Understanding the Thermal Regime of Rivers Influenced by Small and Medium Size Dams in Eastern Canada

Although small and medium‐size dams are prevalent in North America, few studies have described their year‐round impacts on the thermal regime of rivers. The objective of this study was to quantify the impacts of two types of dams (run‐of‐river, storage with shallow reservoirs) on the thermal regime of rivers in eastern Canada. Thermal impacts of dams were assessed (i) for the open water period by evaluating their influence on the annual cycle in daily mean water temperature and residual variability and (ii) for the ice‐covered winter period by evaluating their influence on water temperature duration curves. Overall, results showed that the run‐of‐river dam (with limited storage capacity) did not have a significant effect on the thermal regime of the regulated river. At the two rivers regulated by storage dams with shallow reservoirs (mean depth < 6 m), the annual cycle in daily mean water temperature was significantly modified which led to warmer water temperatures in summer and autumn. From August to October, the monthly mean water temperature at rivers regulated by storage dams was 1.4 to 3.9°C warmer than at their respective reference sites. During the open water period, the two storage dams also reduced water temperature variability at a daily timescale while increased variability was observed in regulated rivers during the winter. Storage dams also had a warming effect during the winter and the winter median water temperature ranged between 1.0 and 2.1°C downstream of the two storage dams whereas water temperature remained stable and close to 0°C in unregulated rivers. The biological implications of the altered thermal regimes at rivers regulated by storage dams are discussed, in particular for salmonids. Copyright © 2016 John Wiley & Sons, Ltd.     

Classification of US Hydropower Dams by their Modes of Operation

A key challenge to understanding ecohydrologic responses to dam regulation is the absence of a universally transferable classification framework for how dams operate. In the present paper, we develop a classification system to organize the modes of operation (MOPs) for US hydropower dams and powerplants. To determine the full diversity of MOPs, we mined federal documents, open‐access data repositories, and internet sources. We then used CART classification trees to predict MOPs based on physical characteristics, regulation, and project generation. Finally, we evaluated how much variation MOPs explained in sub‐daily discharge patterns for stream gages downstream of hydropower dams. After reviewing information for 721 dams and 597 power plants, we developed a two‐tier hierarchical classification based on (i) the storage and control of flows to powerplants, and (ii) the presence of a diversion around the natural stream bed. This resulted in nine tier‐1 MOPs representing a continuum of operations from strictly peaking, to reregulating, to run‐of‐river, and two tier‐2 MOPs, representing diversion and integral dam‐powerhouse configurations. Although MOPs differed in physical characteristics and energy production, classification trees had low accuracies (≤62%), which suggested that accurate evaluations of MOPs may require individual attention. MOPs and dam storage explained 20% of the variation in downstream subdaily flow characteristics and showed consistent alterations in subdaily flow patterns from reference streams. This standardized classification scheme is important for future research including estimating reservoir operations for large‐scale hydrologic models and evaluating project economics, environmental impacts, and mitigation. Copyright © 2016 John Wiley & Sons, Ltd.     

THE MANAGEMENT MODES OF SEASONAL FLOODS AND THEIR IMPACT ON THE RELATIONSHIP BETWEEN CLIMATE AND STREAMFLOW DOWNSTREAM FROM DAMS IN QUEBEC (CANADA)

The goal of the study was to compare the modes of management of seasonal floods for different dams and to constrain their impact on the relationship between climate variables and streamflow downstream from the dams. At the Rawdon dam, downstream from which the Ouareau River is characterized by a natural‐type regulated flow regime, a ‘type A’ flood management mode prevails, in which the same rainfall and/or snowmelt events account for seasonal floods both in the unregulated (natural) stretch of river upstream from the dam and in the river downstream from the dam. As a result, seasonal floods in the natural setting and downstream from the dam are nearly synchronous. In contrast, downstream from the Matawin dam (Matawin River), which produces an inversion‐type regulated flow regime, the prevalent flood management modes are of types B and D, whereby seasonal floods observed upstream and downstream from the dam are not caused by the same rainfall and/or snowmelt events and, as a result, are not synchronous. This difference in seasonal flood management modes affects the interannual variability of the magnitude of seasonal daily maximum flows related to the seasonal floods. Thus, the interannual variability of these flows downstream from the Matawin dam differs significantly from that of flows upstream. No correlation is observed between climate variables and streamflow downstream from the Matawin dam. This absence of correlation disappears gradually at the annual scale, at which streamflow is correlated with rainfall, as is observed upstream from the dam. Copyright © 2013 John Wiley & Sons, Ltd.     

Peaking hydropower and fish assemblages: an example from the Tallapoosa River,AL

Dams alter many aspects of riverine environments and can have broad effects on aquatic organisms and habitats both upstream and downstream. While dams and associated reservoirs can provide many services to people (hydropower, recreation, flood control, and navigation), they can also negatively affect riverine ecosystems. In particular, hydropeaking dams affect downstream fish habitats by increasing variability in discharge and temperature. To assess the effects of Harris Dam on the Tallapoosa River, AL, operating under an adaptive management plan implemented in 2005, we sampled fish for community analyses from four sites on the river: three in the regulated reach downstream of the dam, and one unregulated site upstream. Fish were collected every other month using boat/barge electrofishing. We used Shannon's H, nonmetric multidimensional scaling (NMDS), a multiresponse permutation procedure (MRPP), and indicator species analysis to quantify patterns in fish assemblage structure and determine how assemblages varied among sites. NMDS and MRPP indicated significant fish assemblage differences among sites, with the tailrace fish assemblage being distinct from the other downstream sites and sites becoming more similar to the upstream, unregulated site (relative to fish assemblages) with distance downstream of the tailrace. The tailrace fish assemblage included higher proportions of rheophilic species that may be better suited to variable and/or high flows. Altered fish assemblages demonstrated continued effects of Harris Dam on the downstream aquatic systems, particularly close to the dam. These effects may indicate that further mitigation should be considered depending on conservation and management goals.     

SHIP‐LOCK–INDUCED SURGES IN AN IMPOUNDED RIVER AND THEIR IMPACT ON SUBDAILY FLOW VELOCITY VARIATION

The majority of large rivers are fragmented by dams, and navigation is often supported by the installation of ship locks. Despite their ubiquitous existence, the effect of ship locks on river basin hydrodynamics is rarely considered in an environmental context. Ship‐lock operation induces single‐wave crests or troughs called surges, which propagate along the basin and are subject to reflection at the up‐ and downstream impoundments. We used pressure sensors and acoustic Doppler current profiler measurements to investigate the effects of up‐ and downstream ship locking on the water level and the current velocity dynamics in a 12.9‐km‐long basin of the impounded river Saar (Germany). Ship lifting at the lower dam and the associated water export from the basin results in a negative surge propagating upstream, whereas a descending ship‐lock operation at the upper dam of the basin creates a positive surge propagating downstream. Both types of waves are subject to positive reflection at the opposing dams. Frequent lock operations lead to a complex pattern of multiple superimposing surges. The resulting water level fluctuations are comparable in magnitude with those associated with discharge variations due to hydropower peaking but occur at much shorter timescales. Associated with the water surface displacement during wave passage is a corresponding increase or decrease of the longitudinal current velocity. The magnitude of wave‐induced velocity fluctuations can exceed mean flow velocities by a factor of three and, depending on wave type and direction, can result in a reversal of the main flow direction of the river. Because of their longevity of several hours and superposition effects, ship‐lock–induced surges govern 66.3% and 45.4% of the subdaily variations in flow velocity and water level, respectively. This article concludes with a discussion of the potential effects of lock‐induced flow dynamics in impoundments on oxygen dynamics and methane ebullition. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of dams on river water‐quality signatures at event and seasonal scales: The Sélune River (France) case study

Dams and reservoirs are known to disturb river‐water composition, among other impacts, with potential implications for downstream river ecosystems and water uses. Existing studies have emphasized the variable influence of dams on water composition according to the element, its speciation (dissolved vs. particulate), reservoir properties (residence times), reservoir functions (e.g., hydropower, irrigation), and management (water releases). A now common approach to analyzing hydrological, geochemical, and biological controls on element export from unregulated rivers is to study hydrochemical signatures such as concentration‐flow relationships. We investigated a case study to analyze hydrochemical signatures of a regulated river (Sélune River, western France) upstream and downstream of a chain of two hydropower dams, assuming that the dams disturbed the river's signatures, and that those disturbances would provide information about processes occurring in the reservoirs. Both seasonal and event‐scale signatures were analyzed over two contrasting hydrological years and a range of storm events. The dams induced a chemostatic downstream response to storm events whenever elements were diluted or concentrated upstream. Dams did not disturb the seasonality of major anions but did modify silica and phosphorus concentration‐flow relationships, especially during low flow. Such changes in dynamics of river‐water composition may affect downstream biological communities. This study presents an initial state of the hydrochemical signature of the downstream river, before the removal of the two dams.     

Site modelling methods for detecting hydrologic alteration of flood frequency and flood duration in the floodplain below the carlyle dam,lower Kaskaskia River,Illinois, USA

Reports concerning the influence of dams on river hydrology vary among researchers, interest groups and government agencies. These often contradicting statements may occur because changes in hydrology caused by dams are distinct for each dam and river watershed. The objective of this research was to use site specific techniques to determine if the 1967 installation of the Carlyle Dam, lower Kaskaskia River, Illinois, altered flood frequency and duration within the forested floodplain located below the dam. Results indicated a decrease in flood duration and frequency, and a decrease in annual flood frequency variation at a site 6.4 km below the dam. Pre‐dam versus post‐dam differences in flood frequency and duration at the site 32.2 km below the dam were related to climate rather than dam effects. Although dam impacts are a concern, this research shows that distance downstream from the dam and downstream tributary and watershed characteristics should be considered before assuming that the dam has changed hydrologic parameters for portions of rivers. This research also indicates that areas of the lower Kaskaskia River may still maintain hydrologic ecological integrity, and could be targeted for restoration and adaptive management purposes. Hydrologic modelling combined with river gage and on‐site well measurement techniques presented in this study could provide detailed flood frequency and duration information for land use, sociological and geomorphological questions in focus areas within river floodplains. Copyright © 2008 John Wiley & Sons, Ltd.     

Effects of hydropeaking operations on the growth of Alabama bass Micropterus henshalli and redeye bass Micropterus coosae in the Tallapoosa River,Alabama, USA

Anthropogenic factors such as dam construction and hydropower generation can dramatically alter the flow regime of rivers and may impact growth of aquatic organisms. Using incremental growth techniques, annual growth of Alabama bass Micropterus henshalli and redeye bass M. coosae in the Tallapoosa River, Alabama, USA, was evaluated in response to variation in flow regime. Fish were collected from the Tallapoosa River above Harris Dam (unregulated site) and at two sites downstream of the dam (regulated sites), as well as Hillabee Creek (unregulated tributary). Flow variables were calculated for each growth year, and the best model that described growth for each species at each location was determined using Akaike's Information Criterion. Additionally, growth increments of each species at ages 1, 2, and 3 were compared between years characterized by low and high flow variability. Age was the best explanatory variable that described growth in all models, although flow variables were included in more than half the models. In all cases, annual and seasonal flow variables had low predictive power and explained <2% of the variation in growth. Growth was higher for age‐1 fish in years with less flow variation but was similar among years for age‐2 and age‐3 fish. Overall, this study provided little evidence that annual growth of either species was heavily influenced by flow in this regulated river.     

Small hydropower dam alters the taxonomic composition of benthic macroinvertebrate assemblages in a neotropical river

Hydropower dams substantially modify lotic ecosystems. Most studies regarding their ecological impacts are based on large dams and provide little information about the far more abundant effects of small hydropower dams. Our aim was to characterize the ecological effects of a small hydropower dam and run‐of‐the‐river reservoir on the structure of benthic macroinvertebrate assemblages in the Pandeiros River located in the neotropical savanna of Brazil. We tested the hypothesis that benthic macroinvertebrate assemblages in sites directly affected by the dam and reservoir would show a different taxonomic structure compared with those in free‐flowing sites. We expected to find sensitive native species associated with the free‐flowing sites, whereas resistant and non‐native invasive taxa were expected in impounded sites. We also explored associations between the presence of native and non‐native invasive taxa to each habitat type. We found that the structure of benthic macroinvertebrate assemblages was significantly different between free‐flowing and impounded sites. Also, we found that the dam and reservoir facilitated colonization of non‐native invasive species (Corbicula fluminea and Melanoides tuberculata) because only in those sites they were found in high abundance, in contrast to the free‐flowing sites. Although the environmental conditions imposed by the impoundment altered the structure of benthic macroinvertebrate assemblages, the effects were limited to sites closest to the dam. Our results highlight the necessity of understanding physical habitat changes caused by the presence and management of run‐of‐the‐river dams and reservoirs.     

The Sanaga discharge at the Edea Catchment outlet (Cameroon): An example of hydrologic responses of a tropical rain‐fed river system to changes in precipitation and groundwater inputs and to flow regulation

The Sanaga River is one of Sub‐Saharan Africa's largest and greatly regulated rivers. Available flow data for this hydrosystem largely cover the pre‐ and post‐regulation periods. From comparisons between unregulated (hypothetical) and observed scenarios, it has been possible to separate and to quantify hydro‐climatic (groundwater + rainfall) change effects from anthropogenic impacts (especially dam‐related alterations). To appreciate shifts in the river regime, discontinuity detection tests and the IHA model were applied to discharge data series reflecting average and extreme flow conditions, respectively. Results obtained principally from the Hubert segmentation method reveal that a major discontinuity occurred in 1970–1971 separating a surplus phase between 1945–1946 and 1969–1970, and a deficient and much contrasted one, from 1971/1972. This implies that the Sanaga catchment is dominantly affected by hydro‐climatic changes. However, wide land cover/land use changes experienced here since 1988 have resulted in an increase in surface runoff. Additional quickflows linked to these changes may have partly compensated for the substantial decline in the dry season rainfall and groundwater inputs observed from this date. Although at the monthly scale, dam‐related impacts on average flows increase with stage of regulation, the seasonal variability of the river regime remains generally unaffected. A comparison of the IHA statistics, calculated from unregulated and observed streamflow data, show that hydrologic shifts occurring in maximum and minimum discharges are mostly significant from 1971/1972 and are mainly due to the action of dams. Minimum flows appear, however, widely impacted, thus reflecting the prime objective assigned to the existing reservoirs, constructed to supplement flows for hydroelectricity production during the dry season. Copyright © 2010 John Wiley & Sons, Ltd.     

Low‐Head Dam Impacts on Habitat and the Functional Composition of Fish Communities

The natural flow regime of many rivers in the USA has been impacted by anthropogenic structures. This loss of connectivity plays a role in shaping river ecosystems by altering physical habitat characteristics and shaping fish assemblages. Although the impacts of large dams on river systems are well documented, studies on the effects of low‐head dams using a functional guild approach have been fewer. We assessed river habitat quality and fish community structure at 12 sites on two rivers; the study sites included two sites below each dam, two sites in the pool above each dam and two sites upstream of the pool extent. Fish communities were sampled from 2012 to 2015 using a multi‐gear approach in spring and fall seasons. We aggregated fishes into habitat and reproductive guilds in order to ascertain dams' effects on groups of fishes that respond similarly to environmental variation. We found that habitat quality was significantly poorer in the artificial pools created above the dams than all other sampling sites. Fast riffle specialist taxa were most abundant in high‐quality riffle habitats farthest from the dams, while fast generalists and pelagophils were largely restricted to areas below the downstream‐most impoundment. Overall, these dams play a substantial role in shaping habitat, which impacts fish community composition on a functional level. Utilizing this functional approach enables us to mechanistically link the effects of impoundments to the structure of fish communities and form generalizations that can be applied to other systems. Copyright © 2017 John Wiley & Sons, Ltd.     

A customised approach to determining the geomorphic effectiveness of small flood events in a regulated river

The construction of dams significantly alters flow and sediment regimes with subsequent deleterious effects on the morphological and ecological character of rivers. Effective experimental floods can ameliorate the downstream geomorphic impacts of dams. The traditional view is that large floods are required to perform effective geomorphic work, and the geomorphic outcomes of small floods are often overlooked. Many river restoration frameworks do not consider small floods. Yet, there is evidence that the hydrological characteristics that ameliorate specific geomorphic impacts in a river are unique to each river, and a customised approach to setting the right mix of floods (including small experimental floods) is needed. In this study, we modify an existing flood effectiveness model developed for large floods, for determining the geomorphic effectiveness of small floods in a highly regulated Australian river. Two flood classes were added to the model (medium peak stream power and moderate total energy expenditure), and the flood power characteristics were rescaled to reflect the relative difference in the magnitude of the small floods and the magnitude of the geomorphic work performed. Using a step‐wise approach, this customised model determined the geomorphic effectiveness of small floods. The best flood for ameliorating the geomorphic impacts of flow regulation had medium to long duration (10 to 51 days), high peak unit stream power (77 to 123 Wm^?2) and moderate to large total energy expenditure (78,600 to 342,320 × 10³ J). This approach to determining flood effectiveness for small floods is applicable to other geomorphically impacted river channels downstream of dams and can be used to inform experimental flood releases for geomorphic outcomes.     

COMPARISON OF THE HYDROMORPHOLOGICAL EVOLUTION OF THE L'ASSOMPTION AND OUAREAU RIVER CHANNELS (QUEBEC,CANADA)

The monitoring station method, which is based on comparing data before and after dam construction, is commonly used to quantify the hydromorphological impacts induced by dams. However, in the absence of pre‐dam construction data, other analytical methods may be used to detect changes downstream from dams that remain more or less constant over time. The study used one such method, the control station method, to constrain changes which may be linked to construction of the Rawdon Dam, in 1913, on the Ouareau River. Thus, a comparison of the hydrological (seasonal daily maximum flows) and morphological (mean bankfull width and sinuosity) evolution of the Ouareau and L'Assomption Rivers during the period 1930 to 2008 was carried out. The surface area as well as the climatic, lithological, land use and physiographic features of the two watersheds are nearly identical in the study area. The comparison revealed three changes that may be linked to the Rawdon Dam: an increase in the magnitude of daily maximum flows downstream from the dam for all four seasons, which is inferred to have resulted in extensive widening and low sinuosity of the Ouareau river channel downstream from the Rawdon Dam relative to the L'Assomption river channel. These types of morphological changes are consistent with changes observed downstream from some dams. The Rawdon Dam had no effect on the interannual variability of daily maximum flows, which are characterized by a significant increase in mean in winter in both watersheds. This increase, which is abrupt, occurred in 1973 for both rivers. Copyright © 2012 John Wiley & Sons, Ltd.     

Impact Assessment of Mtera and Kidatu Reservoirs on the Annual Maximum Floods at Stiegler's Gorge of the Rufiji River in Tanzania

Abstract

The impoundment of any river causes changes in the downstream flow regime. The effect of a dam on flow regime depends on both the storage capacity of the reservoir relative to the volume of river flow and the way the dam is operated. The most common attribute of flow regulation is a decrease in the magnitude of the flood peaks and an increase in low flows. This paper reports the findings of a study to assess the cumulative impact of two dams on the Great Ruaha River in Tanzania (the Mtera and Kidatu Reservoir System) on flood flows. The method used was to compare the observed annual maximum flows at downstream locations after the construction of the dams with mathematically modeled estimates of the annual maximum flows at the same locations, assuming that the dams were not built (i.e. generating naturalized flows). Contrary to expectations, the study found that annual maximum peaks were supposed to be less than those actually observed. For instance, in 1989, annual maximum peak flow of 1,400 m³/s was recorded at Kidatu, but the estimated uncontrolled peak flow should have been 800 m³/s, signifying an artificially-induced flood wave. Although, there was a significant impact on the peak flows at Kidatu awing to the dams, the impact was found to be minimal on the flow peaks at Steigler's Gorge.     

Differences in river ecological functions due to rapid channel alteration processes in two California rivers using the functional flows model,part 2—model applications

This study applies the functional flows model (FFM) that integrates hydrogeomorphic processes and ecological functions to assess physical habitat. Functional flows are discharge values that serve ecological uses. The model was adjusted to evaluate gravel‐bed riffle functionality for fall‐run Chinook salmon with respect to river rehabilitation on the Mokelumne River and flood‐induced channel change on the Yuba River. The goal was to test if differences in ecological performance were traceable to differences in hydrogeomorphic conditions. Ecological functions studied were bed occupation (spawning, incubation and emergence) and bed preparation (river bed reworking periods)‐ both reliant on shear stress dynamics. Model outputs included number of days that have functional flows, ranges of functional flows that provide favourable sediment transport stages and the efficiency of a site to produce functional flows. Statistical significance of results was tested using non‐parametric tests. Functional flows analyses before and after geomorphic alteration indicate that river rehabilitation on the lower Mokelume River increased the number of days with functional flows, while the Yuba River May 2005 flood increased the functional ranges of flows for the test sites. Reach‐scale analyses indicated similar ecological performance at reference sites in both rivers. A comparison between both rivers showed that despite a greater geomorphic potential of the Mokelumne River sites to have functional flows, Yuba River sites actually experienced better ecological performance for fall‐run Chinook salmon freshwater life stages due to greater flow availability. The FFM provided an objective tool to assess changes in ecological functionality at hydrogeomorphically dynamic sites. Copyright © 2010 John Wiley & Sons, Ltd.     

Assessing the benefit of flow constraints on the drifting invertebrate community of a regulated river

The downstream effects of hydroelectric dam operations on the abundance and diversity of the macroinvertebrate drift community of a regulated river were compared to that of an unregulated river, longitudinally and across three seasons. The regulated river operated under minimum flow and ramping rate (rate of change of flow) restrictions resulting in a ‘modified peaking’ regime, which means the facility could still peak, but at a slower rate and may not reach maximum turbine flows in the short time typically required to respond to market energy demand. The unregulated river had no dams or other water control structures. There was a trend of increasing abundance and diversity with distance from the dam on the regulated river, with no discernable trend along the unregulated river. While feeding guild proportions did not vary along the unregulated river, within the regulated river feeding guild proportions changed longitudinally as scrapers and collector gatherers increased, and filterers and predators decreased with distance downstream. The regulated river had similar or higher abundance across all seasons, with lower diversity in the spring. Seasonal average discharge was found to be lowest in summer on both rivers, with the regulated river benefiting from a minimum flow to help maintain higher abundance and diversity. Overall, our examination of the drifting invertebrate community on a regulated river support that operational constraints associated with modified peaking regimes helped mitigate the typical negative effects associated with river regulation. Copyright © 2010 Crown in the right of Canada and John Wiley & Sons, Ltd.     

THE COMBINED EFFECTS OF FLOW REGULATION AND AN ARTIFICIAL FLOW RELEASE ON A REGULATED RIVER

Damming and regulating the flow of rivers is a widespread issue and can have a significant impact on resident biota. The Tongariro River, central North Island, New Zealand, has a flow regime that is regulated by two hydroelectric dams along its length, and it has been suggested that ‘flushing flows’ would assist benthic communities by removing ‘nuisance’ periphyton growth forms that typically occur in autumn. We assessed whether (i) damming has altered periphyton and macroinvertebrate communities downstream of the Rangipo Dam and (ii) whether the release of a flow pulse equivalent to 50 times the baseflow is sufficient to (a) move the substrate in the section of river downstream of this dam and (b) impact benthic periphyton and macroinvertebrate communities. Downstream macroinvertebrate communities were impacted by the presence of the dam, but periphyton was not. No movement of substrate occurred downstream of the dam as a result of the flow release, which was likely because of naturally high embeddedness and armouring of substrate. Periphyton biomass and macroinvertebrate density were not affected by the release indicating that larger releases would be required to have any effect on benthic communities downstream of this dam. This study highlights the importance of considering natural bed structure and sediment dynamics when using flow releases downstream of dams to control periphyton. Copyright © 2013 John Wiley & Sons, Ltd.     

Mitigation of cold‐water thermal pollution downstream of a large dam with the use of a novel thermal curtain

Hypolimnial releases from dams during periods of thermal stratification modify the downstream riverine thermal regime by decreasing water temperature and reducing natural diel thermal variability. This cold‐water thermal pollution in rivers can persist for hundreds of kilometres downstream of dams and impact important ecological processes such as fish spawning. To mitigate this problem, a first‐of‐its‐kind thermal curtain was fitted to the large bottom release Burrendong Dam on the Macquarie River, Australia. The thermal curtain acts by directing warmer, near‐surface epilimnial water to the low‐level hypolimnial offtake. This study aimed to test the efficacy of the thermal curtain by measuring temperatures before and after the curtains installation, quantifying the magnitude and extent of cold‐water thermal pollution along the Macquarie River downstream of Burrendong Dam. Epilimnial releases with use of the curtain increased diel temperature ranges and the mean monthly water temperature below the dam. Epilimnial releases with use of the curtain increased diel temperature ranges from 0.9°C to 2.5°C and reduced the difference between the mean monthly water temperature of an upstream control and a downstream site by up to 3.5°C. A comparison of the monthly temperature means along the river, indicated that thermal recovery, whereby temperatures returned to within the natural range of upstream temperatures occurred 45 km downstream of the dam during summer when the thermal curtain was deployed, compared with approximately 200 km prior to deployment of the curtain. Our study suggests that the use of thermal curtains can reduce cold‐water thermal pollution and improve ecological outcomes for river ecosystems downstream of dams.     

MORE EXOTIC AND FEWER NATIVE PLANT SPECIES: RIVERINE VEGETATION PATTERNS ASSOCIATED WITH ALTERED SEASONAL FLOW PATTERNS

Natural flow regimes are important for sustaining riverine vegetation. The regulation of river flows to provide water for agriculture often results in changes to flow timing. This study assesses the impact of altered seasonal flow patterns on riverine flora. Within temperate Australia, we surveyed the vegetation of five lowland rivers, three of which have large dams that alter their seasonal flow patterns; the other two are unregulated. From four to six sites were selected on each river, and these were classified into three levels of regulation based on the extent to which the timing of their seasonal flow patterns were altered. Sites were surveyed in winter and the following summer. Permanent quadrats were also established at a number of the surveyed sites and resurveyed every 3 months. Of the 267 plant taxa identified, 145 were exotic (non‐native). More exotic taxa and fewer native taxa were associated with increasing level of seasonal flow inversion (regulation). In particular, greater numbers of short‐lived exotic terrestrial taxa and fewer native woody taxa were associated with increasing level of regulation. Some exotic woody species (e.g. willows) were more common in the unregulated rivers and may have life‐history traits favoured by the natural seasonal flow patterns of study area. Multivariate analyses showed that level of regulation had a significant effect on the overall composition of the riverine vegetation. Our results provide support for the hypotheses that flow regulation adversely affects native species diversity and increases the vulnerability of riparian zones to invasion by exotic species; however, these effects are dependent on plant species' life‐history strategies. Our study highlights the importance of natural seasonal flow patterns for sustaining native riverine plant communities. Flow management aimed at maintaining or restoring ecological values should consider seasonal flow patterns. Winter/spring flow peaks may be particularly important for the recruitment of native riverine plants, especially trees and shrubs, and reducing the extent of exotic annuals and grasses. Copyright © 2012 John Wiley & Sons, Ltd.     

EFCAM: A METHOD FOR ASSESSING ALTERATION OF ENVIRONMENTAL FLOW COMPONENTS

The environmental flow components (EFCs) are a set of 34 streamflow statistics computed by the Indicators of Hydrologic Alteration software, which are used for environmental flow assessments and developing environmental flow recommendations. The objective of this paper is to demonstrate a methodology for analysing and summarizing the alteration of EFCs, called the EFC assessment method (EFCAM). EFCAM uses non‐parametric rank‐sum statistical tests, sometimes in conjunction with per cent change, to produce ratings of alteration for each EFC statistic, for both central tendency and range of variability. These ratings are combined into summary indexes of alteration, by event type (extreme low flow, low flow, high‐flow pulse, small flood and large flood), flow characteristic (frequency, magnitude, duration, timing and rise/fall rates) and overall. EFCAM is demonstrated by analyzing typical patterns of EFC changes in 66 rivers across the United States where flow is altered by dams. The results reported here show that EFCAM is an effective method for efficiently summarizing flow alteration of ecologically relevant components of flows, and hence would be a useful addition to the Indicators of Hydrologic Alteration software. Results of this study show an overall trend towards compression of the hydrograph from above and below into a more limited overall range on these 66 rivers. The results shown here add to previous findings on dam impacts on river flows by separating out alteration in different segments of the hydrograph, and identify some patterns of alteration not previously reported. Although the version of EFCAM used here is believed to be appropriate for most situations, it is flexible and can be tailored to the needs of a particular project by including or excluding certain flow statistics or using a different weighting system. Copyright © 2013 John Wiley & Sons, Ltd.