Trends in low flows of German rivers since 1950: Comparability of different low‐flow indicators and their spatial patterns

Climate change, land‐use shifts, reservoir storage, and water withdrawals impact low flows in rivers, creating challenges for ecological integrity and human uses. A systematic investigation of river discharges was carried out for 79 stream gauges in Germany. Available time series between 1950 and 2013 were analysed for trends in annual minimum low flows, discharge deficits, and low‐flow durations. The application of different low‐flow indicators led to similar spatial patterns, although each metric is used for different purposes in water management applications. Statistical tests identified significant discharge trends at more than half of the stations investigated. Low‐flow trends since 1950 tended to be catchment specific, suggesting that climate change has not been the dominant driver. Most of the gauges investigated showed statistically significant increases in low flows. This can be mainly attributed to reservoir management. For rivers showing snow‐ and icemelt‐dominated flow regimes, such trends are probably overlain by climate‐driven changes (increasing amounts of rainfall, earlier snowmelt in spring). In contrast, stations showing statistically significant decreases in low flows were correlated with areas of decreasing mining activity. Hydrologic impacts of climate change are widespread and significant, but the results here suggest that human river management remains the dominant hydrologic driver on many rivers.     

INTRA‐ANNUAL VARIATION IN RIVER–RESERVOIR INTERFACE FISH ASSEMBLAGES: IMPLICATIONS FOR FISH CONSERVATION AND MANAGEMENT IN REGULATED RIVERS

While much is known about the fish assemblages, habitats, and ecology of rivers and reservoirs, there has been limited study of the fish assemblages in transitional habitats between these lotic and lentic habitats. Data about these river–reservoir interface (RRI) fish assemblages are needed to guide integrated management efforts of river–reservoir ecosystems. The aim of these efforts is to recommend flows for natural river function, conserve native riverine fish assemblages, and maintain reservoir sport fisheries. We used a multigear approach to assess the fish assemblages of four RRIs in the Colorado River Basin, Texas. In addition to characterizing RRI fish assemblages using species richness and evenness metrics, and habitat‐use guilds, we used a multivariate approach to evaluate intra‐annual shifts in species composition and abundance. All RRIs had high species richness and evenness values and included both macrohabitat generalist and fluvial species. RRIs also contained high proportions of the fish species available within each river–reservoir ecosystem, ranging from 55% to 80%. Observed intra‐annual shifts in RRI fish assemblages resulted from changes in abundance of dominant species rather than changes in species composition, with abundance of most species increasing from early spring to summer. Fish species responsible for intra‐annual shifts included mostly floodplain and migratory species, suggesting that species both used littoral habitats within RRIs and migrated through RRIs to river and reservoir habitats. The diversity of fishes found within RRIs highlights the importance of including these areas in future conservation and management efforts of river–reservoir ecosystems. Copyright © 2013 John Wiley & Sons, Ltd.     

Simulating system‐wide effects of reducing irrigation withdrawals in a disputed river basin

Stakeholders in river systems often target larger upstream water consumers as an intuitive solution for increasing flows for downstream ecological needs. Within regulated river systems, simplistic panaceas may have unexpected and unintended results at a watershed level. The Apalachicola–Chattahoochee–Flint River Basin is a large watershed in the south‐eastern United States whose management has been the source of conflict for several decades. This paper tests the hypothesis of whether a reduction in consumptive losses to Flint River flows through the large‐scale implementation of water‐saving agricultural irrigation technologies and practices will have a positive effect on downstream ecosystem water requirements in the Apalachicola River. An existing integrated reservoir/reach model was used to explore multiple irrigation water use scenarios. Because of current federal reservoir operating rules in the Chattahoochee River, irrigation decreases in the Flint River do not always directly translate to elevated flows downstream in the Apalachicola River. In drought years, a large percentage of the Flint River water savings is captured as greater storage volume at upstream Chattahoochee reservoirs because of a requirement to supplement downstream flows to a prescribed minimum level. In nondrought years, the majority of irrigation decreases translate to increased flow in the Apalachicola River. Given these simulation results, public policy decisions need to be formulated with regard to what portion of the Flint River water savings from changing irrigation practices in drought years should be allocated to the upstream Chattahoochee storage reservoirs and what portion to supporting downstream environmental and social needs in the Apalachicola watershed.     

RESERVOIR FLOODPLAINS SUPPORT DISTINCT FISH ASSEMBLAGES

Reservoirs constructed on floodplain rivers are unique because the upper reaches of the impoundment may include extensive floodplain environments. Moreover, reservoirs that experience large periodic water level fluctuations as part of their operational objectives seasonally inundate and dewater floodplains in their upper reaches, partly mimicking natural inundations of river floodplains. In four flood control reservoirs in Mississippi, USA, we explored the dynamics of connectivity between reservoirs and adjacent floodplains and the characteristics of fish assemblages that develop in reservoir floodplains relative to those that develop in reservoir bays. Although fish species richness in floodplains and bays were similar, species composition differed. Floodplains emphasized fish species largely associated with backwater shallow environments, often resistant to harsh environmental conditions. Conversely, dominant species in bays represented mainly generalists that benefit from the continuous connectivity between the bay and the main reservoir. Floodplains in the study reservoirs provided desirable vegetated habitats at lower water level elevations, earlier in the year, and more frequently than in bays. Inundating dense vegetation in bays requires raising reservoir water levels above the levels required to reach floodplains. Therefore, aside from promoting distinct fish assemblages within reservoirs and helping promote diversity in regulated rivers, reservoir floodplains are valued because they can provide suitable vegetated habitats for fish species at elevations below the normal pool, precluding the need to annually flood upland vegetation that would inevitably be impaired by regular flooding. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Reservoirs Effects on the Interannual Variability of Winter and Spring Streamflow in the St-Maurice River Watershed (Quebec,Canada)

The aim of this study was to test three main hypotheses about the interannual variability of streamflow downstream from dams: (1) an almost similar long-term trend in interannual variability, (2) low variability of flow, and (3) its independence (no link) from climate variability. To test these hypotheses, the interannual variability of winter and spring streamflow downstream from three reservoirs (Gouin, Manouane, and Matawin) which induce an inversion of the natural cycle of streamflow (maximum flows in winter and minimum flows in spring) was compared to the interannual variability of streamflow in natural rivers (measured at the Matawin and Vermillon stations) over the period from 1932 to 2008 in the St-Maurice River watershed. As far as the interannual variability of flow is concerned, its long-term trend is not homogeneous downstream from the three reservoirs in both seasons. However, downstream from two reservoirs, changes in streamflow were observed to be different from those in natural rivers (no significant trend downstream from the Taureau reservoir, on the Matawin River, and significant decrease in spring flow downstream from the Manouane reservoir). Finally, coefficient of variation values for minimum flows are higher downstream from reservoirs than in natural rivers, despite the fact that watershed surface area is larger for regulated rivers than for natural ones. As for the link with climate variability, analysis of the correlation between climate variables (temperature and precipitation) and mean winter and spring daily streamflow reveals that winter streamflow downstream from the Taureau reservoir is not correlated with any climate variable, whereas spring streamflow is positively correlated with rainfall and negatively correlated with maximum temperature. Thus, downstream from reservoirs, the interannual variability of streamflow depends on climate during the spring, but not during winter.     

Influence of cascade reservoirs on spatiotemporal variations of hydrogeochemistry in Jinsha River

River hydrogeochemistry offers necessary guidance for effective water environmental management.However,the influence of cascade reservoirs on river hydrogeochemistry remains unknown.In this study,the Jinsha River,the headwaters of the Yangtze River of China,was selected to investigate the spatiotemporal variations of hydrogeochemistry after the construction of six cascade reservoirs.Major ions,total dissolved solids,electrical conductivity,and pH values of sampled water in the upper natural reaches and lower reservoir-regulated reaches were analyzed in both flood and dry seasons.The results of Piper diagram and Gibbs plots showed that the hydrogeochemistry of the Jinsha River was naturally controlled by both evaporation-crystallization and carbonate weathering processes,but it was also artificially affected by reservoirs.The impoundment of cascade reservoirs affected the hydrodynamic condition of the river.The river flow in the flood season was reduced by approximately 24.5%,altering the proportions of water sources and leading to notable hydrogeochemical alterations in reservoir-regulated reaches.Conversely,river hydrogeochemistry generally remained unchanged in the dry season,owing to the insignificant effect of cascade reservoirs on river flow.In contrast to what has been observed in previous studies of individual reservoirs,the cumulative influence of cascade reservoirs on the Jinsha River flow regime did not cause abrupt hydrogeochemical changes between the upstream and downstream areas of each reservoir.Moreover,the water quality assessments revealed that the impoundment of cascade reservoirs improved downstream irrigational water quality,with lower Na~+ ratio values in the flood season.This study provides the earliest evaluation of cascade reservoir influence on the hydrogeochemistry of the Jinsha River.The findings of this study can be used as a reference for scientific guidelines for future environmental management of cascade reservoirs in large rivers.     

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.     

Compensation flows in the UK: A hydrological review

A survey of United Kingdom reservoirs identified the development of compensation flow policy from the early 19th Century to the present day. Details on the level and pattern of releases below 261 impounding reservoirs provided the basis for assessing the spatial and temporal variation in compensation flows and the impact of impoundment on downstream flow regimes. By standardizing compensation releases by the natural pre-impoundment mean discharge, release patterns from reservoirs with a wide range of both catchment area and climate could be compared. Analysis of the change in flood and low flow frequency following impoundment enabled the impact of reservoir regulation to be evaluated at the regional scale. For individual reservoirs the natural downstream increase in river flow provides a basis for determining the length of river reach which is sensitive to regime changes. The majority of present day compensation discharges were set to satisfy river interests which no longer apply or were based on inadequate hydrological or biological information. This has led to a wide range of release patterns and it is therefore recommended that the re-evaluation of existing water resource schemes should include an assessment of the suitability of current release patterns for present day downstream demands and water resource requirements.     

Water quality changes in hyporheic flow paths between a large gravel bed river and off‐channel alcoves in Oregon,USA

Changes in water quality that occur as water flows along hyporheic flow paths may have important effects on surface water quality and aquatic habitat, yet very few studies have examined these hyporheic processes along large gravel bed rivers. To determine water quality changes associated with hyporheic flow along the Willamette River, Oregon, we studied hyporheic flow at six‐bar deposit sites positioned between the main river channel and connected lentic alcoves. We installed piezometers and wells at each site and measured water levels and water quality in river, hyporheic and alcove water. Piezometric surfaces along with substrate characteristics were used to determine hyporheic flow path direction and hyporheic flow rate. At all sites, hyporheic flow moved from the river through bar deposits into alcove surface water. Stable isotope analysis showed little influence of upwelling groundwater. At a majority of sites, hyporheic dissolved oxygen and ammonium decreased relative to river water, and hyporheic specific conductance, nitrate and soluble reactive phosphorous increased relative to river water. At three sites, hyporheic temperature decreased 3–7°C relative to river water; there was less temperature change at the other three sites. At the two sites with the highest hyporheic flow rates, hyporheic cooling was propagated into the alcove surface water. Hyporheic changes had the greatest effect on alcove water quality at sites with highly permeable substrates and high‐hyporheic flow rates. The best approach to enhancing hyporheic flows and associated water quality functions is through restoring fluviogeomorphic channel processes that create and maintain high‐permeability gravel deposits conducive to hyporheic flow. Copyright © 2006 John Wiley & Sons, Ltd.     

Flow regime alterations under changing climate in two river basins: implications for freshwater ecosystems

We examined impacts of future climate scenarios on flow regimes and how predicted changes might affect river ecosystems. We examined two case studies: Cle Elum River, Washington, and Chattahoochee–Apalachicola River Basin, Georgia and Florida. These rivers had available downscaled global circulation model (GCM) data and allowed us to analyse the effects of future climate scenarios on rivers with (1) different hydrographs, (2) high future water demands, and (3) a river–floodplain system. We compared observed flow regimes to those predicted under future climate scenarios to describe the extent and type of changes predicted to occur. Daily stream flow under future climate scenarios was created by either statistically downscaling GCMs (Cle Elum) or creating a regression model between climatological parameters predicted from GCMs and stream flow (Chattahoochee–Apalachicola). Flow regimes were examined for changes from current conditions with respect to ecologically relevant features including the magnitude and timing of minimum and maximum flows. The Cle Elum's hydrograph under future climate scenarios showed a dramatic shift in the timing of peak flows and lower low flow of a longer duration. These changes could mean higher summer water temperatures, lower summer dissolved oxygen, and reduced survival of larval fishes. The Chattahoochee–Apalachicola basin is heavily impacted by dams and water withdrawals for human consumption; therefore, we made comparisons between pre‐large dam conditions, current conditions, current conditions with future demand, and future climate scenarios with future demand to separate climate change effects and other anthropogenic impacts. Dam construction, future climate, and future demand decreased the flow variability of the river. In addition, minimum flows were lower under future climate scenarios. These changes could decrease the connectivity of the channel and the floodplain, decrease habitat availability, and potentially lower the ability of the river to assimilate wastewater treatment plant effluent. Our study illustrates the types of changes that river ecosystems might experience under future climates. Copyright © 2005 John Wiley & Sons, Ltd.     

Man's Impact on Water Resources

ABSTRACT

The SSARR model is applied to simulate flood levels in the Mekong river delta. The Mekong and Bassac rivers which are two main rivers and the Vaico river are interlaced together with canals and floodplains with storage. The flow exchanges between these rivers, and the tidal influences in their estuarial reaches make it necessary to utilize the backwater mode of the SSARR model in the simulation. In addition to the backwater mode, the diversion algorithm of the SSARR model is also used to take into account the over bank flow conditions.

Since the river storages during the flood periods are considerable, the rivers have been divided into several storage reaches or reservoirs. The flow between these storage reaches depends on the water level differences between each pair. Moreover, the various flood plain storages are also represented by a network of interconnected storage reaches or reservoirs. The upstream boundary condition is the discharge in the Mekong river at Phnom Penh, and the downstream boundary conditions are the tides at the mouths of the three rivers. Rainfall over the study area is also considered as input to the model.

In general, very close agreement is obtained in the calibration and verification except at thte stations Can Tho and My Thuan where tidal influences are significant. However, good results are obtained for the envelope of high tides at these two stations. Sensitivity analysis is made to determine the effects of the overbank flow parameters, and the exchange flow parameters on the flow in the rivers. More data collection of delta discharges and water levels is recommended for the future to improve the existing backwater relationship for better simulation of tidal fluctuation in the estuarial reaches of the delta.     

梯级水库溃坝洪水模拟

为充分利用水能资源和实现流域水资源的综合调控,我国在众多河流上进行了梯级开发,因此,对梯级水库大坝群的溃坝洪水风险开展研究具有重大意义。结合贵阳松柏山、花溪梯级水库实际工程状况,在假设梯级水库大坝出现溃坝事件时,以河道溃坝数学模型进行溃坝洪水演进模拟分析,获得上游松柏山水库溃坝、花溪水库库区及其下游风险区域的水位变化过程。结合数学模型计算成果,分析了梯级水库群不同水位组合溃坝洪水对下游的影响。     

Reservoir Management in the Duero Basin (Spain): Impact on River Regimes and the Response to Environmental Change

The climatic conditions of the Iberian Peninsula result in an imbalance between water availability and demand, which is largely managed through the many dams that were built during the 20th century. However, dam operations modify the natural functioning of rivers and related subsystems. In this study we investigated the effect of reservoirs on river regimes in the Duero basin, which is one of the largest river basins in Spain. This involved calculation of a modified impoundment ratio index, and assessment of the correlations between monthly inflows and outflows. Water resources in the basin have decreased markedly during the last five decades, so we also studied how patterns of management have adapted to less water availability in the region. A significant correlation was found between the level of impoundment and the alteration of river regimes by dams. The degree of regulation was highly dependent on annual inflows into the reservoir, and consequently alterations to river regimes were more intense during dry years. The basic pattern of flow regulation involved the storage of water during winter and spring in preparation for high water demand in summer, when natural flows are low. A combination of trend and cluster analyses revealed three responses of reservoir managers to decreasing inflows during the study period: (i) for several reservoirs the level of storage was reduced; (ii) for many reservoirs, particularly those for hydropower production, the storages were increased; and (iii) for the remainder the storage levels were maintained by adjusting the outflows to the decreasing inflows. The results suggest the absence of a common approach to reservoir management, and the dominance of other interests over environmental concerns, particularly in the context of hydrological change in the basin.     

Characterization of natural and environmental flows in New Brunswick,Canada

A good understanding of the natural flow regime plays an important role in many hydrological studies. Also important in such studies is the quantification of environmental flows. This study focuses on flow metrics that best describe the natural flow regime and the hydrological characteristics for rivers in New Brunswick (Canada) as well as quantifying environment flows for these rivers. New Brunswick rivers have a mean annual flow (MAF) of approximately 23 L s^?1 km^?2, which is also reflective of the water availability. The frequency analysis showed that low flows (T = 2–50 years, where T is the recurrence interval) were all below the 10% MAF. Environmental flow methods based on the MAF and flow duration analysis (median flow) showed good regional regression equations. However, flow duration methods showed high variability especially at flows between Q₈₀ and Q₁₀₀. Flow targets based on the 25% MAF, Q₅₀ and 70% Q₅₀ were used to estimate environmental flows, particularly during low‐flow periods (winter and summer). Results showed that the 70% Q₅₀ method should be used with caution in summer as this method provided flows in the range of 15–16% of MAF. Other methods provided environmental flows higher than 15% MAF, thus, providing better flow protection for aquatic habitat. When comparing water availability for off‐stream use (river flow–environmental flow), different parts of New Brunswick were found to be deficient in flows (i.e., river flows less than environment flows—no extractable water) during the summer and winter low‐flow periods.     

河流生态流量特征图及生态流量评价方法

以水电站大坝下游河道为研究对象,从分析天然日流量特征出发,基于月中值流量和月内典型特征流量绘制了河流生态流量特征图,作为水库下泄生态流量的确定依据。基于此河流生态流量特征图,建立了判断生态流量满足程度的7 d流量偏差率、7 d生态需水保证率、月均生态需水适宜度及基于此3个指标的生态需水综合指标,并给出了各指标的评价依据和评价方法。结果表明,河流生态流量特征图符合各时段河流天然流量的基本特征,可作为指导水库生态泄流的依据;所建立的生态流量评价方法能够反映实际下泄流量与天然流量的变化程度,可用于评价河流的生态流量满足程度。     

河流水电梯级开发水温累积影响研究

从流域开发对河流水温结构的影响方面进行分析,采用现场观测和数学模型计算相结合的方法,对水库水温结构进行研究,同时,对梯级电站下泄水温的累积影响进行数值模拟研究。研究结果表明,高坝大库对河流水温改变大,对水温累积影响的贡献大;流域开发程度越高,累积影响越大,5个梯级比3个梯级累积影响大。这一定量研究成果体现了梯级电站的水温累积影响和群体效应,可推进梯级水电站对河流水温影响的研究进程,并为研究大型电站运行减缓下泄低温水的对策措施提供依据和参考。     

分汊型河道水流运动特性和污染物输移规律研究进展

对分汊型河道的水流运动特性和污染物输移扩散规律的相关研究进行综述,分别从分汊型河道中分汊口、交汇口、整个分汊河道的水流特性及分汊型河道中的污染物输移特性等4个方面进行总结。介绍分汊口和交汇口处的水流从其能量损失、水位变化、水流分离等一维、二维过流特征到三维流动结构和紊动特性的研究过程;从野外现场试验、室内物理模型试验和数学模型模拟等角度出发阐述了整个分汊河道的水流特性;认为污染物在分汊河道中的输运规律研究主要集中在数值模拟上,并进行了回顾和分析;提出分汊河道水流水质输移特性研究中一些有待深入探究的问题,尤其亟待加强不同排放方式、不同污染物密度和不同分汊形态下污染物输移机制的试验研究。     

From a continuous thermal profile to a stepped one: The effect of run of river hydropower plants on the river thermal regime

Both reservoirs and run of river power plants affect the thermal regime of rivers but despite the higher number of the latter few studies have focused on their effect. In this study, we investigated the water thermal regime of Serio River (Northern Italy), a subalpine river regulated by a reservoir and characterized by a cascade system of run of river power plants. Water temperature has been monitored continuously for more than 4 years at the extremes of 4 stretches subjected to water diversion and thermal alterations have been quantified. Our results show that hydroelectric power plants act locally causing a considerable thermal alteration that increases with the distance from the diversion weir. Indeed, within the by-passed stretch, the rate of warming doubles the natural gradient (0.47°C/km vs. 0.19°C/km annually) with peaks in summer (0.73–0.90°C/km on average). By contrast, the run of river power plants keep the water temperature almost constant in the diversion channels. Thus, a cascade system of run of river plants shifts the overall riverine thermal regime from a continuous to a “stepped” longitudinal profile. Results highlight that the thermal effects of run of rivers plants are not negligible and should be considered and monitored continuously. Since there are thousands of hydropower plants powered by flowing waters it is time to consider their thermal impacts in environmental flow policies and bioassessment programs.     

Biogeomorphic characterization of floodplain forest change in response to reduced flows along the Apalachicola River,Florida

The Apalachicola–Chattahoochee–Flint (ACF) River basin is an important ecological and economic component of a three‐state region (Florida, Alabama and Georgia) in the southeastern U.S. Along the Apalachicola River in northwest Florida, the duration of floodplain inundation has decreased as a result of declining river levels. Spring and summer flows have diminished in volume because of water use, storage and evaporation in reservoirs, and other anthropogenic and climatic changes in the basin upstream. Channel erosion from dam construction and navigation improvements also caused river levels to decline in an earlier period. In this paper, we document trends in floodplain forest tree species composition for the interval spanning these influences. Historic tree inventories from the 1970s were compared to present‐day forests through non‐metric multidimensional scaling, indicator species analysis (ISA) and outlier detection. Forests are compositionally drier today than in the 1970s. Overstory to understory compositional differences within habitats (levees, high/low bottomland forest and backswamps) are as large as the species contrasts between habitats. Present‐day forests are also compositionally noisier with fewer indicator species. The largest individual declines in species density and dominance were in backswamps, particularly for Fraxinus caroliniana Nyssa ogeche and Nyssa aquatica. We discuss how contrasts in the compositional change signal for levee and backswamp landform habitats reflect a complex biogeomorphic response to fluctuating river flows for alluvial rivers in humid climates. Copyright © 2009 John Wiley & Sons, Ltd.     

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.