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Draft Guidelines for the Selection of Methods to Determine Ecological Flows and Water Levels

Acknowledgements

Executive Summary – Recommendations

1 Introduction

2 Rivers

3 Lakes and Wetlands

4 Groundwaters

5. References of Main Text and Appendices

Appendix 1: Relationship between Total Allocation and Ecological Flow Requirements in Rivers

Appendix 2: Effects of Flow Regime on Stream Ecology

Appendix 3: Methods Used in Ecological Flow Assessments of Rivers

Appendix 2: Effects of Flow Regime on Stream Ecology

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). Currents distribute nutrients and food down a river system – detritus for invertebrates and drifting insects for fish and birds – and aid species dispersal. Biologists and anglers who study rivers are well aware that aquatic species are likely to be found associated with specific habitats; many aquatic species are found in similar hydraulic conditions in a wide range of rivers. These have been termed habitat niches and include both physical and biotic characteristics of the environment (Odum 1971). Such concepts have been widely applied in both terrestrial and aquatic biological studies and the presence of suitable habitat for any species is a necessary condition for survival.

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. Periodic disturbances, such as floods and droughts, affect stream biota. Floods can reduce trout stocks (Jowett and Richardson 1989), invertebrates (Quinn and Hickey 1990), and periphyton (Biggs et al 1990). However, the effect of disturbance frequency differs between aquatic species. If disturbances are too frequent and severe, most biota will be unable to establish self-sustaining populations. Native fish and brown trout seem to be particularly well adapted to surviving large floods, even taking advantage of the situation to feed (Jowett and Richardson 1994). Aquatic insects are also relatively robust, colonising a stream within 4–6 weeks of a severe disturbance (Sagar 1983; Scrimgeour and Winterbourn 1989). Some stream insects recolonise streams relatively quickly: as drifting insects from upstream, from within the gravels, or from eggs laid by the terrestrial adult insects. The recolonisation rate of fish is slower than of stream insects. Floods particularly affect juvenile trout and adult rainbow trout, presumably because they do not utilise cover as well as adult brown trout, and juvenile trout – especially recently emerged fry – are weaker swimmers.

The biota present in a stream have survived series of disturbances and, presumably, will continue to survive provided that the frequency of these disturbances does not change appreciably. Some stream ecologists hypothesise that stream biota have adapted to the flow regime of particular streams or rivers, and in particular, they believe that biota have adapted to, or survive, the low flows that occur in the river every year or so. 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 would have a detrimental effect on that species, but if the species is not limited by low flows then reduction in flow will have little effect. Given that the life of most stream fish is between three and 15 years, fish will have survived droughts that occur about once every two years; the status quo, in terms of stream biology, is likely to be retained if the minimum flow does not fall below the average natural low flow: the mean annual minimum flow.

New Zealand river flow regimes

Rivers in New Zealand vary greatly, influenced by our geographic and climate features, including the maritime location and tectonically young/active landscape. However, it is possible to categorise flow regimes into broad groups based on local climate, topography, watershed geology and land cover (Snelder et al 2005). Examples of the flow regimes from three main river types in the South Island are:

• mountain source of flow: having relatively high minimum flows, with very frequent (often weekly) high flows of three to six times the low flow, and large amounts of transported gravel. Low flows typically occur in winter and seldom last for more than six weeks

• hill source of flow: having relatively low minimum flows compared with the high flows, and moderately frequent high flows of greater than about six times the low flow. Low flows typically occur during late summer/autumn and can last for 12–16 weeks, with high flows more common in late winter. Flood flows can have a very high magnitude (about 3,000 times the lowest flows)

• low elevation/spring source of flow: having relatively high minimum flows, a low to moderate frequency of low-intensity high-flow events (greater than about 1.5 times the low flows), with few high-magnitude flood events resulting in stable velocities and bed sediments.

For most river types, floods/freshes occur throughout the year and in most parts of New Zealand winter flows are generally higher than summer flows (this is reversed where rivers drain from the Southern Alps). The critical question for management becomes: is there a concomitant change in ecosystem structure and function that reflects these broad divisions in flow regimes? Are any biota/values dependent on these different flow regimes in a way that cannot be recognised or predicted, which would then justify promoting the retention of a ‘natural flow regime’?

The relationship between the relative magnitude of natural minimum flows and the source of flow means that ecological flow requirements relative to natural flows will vary with the source of flow. Thus, the ecological flow requirements of rivers draining from hill sources are likely to be higher in terms of the natural minimum flow than those with mountain sources, with relatively more water available for allocation in rivers with mountain or spring sources than in rivers with hill sources.

New Zealand river ecosystem dependence on flow variability

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). 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). Similarly, most common periphyton taxa (eg, Ulothrix zonata, Gomphoneis herculeana, Spirogyra spp.) live, and can prosper, across a similar range of flow regimes (Biggs and Price 1987; Biggs 1990). This indicates that they are well adapted to tolerate a range of flow conditions. Indeed, it appears that New Zealand aquatic systems (at least for invertebrates and periphyton) are characterised by populations that have evolved to be resilient and opportunistic, with flexible life-histories and (in broad terms) only poor specialisation to specific spatial or temporal habitats (Biggs 1990; Death and Winterbourn 1995; Thompson and Townsend 2000).

Artificially enhanced flow variability over daily scales, such as that which occurs in some hydro-electric controlled rivers, can be detrimental to species with lower mobility such as cased caddisflies due to drying effects (Irvine and Henriques 1984). Indeed, if high flows are too frequent, some biota will be unable to establish self-sustaining populations.

Aquatic macrophytes are only found in New Zealand waterways where floods are infrequent; thus they are dependent on low flow variability to prosper. Therefore, they tend to dominate benthic production in lakes or spring-fed rivers (Riis and Biggs 2003), and can become troublesome downstream of impoundments with constant flows (Biggs 1995).

Many common fish species also have flexible flow regime requirements. Brown trout, in particular, have very wide-ranging habitats from lakes and springs (ie, no flow variability) through to flashy mountain-fed rivers, although rainbow trout generally 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).

Spawning, egg hatching and migratory movements of some fish may be restricted to a few months of the year (McDowall 1995) and linked to the occurrence of suitable flow conditions. Early 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). Species with asynchronous or extended periods of reproduction, such as upland bullies, will be influenced less by flow changes. 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). 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 (eg, through high abstraction rates) then the spawning will fail.

Fish migration is often considered to be cued by flow variability. However, this appears uncommon in New Zealand rivers with their relatively frequent and short floods (an exception is salmon migration in shallow rivers). 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). 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 (Jowett et al 2005). The timing of hydrological events can also have negative effects. Studies in the Kakanui River indicated that the adult trout population was regulated by variable recruitment and that in turn was associated with the occurrence of floods during spawning and incubation, with relatively small spring floods causing high mortality in emergent fry (Jowett 1995; Hayes 1995).

Flow variability in New Zealand rivers probably has its greatest impact on community structure and functioning. In streams with frequent floods, fish and invertebrates that are small and can colonise new areas rapidly, are often dominant (Scarsbrook and Townsend 1993). In such rivers, the periphyton community is usually sparse, with low species richness and diversity (Biggs 1990). In rivers with low flows and infrequent floods, communities are usually dominated by large, 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 have been reported to have the highest diversity and biomass of benthic invertebrates (Townsend et al 1995; Clausen and Biggs 1997), although other studies have not supported this conclusion (Death and Winterbourn 1995; Death 2002; Death and Zimmermann 2005).

Arguably, the most important requirement for flow variability is for the removal of accumulations of silt and periphyton on the river bed. Such accumulations can strongly degrade the quality of benthic habitats, but on flow-controlled (dammed) rivers can be dealt with 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). In some systems, stopping abstractions to develop high flows may be ineffective for significant cleansing: flows > 10 × baseflow will often be required.

Although flow variability is often thought of as an essential element of the flow regime that should be maintained, there is little published biological evidence that flow variability, in addition to uncontrolled floods, is essential for the maintenance of most instream values in New Zealand. Valued biological communities can be maintained in rivers where the flow regime has been extensively modified, but the needs of the instream values have been specifically identified and targeted in the management regime (which may include flushing flow releases) (Jowett and Biggs 2006).

The natural flow paradigm is a simple construct, based on the assumption that if you don’t change the flow regime (and non-flow related factors also remain unchanged), the natural ecosystem will be maintained. Adoption of such an approach could place unnecessary restrictions on the use of water for out-of-stream purposes and may be suboptimal for the maintenance of key instream values. While some species may be adapted to a specific aspect of flow, this does not imply that the entire flow regime is necessary. This also doesn’t allow for flexibility in habitat requirements and life-history strategies of biota that will enable them to cope with certain degrees of change. New Zealand flow regimes do differ according to climate and river type, yet the aquatic communities are broadly similar across these regimes. Flow regime decisions need to be guided by community values and the requisite ecosystem composition/functionality. Effort should be given to designing regimes that specifically support these values rather than relying on the 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 management objectives, with levels of protection set according to the relative values of the in- and out-of-stream resources. 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 – with appropriate monitoring to verify whether the biota responds as expected.

Habitat requirements and relationships with abundance of aquatic fauna

It is the quality of the habitat that is provided by the flow that is important to stream biota, not the magnitude of the flow per se. In many streams, flows less than the naturally occurring low flow are able to provide good-quality habitat and sustain stream ecosystems. The flows that provide good habitat will vary with the requirements of the species and with the morphology of the stream; water velocity is probably the most important characteristic. Without it, the stream becomes a lake or pond. An average velocity of 0.3 m/s tends to provide for most stream life and will prevent the accumulation of fine sediment. Velocities lower than this are unsuitable for a number of fish species and stream insects and allow the development of nuisance growths of periphyton. In large rivers, water depth of more than 0.4 m provide habitat for adult brown trout, but in small streams depths in excess of 0.05 m are adequate for most stream insects and native fish. The flow at which these limiting conditions occur varies with stream morphology. Generally, minimum flow increases with stream size, because stream width increases with stream size. However, the relationship is not linear. Small streams require a higher proportion of the natural stream flow to maintain minimum habitat than do large streams.

Minimum flows do not necessarily influence fish populations nor are they the only factors controlling the fish population. Studies of trout in the Kakanui River showed that the total adult population was regulated by recruitment; that in turn was controlled by the occurrence of floods during spawning and incubation (Jowett 1995; Hayes 1995). Over the study period, low flows in the Kakanui River had no discernable effect on the trout population: lowest flow in the study period, 0.62 m³/s, was a little higher than the MALF, 0.58 m³/s.

Food availability may limit trout populations, as in the Horokiwi Stream (Allen 1951). Benthic invertebrate biomass was shown to be the most important factor relating to trout abundance in different rivers (Jowett 1992). In the Kakanui River the distribution of adult trout mirrored benthic invertebrate abundance, suggesting that it might be a limiting factor (Jowett 1995).

Less is known about the factors controlling native fish populations. Studies have been carried out to determine habitat preferences of native fish (eg, Jowett and Richardson 1995) and these have been independently verified by studies that show that native fish are more abundant where the average stream characteristics are close to the preferred habitat for the fish species (Jowett et al 1996). Native fish densities are therefore often higher in small streams than in larger streams or rivers because the preferred habitat of native fish is usually for relatively shallow water. New Zealand native fish have evolved to cope with the conditions they experience in our rivers. Most galaxiids and eels are able to survive relatively long periods out of water and are capable of some overland movement. Many are also capable climbers and can penetrate to the headwaters of most rivers. Diadromous native species spend their early life stages in the ocean, thus avoiding the harsh riverine environment associated with frequent floods and freshes and unstable gravel substrate. Native fish live at densities of up to about 2/m2 in lowland areas, fish density reducing with elevation. The overwhelming influence of diadromy, the widespread distribution of the more common native species, and their well-defined preferences for relatively shallow water habitats, suggest that the total fish numbers and diversity will be controlled by diadromy, while instream habitat will control the distribution of fish within a river (Jowett and Richardson 1996). Native fish distribution and abundance does not appear to be related to benthic invertebrate abundance. Flows that provide adequate native fish habitat are therefore likely to maintain native fish populations. Juvenile trout, like native fish, occupy shallow water and feed on smaller food items than adult trout. Their abundance was more closely related to the availability of food than to their habitat requirements, which are broad (Jowett et al 1996). Predation can also limit native fish populations, as in Otago where many non-migratory galaxiid populations have been heavily impacted on by the introduction of trout (LePrieur et al 2006).

Stream size and flow requirements of aquatic communities

The composition of the fish community varies with stream size. Small streams are more suited to small fish than large, and vice versa. Small fish have lower swimming speeds and lower velocity and depth preferences than large fish. Adult salmonids usually move upstream or into tributaries to spawn and the juvenile fish rear in these areas, whereas the adults usually move back downstream to deeper waters after spawning. Because water depth and velocity generally increase with flow, there tends to be a flow that provides a maximum amount of habitat for a particular fish species and life stage. The amount of habitat (weighted usable area) at mean annual low flow in 71 New Zealand rivers was calculated for a range of fish species and life stages. When available habitat was plotted against flow and a smooth curve fitted, the peak of the curves gave an indication of the streams sizes that provided the most habitat for the species and life stages (Figure A2.1).

There is a general relationship between fish community, physical habitat requirements, and optimum size of river. Habitat increases with flow as streams become wider, until the stream reaches a size where further increases in stream size do not increase the amount of available habitat. The optimum size of a river for food producing habitat was about 15 m³/s, for adult brown trout habitat 10 m³/s, and the optimum size for trout fry/juvenile habitat (≤ 15 cm) was about 2 m³/s. This is in agreement with general observations of the distribution of trout with adult trout in the larger streams and rivers, and trout rearing either in small streams or headwaters. The analysis can be extended to native fish and indicates that the optimum size of river for torrentfish, which are common in large braided rivers, is 10–15 m³/s, whereas streams less than 1 m³/s contain maximum physical habitat for many of the other native fish species.

A generalised analysis of habitat requirements (Jowett and Hayes 2004) produces similar results, with small streams suited to biota with low velocity and low depth requirements, and larger streams and rivers suited to larger species that prefer deeper water and higher velocities.

Relative importance of flow variability versus minimum flow

Before the effect of flow abstraction can be examined, it is necessary to appreciate the inter-relationships between flow variability and the magnitude and duration of low flows. Although flow variability is often thought an essential element of the flow regime that should be maintained, there is little published biological evidence that flow variability is essential. Similar biological communities are often found in streams and rivers with very different patterns of flow variability. Valued biological communities can be maintained in rivers where the flow regime has been extensively modified by hydro-electric operations, such as in the Monowai, Waiau, and Tekapo Rivers. The term ‘flow variability’ tends to confuse the discussion because high flow variability is often bad for the aquatic ecosystem and low flow variability good, depending on how flow variability is measured. Jowett and Duncan (1990) used hydrological indices, particularly the coefficient of variation, to define flow variability. They found that rivers with high flow variability had long periods of low flow and occasional floods, rivers with low flow variability were lake- or spring-fed, and rivers with moderate flow variability had frequent floods and freshes that maintained relatively high flows throughout the year. Rivers with high flow variability (ie, long period of low flow interspersed with occasional floods) contained poorer ‘quality’ aquatic communities than rivers with low to moderate flow variability. This suggests that the magnitude and duration of low flows is more important than flow variability per se. However, flow variability can also be associated with the frequency of floods and freshes. Clausen and Biggs (1997) used the frequency of flows greater than three times the median (Fre3) as an index of flow variability and showed, not surprisingly, that periphyton accumulation was less in rivers with more frequent floods (high Fre3) and that invertebrate densities in rivers with moderate values of Fre3 (10–15 floods a year) were higher than those in rivers with high and low Fre3 values. However, as with the Jowett and Duncan (1990) study, the rivers with low Fre3 were also rivers in which there were long periods of low flow without floods.

Wetland inundation may occur in a very specific flow band. For example the upper Taieri River breaches its river channel at a flow of around 10 m³/s and begins to enter the scroll plain wetland (the area between the existing river and old meandering channels) and, at around 15–20 m³/s the scroll plain is fully inundated. Therefore it is a relatively small flow band that is critical to maintaining this wetland and any reduction in the frequency of the occurrence of flows between 10 and 20 m³/s would need to be investigated.

The effect of flow abstraction on the frequency of floods and freshes and the duration and magnitude of low flows depends on the specific proposals for use of the river – damming, large-scale run-of-river abstraction, or minor abstractions. Potentially, damming can have the greatest effect both on the frequency of floods and freshes and the duration and magnitude of low flows. In fact, damming is the only way the flow regime can be modified sufficiently to affect the channel-forming floods that maintain the character and morphology of the river significantly. Large-scale diversions can increase the duration and decrease the magnitude of low flows significantly and can also reduce the frequency of freshes, but 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 during periods of low flow.

Large-scale projects like damming and major diversions will usually require detailed and specific studies to determine downstream flow requirements, such as minimum flows and their seasonal variation and flushing and channel-forming flows. Because minor diversions have little effect on floods and freshes, the main ecological concern is the minimum flow.

Flow variability and movement of bed sediments can have profound effects on stream ecosystems. Stable, spring-fed streams are subject to few floods, and the fish and plants that live in such streams are often unable to develop similarly or even to survive in less stable environments (Figure A2.2). On the other hand, gravel-bed rivers and their aquatic biota are in a constant state of change, caused by extreme flows (floods and droughts) and mobile bed sediments. Floods are the most important element of flow variability; flood frequency has been used in several biological models as the primary axis for classifying biological communities (Biggs et al 1998). In streams with frequent floods, fish and invertebrates that are small and can colonise new areas rapidly are often dominant (Scarsbrook and Townsend 1993), and the periphyton community is usually sparse, with low species richness and diversity (Clausen and Biggs 1997; Biggs and Smith 2002). In streams with stable flow regimes, aquatic communities are thought to be influenced more by biological processes such as competition between species and grazing/predation than by external environmental factors (Poff and Ward 1989; Biggs et al 1989).

The biological effects of flow variability usually refer to the effects of floods or the effects of long periods of low flows (eg, Figure A2.3). However, we are not aware of any studies that demonstrate that small-scale flow variation is biologically important. In fact, frequent flow variations are usually considered detrimental. Daily and weekly flow fluctuations are often a feature of rivers downstream of hydropower stations. These fluctuations in flow create a ‘varial’ zone that is wetted and dried as water levels rise and fall. With frequent flow fluctuations, this zone will not sustain immobile plant and invertebrate species. Mobile species such as fish, and probably some invertebrate species, can make some use of this zone – especially for feeding in recently inundated areas of river bed, where there may have been some terrestrial invertebrates in the substrate. However, a varial zone that is wetted and dried at more frequent intervals than a week is unproductive and can be regarded as lost habitat.

It can be seen that determining the river flows required to maintain particular instream values 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’, that considers the need to maintain floods, freshes, low flows, and aspects of flow variability. This, of course, means that the manager must have a clear idea of the outcomes that are desired, with regard to instream values, and the time and resources available to conduct an extensive ecological flow analysis.

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