Altered Flow Regimes Research Articles

Spectral Signatures of Flow Regime Alteration by Dams Across the United States

AbstractRiver scientists strive to understand how streamflow regimes vary across space and time because it is fundamental to predicting the impacts of climate change and human activities on riverine ecosystems. Here we tested whether flow periodicity differs between rivers that are regulated or unregulated by large dams, and whether dominant periodicities change over time in response to dam regulation. These questions were addressed by calculating wavelet power at different timescales, ranging from 6 hr to 10 years, across 175 pairs of dam‐regulated and unregulated USGS gages with long‐term discharge data, spanning the conterminous United States. We then focused on eight focal reservoirs with high‐quality and high‐frequency data to examine the spectral signatures of dam‐induced flow alteration and their time‐varying nature. We found that regulation by dams induces changes not only in flow magnitude and variability, but also in the dominant periodicities of a river's flow regime. Our analysis also revealed that dams generally alter multi‐annual and annual periodicity to a higher extent than seasonal or daily periodicity. Based on the focal reservoirs, we illustrate that alteration of flow periodicity is time varying, with dam operations (e.g., daily peaking vs. baseload operation), changes in dam capacity, and environmental policies shifting the relative importance of periodicities over time. Our analysis demonstrates the pervasiveness of human signatures now characterizing the U.S. rivers' flow regimes, and may inform the restoration of environmental periodicity downstream of reservoirs via controlled flow releases—a critical need in light of new damming and dam retrofitting for hydropower globally.

Hydrological impacts of dam regulation for hydropower production: The case of Lake Sibinacocha, Southern Peru

Study regionVilcanota-Urubamba river basin, Southern Peru. Study focusHydraulic infrastructure plays a fundamental role for energy production, drinking and irrigation water storage and flood control in regions with seasonal river flow. The high-Andean Lake Sibinacocha has been regulated since 1988 to increase energy production of the Machupicchu hydropower plant. In this study, river streamflow changes are evaluated by analyzing precipitation and discharge trends using indicators of hydrologic alteration and ecoflow for natural (1965–1987) and altered (1988–2016) flow regimes. New hydrological insights for the regionFor the altered flow regime, an ecodeficit of about 20% (compared to natural river flow) and an ecosurplus > 30% were found during the wet season (December-February) and dry season (June-August), respectively. These changes have reduced the risk of water shortage (dry season) and flood (wet season) and contribute to increasing water use including hydropower production, irrigation and drinking water. However, river alteration might lead to considerable impacts on riverine ecosystems.Despite major limitations related to data scarcity and complex environmental processes in the basin, our results highlight the usefulness of combined methods of hydrological alteration and ecoflow to effectively evaluate water regime changes in regulated basins. An integrated scientific approach is necessary to address uncertainties and develop meaningful future water availability scenarios that guide hydropower projects with improved water and energy security considering minimal impacts on human and natural systems.

Improved indicators of hydrological alteration for quantifying the dam-induced impacts on flow regimes in small and medium-sized rivers

Dam construction is the main factor altering the flow regimes, while few studies have described that in small and medium-sized rivers (SMRs). The universal indicators of hydrological alteration (IHA) that are widely used in large rivers calculate the parameters just on the annual scale and omit the intra-annual seasonal differences of the flow regimes in SMRs. To fully quantify dam-induced impacts on the flow regimes in SMRs, this paper proposes the improved IHA (IIHA) based on the universal IHA. Then two methods of range of variable approach (RVA) and histogram matching approach (HMA) are used to assess the flow regime alteration. Finally, two indicators of water quantity level (WQL)/hydrological alteration (HA) defined by the parameters in IIHA are employed to evaluate the influence of flow regime alteration on the riverine ecosystem. The case study of a typical SMR named Liujiaping River in Hunan Province, China, verifies the necessity of improving IHA where more hydrological parameters calculated in different periods can comprehensively reflect the flow regime alteration. We find that the integrated hydrological alteration of Liujiaping River assessed by RVA and HMA are both at a high level, and the flow regimes have been significantly altered after the dam construction. Also, the indicators of WQL and HA have higher correlation coefficients with 77 of IIHA parameters and thus can retain as much information of the flow regimes as possible to evaluate the influence of its alteration on the riverine ecosystem.

How vulnerable are Wular Lake fishing communities compared to non‐fishing communities in rural Kashmir? Insights and interventions for development

AbstractWular Lake, one of the largest wetlands in Asia (13,000 ha in Kashmir Valley) is a biodiversity‐rich Ramsar site supporting nearly 32,000 rural people involved in harvesting fish, water chestnuts, lotus stems and fodder. With 2914 active fishers, Wular Lake contributes 54% of Kashmir's total fish catch. Large‐scale land‐use changes and global warming have resulted in siltation, reduced lake area and alterations in the water flow regime, threatening lake ecology and human livelihoods. A novel census‐based multi‐criteria Socio‐Economic Vulnerability Index (SEVI) was used to assess the vulnerability status of 23 fishing and 115 non‐fishing adjacent villages. All the fishing villages exhibited ‘low’ (0.21–0.40) vulnerability levels, except Kunas village (0.42). Interestingly, no significant differences could be found in vulnerability levels between fishing (23) and non‐fishing (115) villages, although the former exhibited relatively higher social sensitivity. A graphical decision matrix based on sensitivity and adaptive capacity scores provided a snapshot of vulnerability status for each village. Using SEVI scores, drivers (i.e., key indicators responsible for high sensitivity & low adaptive capacity) were identified and corresponding village‐specific development interventions are suggested with Kunas as an illustrative case village. Improving education levels, strengthening community and rural infrastructure, and expanding livelihood options were key short‐term strategies for improving resilience in Kashmir's Wular region.

Assessment of hydrologic impact on flow regime due to dam inception using IHA framework.

In the last century, thousands of dams and diversions have been built to regulate the streamflow, resulting in water impoundment in the upstream and frequent drought conditions in the downstream. It has pressured researchers to study flow regime change and its complication on the downstream biota. The present study planned to develop a framework for trend analyzing of river flow and detecting flow regime change after the inception of Isapur and Arunavati dams, situated on the upstream side of Penganga bridge. Mann-Kendall (MK) and Sen's slope estimator for trend analysis and Indicators of Hydrologic Alteration (IHA) for flow regime alteration analysis were utilized. A total 26 parameters showed negatively altered flow regime with a magnitude varying from - 5.56 to - 100%. Fourteen altered parameters were modified drastically (more than 50% decrease) with the highest modification in 30-day maximum (100%) post-single dam inception. a total of 13 parameters were negatively altered with alteration value - 9.09 to - 86.36% post-double dam inception, out of which, three parameters were severely altered, with the highest alteration in the month of June. The period (1983-1994) was more altered than 1995-2016. This shows that Isapur dam has higher impact on flow regime change than Arunavati dam. Information about alteration of hydrological parameters will be helpful to improve the water flow regulation at Isapur and Arunavati dams for restoring river ecology on the downstream side.

Flow regime changes in the Lancang River, revealed by integrated modeling with multiple Earth observation datasets

The flow regime change of rivers, especially transboundary rivers, affected by reservoir regulations is evident worldwide and has received much attention. Investigating dam-induced flow regime alterations is essential for understanding potential adverse downstream effects and facilitating dialogue around coordinated water use in transboundary basins, such as the Lancang River Basin (LRB). This study explored the value of combining several types of satellite Earth observation (EO) datasets that monitor different water balance components to constrain the parameter space of lumped conceptual hydrological models. Thus, we aimed to reconstruct the natural flow regimes upstream and downstream of the cascade reservoirs. Specifically, reservoir water storage changes were first estimated using satellite imagery and altimetry datasets. Then, storage changes were combined with hydrological model simulations of reservoir inflow to estimate the regulated flow regime downstream. Our results showed that integrated hydrological modeling combined with EO datasets exhibited better overall performance. Continuous warming and drying of the LRB resulted in a decrease in discharge of approximately 47 %. By comparing the simulated natural and regulated flow regimes, we revealed the pivotal role of the Xiaowan and Nuozhadu reservoirs in regulating natural flows. The wet season shortens (approximately 45 days), the flood peak flattens, and the low flow in the dry season has primarily increases. The two reservoirs attenuated 50 % of the flood peaks in the wet seasons and mitigated droughts by releasing up to 100 % of the natural flows in the dry seasons at the China–Laos border. Overall, these results enhance the understanding of upper reservoir operation, and the approaches can be applied to studies of dammed basins under climate change scenarios when knowledge of the upstream area is limited.

Fish movements in response to environmental flow releases in intermittent rivers

Abstract Variation in river flow is a strong behavioural determinant for the movement of many freshwater fish species and often is linked to key aspects of their life cycle. The alteration of natural flow regimes to meet human water demands can result in changes to this variability, and cause declines in water‐dependent biota. Environmental flows are used as a remediation tool in some regulated rivers with the intention of restoring aspects of the natural flow regime to benefit riverine species, although empirical data are required to inform the efficacy of these interventions. Using acoustic telemetry, we quantified the movement responses of two large‐bodied native fish species (freshwater catfish, Tandanus tandanus and Murray cod, Maccullochella peelii) to variations in river flow over 4 years in two intermittent regulated rivers in the northern Murray–Darling Basin, Australia. Both rivers received periodic environmental flow releases and also there were several large natural flow events during the study period. Both species displayed a range of intra‐specific movement behaviours. Analysis of individuals' movements revealed five distinct functional groups, which were represented in both species. We found that periods of environmental flow delivery played an important role in the movement behaviour of both species. Murray cod were more likely to move during periods of environmental flow releases, whereas freshwater catfish were less likely to move on an environmental release following higher antecedent flows. No large‐scale philopatric movements were observed, yet Murray cod were more likely to move during the breeding period, indicative of nest site selection. We also found that the likelihood of movement in both species was higher in the smaller of the two rivers for a given magnitude of flow. Our results suggest that environmental flows may benefit certain fish species by facilitating, rather than cueing breeding behaviours, allowing individuals improved access to, and provision of, higher‐quality nesting habitats. These findings will aid water managers in creating economical and targeted environmental flow releases, timing larger flow pulses for the species with flow‐cued reproductive strategies, and providing a continuity of smaller baseflows for species whose reproduction is not dependent on flow cues but may be enhanced by greater connectivity.

Causes & effects of upstream-downstream flow regime alteration over Catchment-Estuary-Coastal systems

The construction of large dams along rivers has significantly changed the natural flow regime, reducing the inflow into many lakes and terminal wetlands. However, the question of the impact of dam operation on downstream estuarine wetlands has less been taken into account. Spatio-temporal flow regime alteration in the Mond River shows the complexity of drivers affecting the estuary-coastal system named the Mond-Protected Area in southern Iran. To this end, we applied river impact (RI) and Indicator of hydrological alteration (IHA) methods on monthly and daily river flow data across the basin. Based on the river impact method, a “drastic” impact below two in-operation (Tangab and Salman Farsi) dams, with RI values of 0.02 and 0.08, diminish to a ‘severe’ impact with RI value of 0.35 at the last gauge (Ghantareh) on the main corridor of the Mond river due to the addition of flow from a large mid-basin (about 20,254 km2). Furthermore, the degree of hydrological alteration (daily flow analysis) at mid-stream (e.g., Dehram gauges) was similar to the unregulated upstream tributaries (e.g., Hanifaghan gauges). The remote sensing analysis in the Mond Protected Area showed the prevailing impact of sea-level rise in the Persian Gulf with the inundation of the coastal area and a shift of vegetation in a landward direction which complied with standardized precipitation index (SPI) values as a meteorological drought indicator. Thus, the consequence of climate change (e.g., sea-level rise, draught) has a higher impact on the protected area than the upstream river regulation and land-use change in the Mond basin. The holistic approach and the catchment-level study allowed us to see the complexity of the drivers influencing the estuary-coastal system.

In-river weir effects on the alteration of flow regime and regarding structural stream habitat

In-river hydraulic structures may negatively impact aquatic ecosystems, including changing river flow regimes and the habitat quality of endemic fish habitats. This study integrates the wavelet analysis and the time-series exceedance probability method to quantify the degree of evolution in the river flow regime caused by weirs and determine the critical time scale most affected in the environmental flow. Hydrodynamic and habitat models were conducted to analyze the influence of the variant flow characteristics between the dry and wet seasons on the habitat structure. Four endemic fishes were selected as the indicator species to represent different habitat preferences and reveal the spatiotemporal changes in habitat quality. The flow difference between dry and wet seasons became more extensive after the weir construction, indicating the alternative flow regime downstream. The annual deviation of daily flow dramatically raised, showing the flow rates decline in the dry season and enlarge in the wet season. The findings exhibit that the downstream flow amplitude changes regardless of time scale after constructing the weir. The flow data from the 2-days scale to the 1-week scale were most adjusted, meaning that these periods to meet the environmental flow decline significantly. The critical environmental flow corresponding to lower flow events was apparently influenced in the dry season, in which the Q75 and Q90 displayed the highest degradation in fish habitat quality. In addition, the simulation results of the habitat model found that glide is the primary habitat type. The total water area and the habitat diversity were reduced, while the habitat quality also had a lowered tendency after the weir operation. The habitat run was most degraded, and the riffle was also significantly negatively impacted. Regarding the endemic fish species,F. lacustresuffered the most harmful effects on suitable-habitat quality and quantity from the weir. This study suggests considering natural flow regimes at the planning and design phases of weir construction and rehabilitating these essential habitat patterns to mitigate the downstream ecosystem impacts.

Quantifying flow and nonflow management impacts on an endangered fish by integrating data, research, and expert opinion

AbstractManagers charged with recovering endangered species in regulated river segments often have limited flexibility to alter flow regimes and want estimates of the expected population benefits associated with both flow and nonflow management actions. Disentangling impacts on different life stages from concurrently applied actions is essential for determining the effectiveness of each action, but difficult without models that integrate multiple information sources. Here, we develop and fit an integrated population model for endangered Rio Grande Silvery Minnow (Hybognathus amarus) in the Middle Rio Grande, New Mexico. We integrate catch per unit effort monitoring data collected during 2002–2018 with population estimates, data collected during rescue of minnow from drying pools, habitat availability estimates, laboratory results, releases of hatchery reared minnow, and expert opinion. We use expert elicitation to develop a larval carrying capacity index as an informed proxy for the complex interactions among flow, habitat, and life history in this species. We evaluate the model using out‐of‐sample forecasts of 2019 and 2020, develop an algorithm to identify supplemental water releases that maximize benefits to the minnow, and quantify the effectiveness of various actions. Experts generally agreed on the duration and timing of flow requirements and disagreed regarding the importance of different magnitudes. The integrated model with the larval carrying capacity index outperformed two alternative models in forecasting catch in 2019 and 2020. The model estimates that minnow abundance varied by more than three orders of magnitude between 2002 and 2018 and that in a few years recruitment was limited by spawner abundance. Evaluation of the expected benefits of flow and nonflow management actions to fall population abundance across different years suggests that efficient addition of water to the base hydrograph is the most effective action in most, but not all years. Many actions are effective only under certain hydrologic and population conditions and the effectiveness of different actions varies in different sections of the study area. Widespread water extraction and river regulation combined with periodic drought and ongoing climate change may necessitate creative management of federally listed fish species in arid systems informed by thorough analyses of management effectiveness.

Spatially structured relationships between white banana prawn (Penaeus merguiensis) catch and riverine flow in the Northern Prawn Fishery, Australia

Water resource development can lead to the significant alteration of natural flow regimes, which can have impacts on the many aquatic species that rely on both freshwater and estuarine environments to successfully complete their lifecycles. In tropical northern Australia, annual catches of commercially harvested white banana prawns (WBP) are highly variable in response to environmental conditions, namely rainfall and subsequent riverine flow. However, little is known about the spatial extent to which flow from individual rivers influences offshore WBP catch. In this study, we quantify how the relationship between WBP catch in the Gulf of Carpentaria is influenced by flow from the Mitchell River, Queensland Australia. We used a Bayesian framework to model both prawn presence and catch per unit effort, and found evidence that multiple components of the flow regime contribute to fishery catch. We also found evidence to suggest that the relationships between prawn presence and flow were spatially structured across the fishing ground. Our results suggest that attributing fishery catch to a single river remains challenging, though highlights the importance of maintaining natural flow regimes to support a highly valuable commercial fishery species in the face of potential water resource development.

Shedding light on the decline of Iberian freshwater fish species over the period 1980–2020

Abstract Freshwater fish biodiversity is experiencing an alarming decline worldwide. Understanding the main factors behind its deterioration is a key step for ecosystem restoration. In this work, large‐scale and long‐term data were used to identify the causes of the decline of native species richness in Castilla‐La Mancha. This region in central Spain covers part of six river basins belonging to four of the 11 biogeographical provinces for freshwater fish in the Iberian Peninsula. Firstly, we built a dataset that associates the presence of several fish species and a wide range of environmental variables (e.g. hydrological and hydromorphological indicators, land use classes, presence of alien fish species) at selected river sites for two different time periods (1980–2000 and 2001–2020). Secondly, we conducted an exploratory data analysis to identify possible temporal trends in the dataset. Finally, we applied the random forest algorithm to predict the response of different ecological guild‐based metrics of fish richness to the selected variables. The exploratory data analysis revealed a decrease in native fish species richness in 74% of the area studied. There was no sustained temporal trend for stressor variables, except for the number of alien species, which increased in most river sites (63%). The models of the richness of native rheophilic, native intolerant, alien rheophilic, and alien limnophilic species performed satisfactorily. Magnitude of maximum discharge, presence of alien species, land use in the catchment area and altitude were the most important predictors of richness of native intolerant and rheophilic species. Alien limnophilic species proved to be sensitive to variables related to flow regime alteration, such as the presence of dams and the number of river flow reversals, while a less degraded habitat was found to be favourable to alien rheophilic species. The results suggest that the cumulative effect of persistent altered flow regimes and water pollution, coupled with a strong increase in the number of alien species, have led to the decline of native species in the area studied. The restoration of near‐natural magnitudes of high flows when implementing environmental flows emerged as a key measure to restore ecosystem integrity. Starting from a long‐term and large‐scale dataset, this study provides new, quantitative insights into stressor–ecosystem relationships in rivers and could inform future environmental policy initiatives because it has identified the main factors leading to native fish decline and alien fish proliferation. Our findings emphasise the importance of considering metrics based on fish assemblage composition and ecological functional groups in order to disentangle the effects of stressors on fish communities.

Modelling water temperature in the lower Olifants River and the implications for climate change

Freshwater systems in southern Africa are under threat of climate change, not only from altered flow regimes as rainfall patterns change, but also from biologically significant increases in water temperature. Statistical models can predict water temperatures from air temperatures, and air temperatures may rise by up to 7 °C by 2100. Statistical water temperature models require less data input than physical models, which is particularly useful in data deficient regions. We validated a statistical water temperature model in the lower Olifants River, South Africa, and verified its spatial applicability in the upper Klaserie River. Monthly and daily temporal scale calibrations and validations were conducted. The results show that simulated water temperatures in all cases closely mimicked those of the observed data for both temporal resolutions and across sites (NSE>0.75 for the Olifants River and NSE>0.8 for the Klaserie). Overall, the model performed better at a monthly than a daily scale, while generally underestimating from the observed (indicated by negative percentage bias values). The statistical models can be used to predict water temperature variance using air temperature and this use can have implications for future climate projections and the effects climate change will have on aquatic species. Significance: Statistical modelling can be used to simulate water temperature variance from observed air temperature, which has implications for future projections and climate change scenarios. While there are many other factors affecting water temperature, air temperature accounts for up to 95% of water temperature variance. The model used can successfully simulate water temperature variance for different rivers.

Climate change and local anthropogenic activities have altered river flow regimes across Canterbury, New Zealand

River flow regimes influence ecologic, cultural, social, aesthetic, and economic values. Detecting changes in river flows and attributing their causes is important but challenging due to the combined influence of climate and relevant local activities, and the lack of data on water abstraction, drainage modification or land use management. This study assessed the degree to which trends in river flows could be attributed to changes in climate versus local anthropogenic activities across Canterbury, Aotearoa New Zealand. Trends were assessed for a period that started immediately after a change in regulatory regime in 1991 and ended in 2020, that coincided with increases in water abstraction and changes in water management practices. Trends in observed summer conditions indicated that rainfall was stable, temperature increased, and flows decreased for many sites during the assessed period. Models representing flow as a function of rainfall and temperature were trained and tested using cross-validation for an earlier baseline period. Predictions for the 1991–2020 period made with the models were used to account for the effect of change in climate. The difference between predicted and observed flows were attributed to changes in local activities. Decreases in summer flows were partially associated with changes in climate, but changes in summer flows in several catchments were also associated with local activities. The findings indicate changes to both climate and local activities have combined to alter flow regimes, suggesting that hydrological impacts of local activities should be considered alongside climate change when making river flow management decisions.

Risks of mining to salmonid-bearing watersheds.

Mining provides resources for people but can pose risks to ecosystems that support cultural keystone species. Our synthesis reviews relevant aspects of mining operations, describes the ecology of salmonid-bearing watersheds in northwestern North America, and compiles the impacts of metal and coal extraction on salmonids and their habitat. We conservatively estimate that this region encompasses nearly 4000 past producing mines, with present-day operations ranging from small placer sites to massive open-pit projects that annually mine more than 118 million metric tons of earth. Despite impact assessments that are intended to evaluate risk and inform mitigation, mines continue to harm salmonid-bearing watersheds via pathways such as toxic contaminants, stream channel burial, and flow regime alteration. To better maintain watershed processes that benefit salmonids, we highlight key windows during the mining governance life cycle for science to guide policy by more accurately accounting for stressor complexity, cumulative effects, and future environmental change.

Effects of flow recession regime on stranding of Rio Grande silvery minnow suggests that conservation actions must overcome evolutionary traps

Abstract Flow intermittence is a major disturbance for riverine fishes. Many species of fishes that evolved in naturally intermittent streams have specialized adaptations to survive drought such as movement to refuge habitats. Conservation of fishes in river systems with highly altered flow regimes requires understanding of how individuals and populations respond to flow intermittence. Here, flow was manipulated in two 16‐km segments of the Rio Grande, New Mexico, to determine whether managed flow recession could reduce stranding imperilled Rio Grande silvery minnow in isolated pools. Slower flow recessions did not decrease stranding in isolated pools over the range tested. Flow recession rate appeared to have an adverse impact on stranding; fewer fish were stranded under faster recession rates, but complex interactions were evident. Fish were stranded throughout both segments regardless of flow recession rate. No evidence of a synchronized movement response was observed. Instead, many Rio Grande silvery minnows were probably trapped within proximal habitats, which dry completely and function as evolutionary traps, rather than moving to areas of perennial flow. Use of proximal habitats during streamflow intermittence has implications for management of the species and mitigation of mortality, including managed flow recession rates, and position and function of refuge habitats. Effective mitigation for flow intermittence and stranding in streams with highly modified flow regimes will depend on the life history and behavioural response of the species. For Rio Grande silvery minnow, facilitating adaptive behaviour would require creating refuge areas that increase fish survival for weeks to months, as reducing the attractiveness of the traps is likely to be impossible given lack of a synchronized movement response to declining flows. Mismatching life‐history strategies, behaviour, and scale of conservation actions may result in ineffective management and continued dependence on intensive management actions such as augmentation with hatchery fish.

Drivers of periphyton biomass in south-western Cape rivers, South Africa, and the implications for management

In open-canopied temperate rivers, periphyton plays a pivotal role by converting dissolved nutrients into food for biota. International research suggests that periphyton is particularly responsive to flow and nutrient alterations and therefore ideally suited to aiding the prediction and monitoring of the effects of altered flow regimes. Given that one of South Africa’s key focus areas for river management has been the provision of flows to sustain aquatic ecosystems, this study aimed to quantify the relative importance of potential environmental drivers of periphyton, particularly flow, under different nutrient conditions in rivers of the south-western Cape. The study focused on two perennial rivers, the Berg and Molenaars Rivers. Quantitative monthly samples of periphyton biomass were collected from four sites, representative of different flow and nutrient regimes, over a 21-month sampling period between September 2007 and May 2009. The frequency of flood disturbance over the wet season was found to be the single most important driver of periphyton communities. It accounted for almost 75% of the total variation in periphyton biomass under natural flow and nutrient-poor conditions and it was therefore identified as the primary driver of periphyton communities. Its role was smaller under moderately enriched conditions and accounted for 45% of the total variation. These findings suggest that periphyton in south-western Cape rivers has great potential as a tool for predicting and monitoring the effects of altered flow regimes under different nutrient enrichment regimes. Together with some insight into community structure, the next step is to develop periphyton indicators that reflect different aspects of ecosystem integrity, for use in scenario modelling and other management aspects.

PairwiseIHA: A python toolkit to detect flow regime alterations for headwater rivers

The PairwiseIHA is an open-source toolkit written in Python that enables the user to quickly obtain the Indicators of Hydrologic Alteration (IHA) from inflow and outflow time series for headwater reservoirs. Specifically, the IHA metrics obtained from inflows (outflows) are considered as natural (regulated) and the differences are used to indicate how the magnitude, frequency, duration, timing and rate of change are altered by reservoir operation. It extends the functionality of the popular IHA software for analyzing concurrent inflows and outflows. The results of three case study reservoirs in California highlight that reservoir operation leads to considerable alterations in the five components of the flow regime. The findings on flow regime alteration are consistent across wet, normal and dry years. Overall, the PairwiseIHA can serve as an effective tool to investigate alterations in flow regime induced by human activities for water managers, scientists, natural resource agencies and water users.

Long-time impact of a large dam on its downstream river’s morphology: determined by sediment characteristics, pollutants as a marker, and numerical modelling

Many river systems are regulated by dams, which causes an altered flow regime and sediment deficit in the downstream reach. The Rur dam (North Rhine-Westphalia, Germany), constructed from 1900 to 1959, serves as a model example of the impact of a large dam in a European low mountain area on downstream morphology. Today, a new equilibrium incorporates flow regulations, a deficit in suspended sediment supply, and an increased mean sediment diameter downstream of the dam. A hybrid examination of field measurements and numerical modelling shows that the sediment deficit and increased mean sediment diameters downstream of the Rur dam are superimposed by the lithostratigraphy and the sediment supply of tributaries. However, the discharge regulations lead to floodplain decoupling downstream of the dam. Furthermore, the Rur dam functions as a pollutant trap. Overall, more studies on the impact of damming on downstream reaches are needed to classify the impact depending on the river type for sustainable water management.

Impact of Hydrologic Alteration on Brazos River Pelagophilic Minnows

AbstractThe lower Brazos River has been profoundly altered since the construction of multiple impoundments. The resulting river regulation poses problems for organisms adapted to a natural flow regime and is thought to play a major role in the imperilment of seven native minnows (Smalleye Shiner Notropis buccula, Sharpnose Shiner N. oxyrhynchus, Shoal Chub Macrhybopsis hyostoma, Plains Minnow Hybognathus placitus, Ghost Shiner N. buchanani, Silverband Shiner N. shumardi, and Chub Shiner N. potteri) that synchronize spawning during high flow pulses. Four of the species are presumably extirpated from the region, but their declines and losses differed across time. To address potential reasons for species' declines, we assessed pre‐ and postimpoundment changes in flow regime from five gauging stations across the lower Brazos River to identify river flow characteristics that depart from the natural preimpoundment flow regime. For the seven minnows, there was a strong relationship between the year they were last documented and their historical percentage occurrence in the lower river. Differences in flow duration curves and flow behavior were generally consistent with position in the drainage such that the greatest changes occurred in the highly fragmented upper reach of the study area. However, extensive hydrologic alteration was apparent at all five localities. The extirpated species in the lower Brazos River may have been more sensitive to altered flow regimes and accompanying geomorphic change or were population sinks dependent on recolonization or drift from upstream populations. The relationships between pelagic‐broadcast‐spawning minnows and the flow regime are complex, and nonflow variables, such as population fragmentation, thermal regimes, geomorphology, and larval habitat dynamics, should be considered in environmental flow assessments to aid our understanding of biodiversity loss in this group of species across the North American plains. Managing flows to support species that depend on natural flow regimes will require not only a better understanding of their reproductive life histories, but also the ecology and physiology of early life history stages.