vegetation

AbstractDefining hydraulic habitat conditions in rivers requires prediction of local flow velocities, which are strongly influenced by the presence of emergent vegetation along the banks. Previously proposed methods for describing the transverse variation of depth‐averaged velocity away from the vegetation edge are shown by comparison with independent laboratory data to be inadequate. One of the methods is modified on the basis of new data to provide realistic descriptions over a wide range of conditions. The new method enables prediction of the maximum velocity in both wide and relatively narrow channels, the apparent shear stress at the vegetation interface, and the variation of velocity from the vegetation boundary to the point where the maximum velocity occurs. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Evaluation and Application of Environmental Flows for Running Waters in Slovenia

Water use invariably results in major impacts on river flows. Environmental Flows (EF) are defined as the quantity and quality of water that is needed to preserve the structure and the function of the river and riparian zone ecosystem and sufficient quantity of water to enable the survival and reproduction of aquatic organisms in different hydraulic habitats. This paper describes the criteria and methods used to determine EF and experiences with their application in Slovenia. The diversity of running waters of Slovenia demand special treatment and determination of EF for each individual section of the river system. Using hydrological, morphological and ecological criteria, two different approaches are used for the determination of EF in Slovenia, a rapid assessment method and a detailed assessment method. For both methods, data are then analyzed by an expert panel in order to determine an EF. Since 1994, more than 180 study sites have been examined for research and application of EF in Slovenia. Determination of EF for existing users has prioritized their water requirements so they can remain economically viable. Where new schemes are proposed, there has been much greater scope to prioritize ecosystem requirements. EF determination is receiving growing attention and will continue to increase in importance, driven by research that aids our understanding of flow-biota relationships and recent environmental policy and legislation at both the national and European level.

Impact of winter flow regulation on pest-level populations of blackfly (Diptera: Simuliidae) and non-target faunal communities in a South African river

The mid-reaches of the Great Fish River in the Eastern Cape Province of South Africa experience regular outbreaks of the pest blackfly Simulium chutteri as a direct consequence of increased flows caused by an inter-basin transfer scheme. There are opportunities to control these outbreaks through flow manipulation by using an upstream impoundment that has an annual shutdown period for maintenance. In this study, the stones-in-current biotope was surveyed at five sites prior to, and immediately succeeding, a flow shutdown. Changes in aquatic macroinvertebrate community composition and densities of larval and pupal blackfly were assessed and related to changes in hydraulic habitat due to the flow reductions. Although flows at those sites where blackfly densities were highest did not decrease to the levels desired for effective pest control, results from two sites indicated that flow reduction can lead to reductions in densities of larval blackfly without detrimental impacts on non-target macroinvertebrates. Simulations using a blackfly outbreak model indicate that flow reduction from an average of over 7 m3 s−1 to below 2 m3 s−1 for three to four weeks would effectively control blackfly outbreaks. Such an approach could be achieved through negotiations among affected Farmers' Associations, Irrigation Boards and the regional Department of Water Affairs and Forestry.