RIVER RESEARCH AND APPLICATIONS

River. Res. Applic. (2008)

Published online in Wiley InterScience

APPLICATION OF THE ‘NATURAL FLOW PARADIGM’ INA NEW ZEALAND CONTEXT

IAN G. JOWETTa* and BARRY J. F. BIGGSb

a National Institute of Water and Atmospheric Research, Hamilton, New Zealandb National Institute of Water and Atmospheric Research, Christchurch, New Zealand

(www.interscience.wiley.com) DOI: 10.1002/rra.1208

ABSTRACT

The natural flow paradigm (NFP) emphasizes the need to partially or fully maintain or restore the range of natural intra- andinterannual variation of hydrologic regimes to protect native biodiversity and the evolutionary potential of aquatic, riparian andwetland ecosystems. Based on our studies of natural and managed flow regimes in New Zealand, we do not believe that allcomponents of the natural flow regime are necessary to achieve the objectives of the NFP, either partially or fully, because manyaquatic species have very flexible niches and life-history requirements (i.e. there is ‘ecological redundancy’). Obviously,maintaining the natural flow regime will maintain the hydrologic and hydraulic conditions necessary for sustaining naturalecosystems. However, if there is adequate knowledge of what ‘values’ need to be maintained in a waterway, and the aspects ofthe flow regime that are required to maintain those values are also known, then regimes can be designed that target theserequirements and thus optimize conditions for the ‘values’. We believe that an assessment of ecosystem requirements usinginformation on river processes together with habitat requirements and life-history strategies of biota can achieve the best balancebetween resource use and sustaining ecosystem function and value, and show examples where changes to natural flow regimeshave maintained, or even improved, instream values in some New Zealand rivers. We caution that simple flow-based rules, suchas those that might be developed under the NFP, could be unnecessarily restrictive on multiple use of water in New Zealandwhile, at the same time, preclude the opportunity for enhancement of key ecosystem values in many waterways. Copyright #2008 John Wiley & Sons, Ltd.

key words: flow regime; natural flow paradigm; flow variability; instream flow assessment; ecohydrology

Received 6 January 2008; Revised 13 July 2008; Accepted 18 September 2008

INTRODUCTION

The natural flow paradigm (NFP) emphasizes the need to partially or fully maintain or restore the range of natural

intra- and interannual variation of hydrologic regimes in order to protect native biodiversity and the evolutionary

potential of aquatic, riparian and wetland ecosystems (Arthington et al., 1992; Poff et al., 1997; Richter et al., 1997;

Arthington et al., 2006). Poff et al. (1997, p. 770) argue that ‘the natural flow regime plays a critical role in

sustaining native biodiversity and ecosystem integrity’ and that ‘decades of observation of the effects of human

alteration of natural flow regimes have resulted in a well-grounded scientific perspective on why altering hydrologic

variability in rivers is ecologically harmful’.

Interest in the use of the NFP as an approach to managing flow regimes has recently increased in New Zealand.

However, interpretation of what the paradigm means for practical purposes currently appears to be at the two

extremes of the spectrum. For example, we have witnessed developers suggesting support for the NFP as it would

permit almost unlimited water abstractions from a river (e.g. >95%) so long as the nature of the variability was

maintained in whatever flow remained. Conversely, conservationists have advocated that under the NFP no, or very

little, water can be taken from a river as it will change the flow regime and thus it must severely degrade/destroy the

stream or river ecosystem. We suspect that neither of these polarized views was intended by the original NFP

authors who recognized that ‘mimicking certain geomorphic processes may provide some ecological benefits’

(Poff et al., 1997, p. 781), and as has been hinted at in more recent developments of the paradigm (e.g. Arthington

*Correspondence to: Ian G. Jowett, National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand.E-mail: ian.jowett@ihug.co.nz

Copyright # 2008 John Wiley & Sons, Ltd.

I. G. JOWETT AND B. F. BIGGS

et al., 2006). However, without specific guidance on how the NFP might be incorporated into flow management

frameworks, these types of assertions will naturally evolve as competing parties seek to reduce engagement in

complex, often expensive, flow allocation debates and decisions. We believe that, while extreme changes in flow

regimes can result in morphological and ecological change, not all components of the natural flow regime are

necessary to maintain either diverse ecosystems or specific instream values in New Zealand. Further, many of our

highest instream values are based on introduced species (trout and salmon) that have obviously not evolved under

local natural flow regimes so it cannot be argued from an evolutionary point of view that maintenance of a ‘natural

regime’ is necessary for trout and salmon protection. The natural flow regime will obviously maintain the

hydrologic and hydraulic conditions necessary for sustaining natural ecosystems and this management approach

may be applicable where the ecosystems are highly valued and the consequences of any change highly uncertain

(Jowett, 1997). However if flow regimes are to be partially modified, what specific components of the flow regime

are necessary and at what magnitude and frequency?

We restrict our commentary to ecosystem flow regime requirements in New Zealand because we believe that flow

regime requirements will vary depending on the ecosystem and climatic context, and considerations that we would

apply to temperate rivers in New Zealand may differ from those that apply to arid flood plain river systems, such as

those in South Africa. There have been relatively few studies that link ecological and hydrological site

characteristics over geographically wide areas (Poff and Ward, 1989; Poff and Allan, 1995; Clausen and

Biggs, 1997; Snelder et al., 2005; Monk et al., 2006) and even fewer that include long-term data (Jackson and

Fureder, 2006) or hydraulic data such as used in the study by Jowett and Duncan (1990). In general, we believe that

ecologically relevant components of a flow regime should be determined from within similar ecosystem and

climatic zones (Jowett and Duncan, 1990; Clausen and Biggs, 1997; Monk et al., 2006), rather than using global

examples to justify all components.

Determination of a flow regime that maintains the natural ecosystem may present significant challenges,

particularly if the proposed hydrologic change results in different, or even opposite, effects on some aspect of the

ecosystem. If the natural ecosystem is to remain unchanged in terms of both community composition and

abundance, the only management option is to maintain the natural flow regime. However, in most situations it

should be possible to develop a set of management goals and a flow regime that maintains the ecosystem in a state

that is indistinguishable from the natural one or even improves upon some valued aspects, recognizing that in some

instances this may be at the expense of less valued aspects (Beecher, 1990). Depending on specific proposals for use

of the river (e.g. damming, large-scale run-of-river abstraction, minor abstractions), the flow regime should

consider the need to maintain floods, freshets, low flows and aspects of flow variability, as in the building block

methodology described by King et al. (2000), and has been done in more recent assessments in New Zealand (e.g.

the Waitaki Water Allocation Plan, Waitaki Catchment Water Allocation Board, 2005). To achieve this, the

manager must have a clear idea of the ecological outcomes that are desired and a good knowledge of river

processes. Jowett and Biggs (2006) cite examples of changed flow regimes that have maintained or even improved

instream values for trout, while maintaining native populations and ecosystem integrity.

Poff et al. (1997) state that the significant elements of the natural flow regime include the seasonal patterning of

flows; timing of extreme flows; the frequency, predictability and duration of floods, droughts and intermittent flows;

daily, seasonal and annual flow variability; and rates of change. Richter et al. (1996) described 32 hydrological

variables and Olden and Poff (2003) examined 171 variables that have been used in various ecological studies.

However, the relative importance of these hydrological characteristics depends on the sediment regime, river and

riparian morphology and their associations with the life cycles of the aquatic biota and, in the opinion of the authors

and Tharme (2003), these have yet to be established. In addition, the hydrological characteristics of natural flow

regimes are often highly correlated, described by Olden and Poff (2003) as hydrological redundancy, and this

makes it difficult to determine the ecological functions of the hydrological components without an understanding of

the physical processes and their relationship to aquatic biota (Monk et al., 2007).

Poff et al. (1997) conclude that too little attention and too few resources have been devoted to clarifying how

restoring specific components of the flow regime will benefit the entire ecosystem and that natural flows should be

the cornerstone of our management approach to river ecosystems. While this may be the case in North America,

studies commencing in the mid-1980s, in New Zealand have focussed on linking the physical, hydrological,

hydraulic, chemical and biological characteristics of rivers (Biggs et al., 1990). One of the goals of this work was to

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. (2008)

DOI: 10.1002/rra

APPLICATION OF THE ‘NATURAL FLOW PARADIGM’

establish quantitative links between the flow regimes and ecosystems to allow for more informed decisions on flow

management that would enable protection of instream values and determining how much water might be available,

and when, for societal abstractive use. Since then numerous survey and experimental based studies have been

carried out in New Zealand to further test these links between biological responses and flow regime, and

understand/model the relevant processes (e.g. see Biggs et al., 2005). This research allows us to examine flow

regime requirements in the context of the NFP.

In this paper, we outline aspects of the natural flow regime that need to be recognized in relation to the application

of the ‘natural flow’ paradigm in New Zealand and conclude that process-based environmental flow evaluations can

enable sustainable resource use while protecting ecosystem values by exploiting ‘ecological redundancy’ in flow

regimes (i.e. parts of the flow regime well outside a threshold which might be necessary to achieve a benefit for the

structure or functioning of the ecosystem).

NEW ZEALAND FLOW REGIMES AND THEIR ECOSYSTEMS

Rivers in New Zealand vary greatly in many of their physical aspects, influenced by our geographic and climate

features, including the maritime location and tectonically young/active landscape. They vary from wide, braided

rivers in the South Island which carry high loads of bed sediment to turbid or clear meandering low gradient rivers

in the North Island and uniform deep clear streams or rivers below springs or lakes. However, the most common

type of river is the gravel bed river, with riffles and runs and moderately straight channels (Mosley, 1992).

Compared to continental climates, there is little seasonality in flow regimes, because New Zealand rivers are

generally dominated by rainfall events that can occur throughout the year, as shown in Figure 1. However, it is

possible to classify flow regimes into broad groups based on local climate, topography, watershed geology and land

cover (Snelder et al., 2005). The main differences between flow regimes in different parts of New Zealand is the

relative magnitude of floods and freshes, with stable flow regimes in the Central North Island, frequent floods and

high base flows in rivers draining from the Southern Alps, and occasional very large floods and low baseflows in

rivers draining from the rainshadow of the Southern Alps (Figure 1). In a national study, Jowett and Duncan (1990)

classified rivers using a set of hydrological variables describing the flow regime and compared biological

communities between different flow regimes. They found that the hydrological indices were highly correlated, with

strong associations between flow regime, periphyton and rainbow trout and a weak association with benthic

invertebrate communities. Flow variability was a factor that influenced invertebrate community composition, algal

biomass and the presence of rainbow trout. However, water velocity was the most important hydraulic variable and

could be linked to water temperature, benthic invertebrate and periphyton community structure, and trout

distribution and abundance. The importance of water velocity is not unexpected because the driving force of a

stream is the current. It is necessary for the respiration of many benthic invertebrates and reproduction of some fish

species (Hynes, 1970; Aadland, 1993; Moir et al., 1998; Gore et al., 2001; Merigoux and Doledec, 2004). Currents

transport sediments and determine the nature of the substratum, as well as distributing food down a river system:

seston for invertebrates and drifting insects for fish.

NEW ZEALAND RIVER ECOSYSTEM DEPENDENCE ON FLOW REGIME

Aquatic life in streams and rivers has developed under a natural flow regime. If the instream environment under

natural flows is unsuitable for a particular species then that species will not be well established in a stream. The

biota present in a stream have survived series of floods and droughts and, presumably, will continue to survive

provided that the frequency of these disturbances does not change. If the abundance of an aquatic species in a

particular stream is limited by the naturally occurring low flows in that stream then further reduction in flow at such

times could have a detrimental effect on that species, but if the species is not limited by low flows then further

reduction in low flows will have no effect. Given that the typical life of stream fish is between 3 and 15 years, fish

will have survived droughts that occur about once every 2 years and the status quo, in terms of stream biology, is

likely to be retained if flow regulation ceases when flow falls below the average natural low flow, the mean annual

minimum flow. Biologically, the mean annual minimum flow in river systems where low flow limits the amount of

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. (2008)

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Figure 1. Flow regimes in terms of the proportion of mean flow for 3 years of record from three rivers in different parts of New Zealand. Top: awest coast, South Island river draining from the Southern Alps. Middle: a central North Island river draining from an absorbent volcanic pumicecatchment. Bottom: an east coast South Island river draining from the rain shadow of the Southern Alps. This figure is available in colour online

at www.interscience.wiley.com/journal/rra

I. G. JOWETT AND B. F. BIGGS

available habitat or some other necessary biological function may be a ‘bottleneck’ for aquatic species that have life

cycles in the order of 3–5 years.

Historically, the focus of instream flow studies has been on determining minimum flows required to maintain

particular instream values, assuming that populations are limited by factors such as competition and stress during

low flows. On the other hand, stable low flows can be a time of high biological productivity (Suren and Jowett,

2006), allowing colonization by invertebrates, reproduction and rearing by fish and re-establishment of aquatic

vegetation. However, other aspects of a river’s flow regime may also influence its ability to maintain particular

instream values. In the following, we discuss the degree to which New Zealand aquatic biota are dependent upon

natural flow regimes.

Ecosystem functions that are independent of flow regime

While trout, native fish, invertebrates and periphyton are all affected by flow variability to some extent, it appears

that this is only at the extremes of intensity and frequency of events (low flows and floods). The ratio of mean to

median flow is an index of flow variability that has been shown to be an excellent indicator of biotic condition

(Jowett and Duncan, 1990; Clausen and Biggs, 1997). For benthic communities, many taxa such as the common

mayfly Deleatidium spp. are able to survive and prosper under a variety of regimes—from spring-fed streams with

almost no flow variability to the flashiest of mountain rivers (Quinn and Hickey, 1990), as illustrated in Figure 2.

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. (2008)

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Figure 2. Relationships between flow variability (ratio of mean to median flow) and benthic invertebrates and trout. Above: numbers ofAoteapsyche coloni and Deleatidium spp. per 0.1 m2 showing little variation with flow variability (R< 0.04, p> 0.72) in 84 rivers. Below: troutbiomass (g m�2) in 90 New Zealand rivers, showing high rainbow trout biomass in rivers of low variability (R¼ 0.36, p¼ 0.0005 lower left) andno significant relationship between brown trout biomass and flow variability (R¼ 0.026, p¼ 0.81 lower right). This figure is available in colour

online at www.interscience.wiley.com/journal/

APPLICATION OF THE ‘NATURAL FLOW PARADIGM’

Most New Zealand stream invertebrates have flexible life-histories with non-seasonal or weakly seasonal patterns

of development (Scarsbrook, 2000), and patterns in invertebrate species richness and diversity as a function of flow

variability are not strong across a very wide spectrum of New Zealand rivers (Jowett and Duncan, 1990; Clausen

and Biggs, 1997). Similarly, most common periphyton taxa (e.g. Ulothrix zonata, Gomphoneis herculeana,

Spirogyra spp.) live, and can prosper, across a similar range of flow regimes (Biggs and Price, 1987; Biggs et al.,

1990). While Clausen and Biggs (1997) found that average periphyton species richness decreased as a function of

flood frequency among 22 streams from around New Zealand, a subsequent study by Biggs and Smith (2002) with

more intensive regional sampling found no significant pattern in mean monthly periphyton taxonomic richness in

relation to flow variability among 12 hydrologically contrasting central South Island streams. This indicates that

periphyton are probably well adapted to tolerate a range of flow conditions within a region, either through resistance

traits or rapid immigration. Indeed, it appears that New Zealand aquatic systems (at least for invertebrates and

periphyton) are characterized by populations that have evolved to be resilient and opportunistic, with flexible

poorly synchronized life-histories with non-seasonal or weakly seasonal patterns of development (Winterbourn

et al., 1981; Biggs et al., 1990; Scarsbrook, 2000; Thompson and Townsend, 2000). This is not surprising given the

lack of strong seasonality in flow regimes. Thus, there is ‘ecological redundancy’ in the flow variability patterns of

New Zealand rivers.

Many common fish species also have flexible flow regime requirements, although spawning, egg hatching and

migratory movements of some fish may be restricted to a few months of the year (McDowall, 1995). Species with

asynchronous or extended periods of reproduction, such as upland bullies, will be influenced less by flow changes.

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They spread reproductive investment over an extended period which may be of adaptive value in unpredictable

environments such as in New Zealand rivers (McDowall and Eldon, 1997).

The widespread occurrence of non-native species in unmodified New Zealand rivers (McDowall, 2003) indicates

that it is simplistic to assume that a natural flow regime will prevent colonization by non-native species. In contrast

to rainbow trout, which are most common in rivers of low flow variability, brown trout have very wide ranging

habitats from lakes and springs (i.e. no flow variability) through to flashy mountain-fed rivers (Jowett, 1990), as

shown in Figure 2. Early trout life stages are particularly vulnerable to high flows which can destroy entire year

classes if they coincide with the egg or larval stage (Allen, 1951; Hayes, 1995). However, there are many New

Zealand rivers where native fish are abundant and where the flow regime limits trout survival because of poor

habitat and unstable substrate during floods (Jowett et al., 2005).

Ecosystem functions that depend on flow regime

Trout, native fish, aquatic invertebrates, macrophytes and periphyton are all affected detrimentally by floods to

some extent (Jowett and Richardson, 1989; Scrimgeour and Winterbourn, 1989; Quinn and Hickey, 1990; Clausen

and Biggs, 1997; Biggs et al., 1999; Riis and Biggs, 2003), and significant macrophyte development and high

species richness only occur where bed-moving floods are rare or absent (Riis and Biggs, 2003). New Zealand native

fish have evolved to cope with the conditions they experience in our rivers, although rare natural disasters, where the

natural ecosystems are devastated by very large floods or other natural events, can and do occur and can influence

the biogeographic distribution of some aquatic species (e.g. McDowall, 1996). The effects of floods can be both

positive and negative—i.e. the effect of ‘flushing’ and ‘refreshing’ the river on the one hand, and disturbance to

parts of the ecosystem on the other. During floods, the stability and movement of sediment accumulation, as well as

the physical stress of high water velocities, influences aquatic organisms.

A few notable exceptions of population level flow regime dependency occur with some fish species. For example,

inanga (Galaxias maculatus) spawn on the banks of river estuaries on high spring tides and rely on subsequent

inundation to stimulate hatching (McDowall, 1990). If this inundation does not occur (e.g. through high abstraction

rates) then the spawning will fail. Floods and freshes in autumn carry larvae of some diadromous native fish to the

sea, but this is largely opportunistic (Ots and Eldon, 1975; Allibone and Caskey, 2000; Charteris et al., 2003).

Similarly, in some streams and rivers, floods in spring can open the mouth to the sea and allow juvenile diadromous

fish to return to the river from the sea. Such annual spring openings are probably essential to the long-term viability

of these populations (Jowett et al., 2005). The timing of hydrologic events can also have negative effects. For

example, studies in the Kakanui River on the east coast of the South Island, New Zealand, showed that the adult

trout population was regulated by variable recruitment and that in turn was controlled by the occurrence of floods

during spawning and incubation, with relatively small spring floods causing high mortality in emergent fry (Hayes,

1995; Jowett, 1995). Similarly, rainbow trout appear to be unable to establish viable populations in rivers with high

flood disturbance unless there are downstream refuges such as lakes (Jowett, 1990; Fausch et al., 2001). Native fish

and brown trout seem to be better adapted to surviving large floods than rainbow trout (Jowett and Richardson,

1989; Jowett et al., 2005).

Trout and salmon spawning migration is strongly seasonal and the initiation of seasonal migration is probably

cued by changes in water temperature rather than flow (Jonsson, 1991; Hodgson and Quinn, 2002). Although

spawning movements are often considered to be in response to flow variability, there does not appear to be a strong

relationship between fish movement and flow variations in New Zealand rivers with their relatively frequent and

short floods (the exception is salmon migration). Studies of rainbow trout spawning migrations in the Tongariro

River showed a weak, if any, link between flow and fish movement (Dedual and Jowett, 1999; Venman and Dedual,

2005).

Artificially enhanced flow variability over daily scales, such as those that occur in some hydroelectric controlled

rivers, can be detrimental to species with lower motility such as benthic invertebrates (Irvine and Henriques, 1984;

Cushman, 1985). Indeed, if high flows are too frequent, some biota will be unable to establish self-sustaining

populations. For example, aquatic macrophytes are only found in New Zealand waterways where floods are

infrequent and are dependent on low flow variability to prosper. Thus, they tend to dominate benthic production in

Copyright # 2008 John Wiley & Sons, Ltd. River. Res. Applic. (2008)

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lake or spring-fed rivers (Riis and Biggs, 2003), and can become troublesome downstream of reservoirs with

constant flows (Biggs, 1995).

Intense flow variability in New Zealand rivers probably has its greatest impact on biomass (usually negative),

community structure (i.e. the relative abundance of species) and functioning of benthic communities, and flood

frequency has been used in several biological models as the primary axis for classifying benthic communities

(Biggs et al., 1998). In streams with very frequent bed disturbing floods (e.g. >15 year�1), invertebrates that are

small and can colonize new areas rapidly are often dominant (Scarsbrook and Townsend, 1993). In such rivers, the

periphyton community is usually sparse, and depending on the state of regeneration may have a low species

richness and diversity (Biggs, 1990; Biggs and Smith, 2002). In rivers with very low variability in flows (e.g. <5

bed disturbing floods/year), communities are usually dominated by large (high biomass), less mobile/more sessile,

taxa such as filamentous green algae, macrophytes and snails (Biggs, 1990; Quinn and Hickey, 1990). Rivers with

an intermediate frequency of bed-disturbing floods tend to have a higher biomass of benthic invertebrates (Clausen

and Biggs, 1997). Townsend et al. (1997) suggested that these rivers also had higher invertebrate diversity based on

studies of Otago streams, but this was not supported by a more broad-scale, New Zealand-wide analysis by Clausen

and Biggs (1997). In UK rivers, Monk et al. (2006) found that specific median flow was a good predictor of the

Lotic-invertebrate Index for Flow Evaluation (LIFE). LIFE is an index related to the water velocity requirements of

benthic invertebrates (Extence et al., 1999) and in New Zealand rivers, at least, the specific median flow is

positively correlated with mean reach velocity (unpublished data, N¼ 109, p< 0.001).

Arguably, the most important requirement for flow variability is for the removal of accumulations of silt and

periphyton. However, there is no simple rule for what degree of flow variation is needed to achieve such removal.

For example, Jowett and Biggs (1997) found that after a period of stable flow, a natural flood causing a fivefold

increase in the flow of one river resulted in the total removal of periphyton, whereas in another river with less

sediment movement, there was little change in periphyton biomass after a natural flood causing a fourfold increase

in flow. It is possible to deal with silt and periphyton accumulations by the use of well-timed flow releases of the

magnitude and frequency that is appropriate for the local reach channel geometry (Jowett and Biggs, 2006).

However, these will often need to be quite large in magnitude in many river systems and (as is often suggested)

simply stopping water abstractions for a short period to develop high flows for flushing and channel maintenance

may be ineffective because of insufficient stream power. Hydraulic calculations from 40 New Zealand rivers

suggest that flows greater than ten times the baseflow may often be necessary for significant riverbed cleansing

(Clausen and Plew, 2004).

FLOW REGIMES TARGETED TOWARDS MANAGEMENT GOALS

Potentially, damming can have the greatest effect both on the frequency of floods and freshes and the duration and

magnitude of low flows (Poff et al., 1997). In fact, damming is the only practical way the flow regime can be

modified sufficiently to affect the channel-forming floods that maintain the character and morphology of the river.

While large-scale diversions can increase the duration and decrease the magnitude of low flows significantly and

can also reduce the frequency of freshes, they usually have little effect on the channel-forming floods. On the other

hand, minor abstractions usually have little effect on the frequency of floods and freshes, even cumulatively, but

certainly can reduce flows significantly during periods of low flow.

The determination of flow regime requirements may present significant challenges, particularly if there are

several values that have different—or even opposite—requirements. Depending on specific proposals for use of the

river—damming, large-scale run-of-river abstraction, minor abstractions, etc.—it may be necessary to develop

what might be called a ‘designer flow regime’ (Jowett and Biggs, 2006) that considers the need to maintain floods,

freshes, low flows, aspects of flow variability and the synergistic effects of water quality (particularly where some

and degradation has occurred). This requires a clear idea of the outcomes that are desired, with regard to instream

values, and the time and resources available to conduct an extensive environmental flow analysis.

Valued biological communities can be maintained in rivers where the flow regime has been extensively modified

and the needs of the instream values have been specifically identified and targeted in the management regime

(which may include flushing flow releases). These values can include salmonid fisheries, native fish communities

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and benthic biodiversity for conservation purposes (Jowett and Biggs, 2006). In these cases the ‘ecological

redundancy’ in the flow regimes has been exploited to provide water for societal out-of-stream use, while

maintaining (or enhancing) instream values and ecosystem services.

CONCLUSION

The NFP is a simple construct, based on the assumption that if you do not change the flow regime (and non-flow

related factors also remain unchanged), the natural ecosystem will be maintained. While some species may be

adapted to a specific aspect of flow, this does not imply that the entire flow regime is necessary to maintain a healthy

aquatic ecosystem or a given value. The paradigm does not take into consideration the flexibility in habitat

requirements and life-history strategies of biota that enable them to cope with certain degrees of change. New

Zealand flow regimes differ according to climate and river type, yet the aquatic communities are broadly similar

across these regimes, demonstrating the existence of ‘ecological redundancy’ in relation to flow requirements.

These communities provide us with knowledge of preferred habitats as well as the ecological functions of flow

regime components. When this information is combined with models of habitat, water quality and other flow-

related variables, we have the necessary tools for flow regime decisions.

There seems to be a general consensus on the need for improving scientific understanding of the flow conditions

necessary to achieve desired ecological changes or processes (Poff et al., 2003; Monk et al., 2006; Richter et al.,

2006). We believe that studies that compare ecological, hydrological and hydraulic data over a range of geographic

and climatic scales, such as described here, will provide the understanding of the physical processes and their

relationships to aquatic biota that is necessary to predict effects of flow changes. In this way, effort can be given to

designing regimes that specifically support instream values rather than relying on a rather nebulous objective of

maintaining a ‘natural flow regime’ in the hope that the values will be protected. The selection of an appropriate

flow regime for a river requires clear goals and targeted objectives, with levels of protection set according to the

relative values of the in- and out-of-stream resources and other ecosystem aspects such as water quality. Attempts to

maintain everything in the existing state invariably lead to the conclusion that flows should not be changed—the

NFP—and preclude the opportunity for enhancement of some aspects of the aquatic environment and the part use

of the water resource by society. The challenge is to determine the aspects of the flow regime that are important for

the various biota associated with their rivers, and to develop flow regimes that meet those needs. This requires an

interdisciplinary approach that is in the best interests of both river conservation and sustainable water resources use.

ACKNOWLEDGEMENTS

We thank the referees for their constructive comments and the Foundation for Research, Science and Technology

(New Zealand) for funding under contract C01X0308.

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