A critical analysis of regulated river ecosystem responses to managed environmental flows from reservoirs

Abstract

Summary The flow regime of a river is fundamental in determining its ecological characteristics. Impoundment of rivers has been documented to severely impact the natural flow regime, resulting in abiotic and biotic changes in downstream ecosystems. Contemporary water legislation is driving increasing concern among environmentalists and water resource managers with respect to how these impacts can be mitigated. This has stimulated research aimed at assessing the relationship between reservoir outflow modification (i.e. managed environmental flows) and downstream ecosystem responses. We carried out a critical review and synthesis of the global literature concerning post‐impoundment reservoir outflow modification and associated downstream biotic and abiotic responses. Seventy‐six studies published between 1981 and 2012 were analysed. In contrast to previous studies of this subject, we systematically assessed the methodological quality of research to identify strengths and weaknesses of the approaches. We also undertook a novel quantification of ecosystem responses to flow modification, thus enabling identification of priorities for future research. We identified that: (i) there was a research bias towards North American and Western European studies; (ii) the majority of studies reported changes in flow magnitude (e.g. artificial floods) and primarily focused on traditionally monitored ecological groups (e.g. fish); (iii) relationships between flow, biota (e.g. macroinvertebrates) and water quality (e.g. electrical conductivity and suspended solids concentration) were evident, demonstrating the potential for managed environmental flows to manipulate river ecosystems; (iv) site‐specific factors (e.g. location, climate) are likely to be important as some ecosystem responses were inconsistent between studies (e.g. fish movement in response to increases in flow magnitude); and (v) quality of study design, methodological and analytical techniques varied, and these factors may have contributed to the reported variability of ecosystem response. To advance scientific understanding and guide future management of regulated flow regimes, we highlight a pressing need for: (i) diversification of study locations as well as flow modification and ecosystem response types assessed; (ii) a focus on understanding flow–ecosystem response relationships at regional scales; (iii) further quantitative studies to enable robust statistical analyses in future meta‐analyses; and (iv) robust monitoring of flow experiments and the use of contemporary statistical techniques to extract maximum knowledge from ecological response data.

Streamflow characteristics from modeled runoff time series – importance of calibration criteria selection

“…As stated by Carlisle et al (2010), with high errors in SFC estimates, only considerable flow departures from natural conditions can be detected. Also, inaccurate SFC values can impede the generation of more robust flow alteration-ecosystem change relationships that are ultimately needed for sustainable flow management guidelines (Arthington et al, 2006;Poff and Zimmermann, 2010;Gillespie et al, 2015;Cartwright et al, 2017). As with regional statistical approaches, incorporating SFCs into model objective functions implies that a modeler knows which SFCs are relevant and that the model must be recalibrated if one is interested in additional SFCs.…”

The Brisbane Declaration and Global Action Agenda on Environmental Flows (2018)

“…Whilst progress with environmental flows science and water management since 2007 has been immense, major challenges remain in protecting and restoring the integrity of freshwater ecosystems and the ecological services that sustain human cultures, economies, livelihoods, and well-being (e.g., Arthington, 2012;Rockström et al, 2014;Hart and Doolan, 2017;Horne et al, 2017b;Kennen et al, 2018). Environmental flow requirements have still not been adequately assessed for most aquatic ecosystems and have been implemented in even fewer (Moore, 2004;Le Quesne et al, 2010;Gillespie et al, 2015;Harwood et al, 2017). In fact, in spite of admirable global efforts, there is no single global record of environmental flow implementations, nor a good understanding of why some projects have succeeded, while other initiatives have failed even to get off the ground.…”

Hydrologic Characteristics of Streamflow in the Southeast Atlantic and Gulf Coast Hydrologic Region during 1939–2016 and Conceptual Map of Potential Impacts

“…The flow alterations are associated with ecological change, and the risk of ecological change increases with increase in the magnitude of flow alteration [1]. Gillespie et al [55] observed evidence of relationships among flow, biota, water quality, and ecosystem responses under flow modifications. They identified that research was primarily focused on traditionally monitored ecological groups (e.g., fish) and the importance of site-specific factors (e.g., climate).…”