What are environmental flows?

You may have heard about the current restoration efforts on the Colorado River: an eight week long ‘pulse flow’ is being released from Hoover Dam on the Arizona-Nevada border (USA). This pulse flow is designed to replicate spring snowmelt driven floods that would have historically occurred (but that no longer do as a result of dam construction to provide water for irrigation & domestic use). It is hoped that the pulse flow will aid wildlife such as fish, plants and birds and can be viewed as an ‘environmental flow’ – but what exactly is an environmental flow?

The term environmental flow started gaining momentum in the academic literature during the early 1990’s (see Figure below), but throughout my PhD I’ve noticed inconsistencies in its definitions and scope of implementation. Here I’ll attempt to clarify its meaning and explore it further.


Use of “environmental flow” in academic literature over time. Source: Scopus.

A quick Google search brings up the Wikipedia definition on the right hand side of my screen:

“Environmental flows describe the quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well being that depend on these ecosystems”

So from that definition environmental flows describe properties of water flow that sustain aquatic ecosystems and human livelihoods. From further reading of the Wikipedia page I gather than environmental flows are essentially flows that strengthen or support aquatic ecosystems and human livelihoods.

I won’t solely rely on Wikipedia so let’s take a look at some alternative definitions (I’ve highlighted key aspects in bold and underscore):

1: “Environmental flows can be described as ‘the quality, quantity, and timing of water flows required to maintain the components, functions, processes, and resilience of aquatic ecosystems which provide goods and services to people” (The World Bank)

2: “Environmental flows are the quantity and timing of water flows required to maintain the components, functions, processes and resilience of aquatic ecosystems and the goods and services they provide to people” (Conservation Gateway)

3: ““Environmental flows” is a system for managing the quantity, timing, and quality of water flows below a dam, with the goal of sustaining freshwater and estuarine ecosystems and the human livelihoods that depend on them” (International Rivers)

4: “Environmental Flows (eFlows) refer to water provided within a river, wetland or coastal zone to maintain ecosystems and the benefits they provide to people” (eFlowNet)

On the face of it these definitions are all pretty similar: they refer to the provision of water to benefit aquatic ecosystems and humans. But, there are some rather subtle differences between them. For example, definitions 1 and 3 mention the quantity, quality and timing of flows, but definition 2 makes no mention of quality and definition 4 fails to mention any of these characteristics. Definitions 1, 2 and 4 refer to maintaining ecosystems while definition 3 uses the word sustain. Clearly, these are similar terms, but it is important to note that sustain can mean to “strengthen or support” whereas maintain can mean to “cause or enable (a condition or situation) to continue” – perhaps this terminological nuance can be disregarded or, perhaps the latter can signify continuation of the status quo whereas the former may represent a more dynamic, evolutionary (dare-I-say ‘natural’) condition?

Further differences between definitions can be noted in their scope of application: definitions 1 and 2 refer broadly to aquatic ecosystems, definition 3 restricts things to within freshwater and estuarine locations while definition 4 is even more specific suggesting the application of environmental flows in rivers, wetlands and coastal zones. Interestingly, definition 3 also states that environmental flows is a system for managing flows below a dam. The variation in scope of these definitions is certainly intriguing and I wonder why there doesn’t appear to be one single, accepted definition. This may in the future be the subject of discussions within court rooms as the term is now building its way into freshwater legislation.

Are environmental flows the same as ‘natural’ flows?

Central to the paradigm of environmental flows is an appreciation of the importance of flow regimes. River flow has been described as a “master variable” that controls biotic abundance and distribution in such systems and stream ecological integrity is said to be primarily affected by characteristics of the natural flow regime. One might expect that environmental flows are therefore a representation of such characteristics, but, Conservation Gateway state:

“Unlike the natural flow regime, the environmental flow regime allows for some degree of hydrologic alteration. However, environmental flows are intended to mimic the patterns and ecological outcomes of the natural flow regime.”

So, with that in mind, environmental flows are certainly not ‘natural’ (the definition of natural is a subject for another day!) as they are hydrologically altered, but the ecological outcome is intended to be natural. This is a key point to recognise as it raises more questions, for instance, is it possible to generate a natural ecosystem with unnatural flows? How do we define the ecological condition we are aiming for? These are tough questions to answer and defining environmental flows to meet the needs of both humans and ecology is not an easy task, especially where anthropogenic pressures on aquatic systems are increasing.

Application of environmental flows

Definition 3 noted that environmental flows can be applied below dams, but are there any other scenarios where environmental flows can be implemented?

In rivers, water is often abstracted for human use. Such abstractions can be subject to environmental flow management, but there is evidence that emphasis is often placed on maintaining flow during periods of drought rather than protection of high flows – this is an example of how environmental flows differ from ‘natural’ flows. Another instance where environmental flow provision in rivers may be required is interbasin transfer. This technique is typically used in large hydropower schemes (e.g. The Tummel scheme) or for provision of water for abstraction/ irrigation (examples here and here). Such schemes present complex challenges for environmental flow provision: flows can be reduced in one waterbody and increased in another – how can these flows be managed to get the balance between ecology and human livelihood right? I have also seen suggestions that canal, weir and sluice operation can contribute to environmental flow implementation.

Wetlands, estuaries and coastal areas were also highlighted in the environmental flow definitions above, yet from my experience, there is very little published literature on environmental flow implementation in these systems when compared to in rivers. Understanding and consideration of the complex interactions between surface and ground water and even tidal flows will therefore be required when implementing environmental flows.


The field of environmental flows is an emerging one. There doesn’t appear to be one accepted definition and nuances between definitions encourage confusion and leave interpretation open to abuse.

Environmental flows are not the same as ‘natural’ flows – they represent a balance between benefiting ecology and humans. This raises some tough questions regarding how targets are defined and future challenges in the face of increasing anthropogenic pressures.

Published literature mainly focusses on implementation of environmental flows in rivers, but definitions also refer to wider ecosystems such as wetlands and coastal areas. Development of understanding of environmental flow provision in such areas is required if future management is to be successful.

If you have any questions regarding the content of this article, or would like to know more about the results of my PhD on environmental flow implementation in an upland regulated British river, please get in touch.

About Ben Gillespie

Researching the impact of reservoirs & environmental flows on downstream hydrology, physical-chemistry, sediment transport & benthic macroinvertebrates. Twitter: @RiversBenG Web: http://www.geog.leeds.ac.uk/people/b.gillespie LinkedIn: http://goo.gl/0ObRnc
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3 Responses to What are environmental flows?

  1. Mike Potter says:

    The thing that immediately stuck me is that all except no 3 could be either natural or man-made flows, while no 3 describes both a ‘system of managing’ and ‘below a dam’, which implies a wholly man made intervention. However, No4 does say ‘water provided’ which implies intervention, but could also be interpreted as ‘provided by natural means’.

  2. Mike Potter says:

    Another thought, although slightly off topic: It’s interesting how we are beginning to quantify and therefore assign a monetary value to natural processes – and of trying to mimic them. As part of my involvement in the Slowing the Flow in Pickering scheme, we spent a whole 3 hour meeting producing such an assessment of the scheme’s component parts. My initial reaction (as a former civil servant) involved the words bureaucracy and bullshit among others! However, by the end, I’d found it to be an interesting and thought provoking exercise. In particular doing it by committee, with people from different agencies (EA, NE, FC etc) raised some very interesting perspectives.
    Our local newspaper’s farming correspondent routinely mentions the ‘green brigade’ in disparaging tones, yet this action of putting a value on ‘eco services’ really highlights the enormous monetary cost of replacing or restoring what we have damaged or destroyed by what were thought to be valid land management practices. Diffuse pollution, sediment supply and pollinators immediately spring to mind.

  3. Pingback: Environmental Flows- eflows | Anotherverse

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