Dear Tavisha We Act for Opuha Water Limited (OWL), Submitter No. PC7

From: Georgina Hamilton To: Plan Hearings Cc: Glenire Farm; "Andrew Mockford"; Julia Crossman; Greg Ryder; Richard Measures; Keri Johnston; Tim Ensor Subject: Plan Change 7: Opuha Water Limited - Evidence Date: Friday, 17 July 2020 5:22:45 pm Attachments: Evidence in chief of Ryan O"Sullivan (OWL) 17.7.20.pdf Evidence in Chief of Andrew Mockford (OWL) 17.7.20.pdf Evidence in Chief of Julia Crossman (OWL) 17.7.20.pdf Quick reference guide (Annexure A to Evidence in Chief of Julia Crossman (OWL)).pdf Evidence in Chief of Richard Measures (OWL) 17.7.20.pdf Evidence in Chief of Keri Johnston (OWL) 17.7.20.pdf Evidence in Chief of Dr Gregory Ryder (AMWG & OWL) 17.7.20.pdf Evidence in Chief of Tim Ensor (OWL) 17.7.20.pdf

Dear Tavisha

We act for Opuha Water Limited (OWL), submitter no. PC7-381.

We attach for filing, in relation to the above matter, statements of evidence in chief of the following witnesses on behalf of OWL:

1. Ryan O’Sullivan (OWL Board Chair) 2. Andrew Mockford (OWL CEO) 3. Julia Crossman (OWL Environmental Manager) 4. Dr Greg Ryder (Lake Opuha - water quality) – note this statement of evidence addresses matters also pertaining to the submissions of the Adaptive Management Working Group (AMWG) and has also been filed with other AMWG evidence today. 5. Richard Measures (water quality) 6. Keri Johnston (hydrology/allocation) 7. Tim Ensor (planning)

We note that:

Annexure A to the evidence of Ms Crossman comprises a “Quick Reference Guide” providing a location map and key information regarding the Opuha Scheme. This is also attached as a separate document for the assistance of the Hearings Commissioners.

a flyover video of the Opihi catchment accompanies Mr Mockford’s evidence. A link is provided within Mr Mockford’s evidence by which the video can be accessed ( https://youtu.be/Kp6IuxCqWsk ). The video is also downloadable in mp3 format from the following link, which can then be shared/posted (e.g. on ECan’s PC7 webpage): https://we.tl/t-YgyExCMmGF

Kind regards

Georgina Hamilton Partner

Level 1, 24 The Terrace, TIMARU 7910 | PO Box 244, TIMARU 7940 PHONE: 03 687 8004 | DDI: 03 687 8065 | MOBILE: 027 686 9252 | FAX: 03 684 4584 EMAIL: [email protected]

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BEFORE INDEPENDANT HEARING COMMISSIONERS APPOINTED BY THE CANTERBURY REGIONAL COUNCIL

UNDER: the Resource Management Act 1991

IN THE MATTER OF: Proposed Plan Change 7 to the Canterbury Land and Water Regional Plan – Section 14: Orari-Temuka-Opihi-Pareora

______

STATEMENT OF EVIDENCE OF DR GREGORY IAN RYDER ON BEHALF OF THE ADAPTIVE MANAGEMENT WORKING GROUP (SUBMITTER NO. PC7-385) AND OPUHA WATER LIMITED (SUBMITTER NO. PC7-381)

Dated: 17 July 2020 ______

______

GRESSON DORMAN & CO Solicitors PO Box 244, Timaru 7940 Telephone 03 687 8004 Facsimile 03 684 4584 Solicitor acting: G C Hamilton [email protected]

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CONTENTS

1 Introduction, background & experience ...... 3

2 Code of Conduct ...... 3

3 Scope of evidence ...... 3

4 Executive summary ...... 4

5 Hydrology and proposed minimum flows ...... 6

6 Overview of ecology and Water quality of the Opuha and lower Opihi Rivers ...... 9

7 Minimum flows ...... 16

8 Lake Opuha ...... 27

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1 INTRODUCTION, BACKGROUND & EXPERIENCE

My full name is Gregory Ian Ryder.

My background, qualifications and experience, and involvement in PC7 were set out in my brief of evidence presented on behalf of the Flow and Allocation Working Party (FAWP).

2 CODE OF CONDUCT

I confirm that I have read the Code of Conduct for expert witnesses contained in the Environment Court’s Practice Note as updated in 2014. My evidence has been prepared in compliance with that Code. In particular, unless I state otherwise, this evidence is within my area of expertise and I have not omitted to consider material facts known to me that might alter or detract from the opinions I express.

3 SCOPE OF EVIDENCE

In this brief of evidence, I address the effects of abstraction and associated flow regimes on the water quality and ecology the Opuha River, lower Opihi River and Milford Lagoon. I also provide some comments on the water quality of Lake Opuha.

My evidence is structured as follows:

(a) An overview of the water quality and ecological values of the Opuha River and lower Opihi River.

(b) Information on flow and habitat requirements for species and life stages relevant to these rivers. Habitat retention is assessed in relation to various proposed flow regimes put forward under PC7 and the Adaptive Management Working Group (AMWG).

(d) Effects on the Milford Lagoon and the Opihi River mouth;

(e) Lake Opuha (relating to the submission of Opuha Water Limited on Tables 14(b), (e) and (f)).

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4 EXECUTIVE SUMMARY

Phosphorus concentrations in the lower Opihi River are generally low while DIN concentrations are elevated and in band C of the draft 2019 NPS-FM. Concentrations of ammoniacal nitrogen and nitrate nitrogen are low with respect to toxicity. E. coli levels place the river at Saleyards in either band A or band B of the draft 2019 NPS-FM.

Phormidium is a recognised problem in the lower Opihi River and is probably linked to nutrient levels, warm temperatures and stable flows. Benthic invertebrate community health index scores have ranged between excellent and fair, and the lower scores are likely to be linked to the combined influence of nutrients, temperature and stable flows.

The fish community of the Opuha River is dominated by brown trout, but its angling amenity is marred by Didymo, which flourishes in the river. The native fish community is similar to that found in the lower Opihi and Te Ana Wai rivers, and includes Canterbury galaxias, common and upland bully, longfin and shortfin eel, lamprey and torrentfish.

Water quality in Milford Lagoon is generally good, with relatively low E. coli and DRP levels, however DIN are elevated and often exceed 1 mg/L. Lagoon temperature appears to be driven primary by the temperatures of incoming water from the lower Opihi River, which in turn is determined largely by climate, and to some extent by the degree of mouth closure.

The effects of monthly minimum flows on the lower Opihi River instream ecology were assessed using instream habitat data and associated modelling undertaken by NIWA on behalf of ECan. Habitat retention under the monthly minimum flows in Table 14(v) and Table 14(w) of PC7 was compared against the monthly minimum flows proposed by the AMWG.

There are some predicted losses in habitat for juvenile brown trout and juvenile salmon under the AMWG's January and February minimum flows relative to PC7 flows, and gains in juvenile brown trout habitat in the months of March, April and October. The AMWG minimum flows over the salmon spawning season provide more habitat than is the case under PC7. The AMWG's minimum flows over the brown trout spawning season provide better conditions than do those in PC7's Table 14(w). All minimum flow

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regimes considered provide similar habitat availability for algae species. The time between freshes is considered to be the main factor controlling nuisance periphyton growths, not the size of the minimum flow.

The AMWG's monthly minimum flow regime provides adequate instream habitat in the Opuha and lower Opihi rivers for key species and life stages under low flow conditions. Minimum flows are considered to have relatively little effect on the extent and duration of nuisance algae growths in the Opuha and lower Opihi rivers. Floods and freshes are the primary mechanism for their removal through increasing the sheer stress on the bed and increasing the abrasive force of sediment movement along the bed.

Modelling of the PC7’s Alternative Management rules by Mr Kerr found that the lower Opihi River would not be able to maintain the PC7's minimum flow regime for the February to April 2015 and March to April in 2016 period when the catchment faced drought conditions, and Lake Opuha would empty. This would not occur under the AMWG’s proposed regime which would maintain ecological flows of 3 cumecs more often than under PC7. From an ecological perspective, the AMWG's Alternative Management rules appear to preferable to those in PC7.

Lake Opuha is well regarded as a recreational asset and is used for bathing, fishing and general boating activities. Regular monitoring over many years show that the lake's TLI scores typically range between 3 and 4 mesotrophic), but have exceeded 4 on occasions. Based on this assessment, the lake would not meet PC7's Table 1b outcome for TLI.

Analysis conducted on the OWL monitoring data of the lake suggests that chlorophyll-a concentrations are trending up over time, as is TN, but to a lesser extent. However, total phosphorus may be decreasing, and the TLI index has remained reasonably static with no clear trend.

OWL monitoring data indicates that PC7 TN and TP limits and targets for Lake Opuha (Table 14(e) and Table 14(f)) would not be achieved in some recent years.

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5 HYDROLOGY AND PROPOSED MINIMUM FLOWS

A summary of key flow statistics for the lower Opihi, Opuha and Temuka rivers is presented in Table 1. The flow statistics show that the bulk of the flow in the lower Opihi River (at the Saleyards Bridge) is made up of water from the Opuha River.

Table 1. Flow statistics based on measured flows from flow recorders associated with Opihi, Opuha and Temuka rivers (source: Dodson & Steele 20181).

River / Flow Record start Median Mean 7D-MALF FRE3 flow FRE3 events Accrual recorder site (m3/sec) (m3/sec) (m3/sec) (m3/sec) (# per year) period (days) Opuha River Skipton 01/03/62 – 2016 6.58 9.32 2.37 19.8 6.5 48.4 Upper Opihi River Rockwood 01/07/63 – 30/6/2015 3.27 5.23 1.21 9.8 7.4 45.5 Lower Opihi River Saleyards Br 07/05/04 – 30/6/2015 9.70 * 4.49 * * * SH1 Br 12/05/98 – 30/6/2015 8.31 13.79 3.22 24.9 5.6 58.3 Temuka River Manse Br 22/07/91 – 30/6/2015 3.36 5.96 1.27 10.1 5.1 68.4 * Not enough data to calculate a statistic/No data at high flows.

The current monthly minimum flows under the Opihi River Regional Plan are presented in Table 2, along with PC7’s proposed monthly minimum flows to be implemented in 2025 (Table 14(v)) and 2030 (Table 14(w)), and the AMWG’s proposed monthly minimum flows (all with full availability). I have not included minimum flow provisions for situations when further restrictions are to be implemented (‘Alternative management’ minimum flows), but I comment on these further in my evidence. I note that observed flows in the Opuha River (as measured at Skipton) will be higher than the minimum flows specified in Table 2 as OWL releases water from the Opuha Dam to meet the minimum flow at the Skipton bridge plus the sum of abstractions downstream of the Saleyards bridge.

1 Dodson, J. & Steele, K. 2006. Current state of surface water hydrology in the Opihi and Temuka catchments Environment Canterbury Report No. R16/46.

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Table 2. Opihi Freshwater Management Unit Environmental Flow Regimes – AA and BA Permits.

Opihi River Location of Lake Opuha Minimum flow for AA and BA Permits Partial (mainstem) recorder or Restrictions site, Management (L/sec) or site where Regime flow is measured Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

Above RL 3,500 3,500 7,500 8,000 4,500 4,000 4,000 4,500 6,000 8,500 7,000 6,000 N/A 375m Current At or below RL 375m, but 3,350 3,350 5,350 5,600 3,850 3,600 3,600 3,850 4,600 5,850 5,100 4,600 50% Opihi River above RL 370m @ PC7 Saleyards Table 14(v) Bridge Full availability 3,500 3,500 7,500 8,000 4,500 4,000 4,000 4,500 6,000 8,500 7,000 6,000 N/A (2025)

PC7 Table 14(w) Full availability 3,800 3,800 7,800 9,000 5,300 4,800 4,800 5,200 6,600 9,400 7,300 6,300 N/A (2030)

PC7 Opuha River Table 14(v) @ 1,500 N/A & Skipton Table 14(w) Bridge

AMWG Full availability 4,500 4,500 7,000 7,000 4,500 4,000 4,000 4,500 6,000 8,000 7,000 6,000 N/A

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A key difference between Table 14(v) and 14(w) monthly minimum flows and those proposed by the AMWG is that the AMWG proposes higher minimum flows for January and February. Mr Webb provides a separate analysis of the ecological and recreational benefits of these higher minimum flows in his brief of evidence presented on behalf of the AMWG.

Derivation of current minimum flows

There have been various reports and investigations into minimum flow requirements in the lower Opihi River for ecological reasons. These have been based loosely around observations of flows to maintain surface connectivity between the Saleyards bridge and the SH1 bridge, providing sufficient trout habitat and food production, and maintaining lagoon health.

Sagar & Palmer (19902) recommended a minimum flow of at least 3 cumecs (m3/s) be implemented at SH1, if flow augmentation proceeded, on the basis that this would ‘probably’ ensure continuous river flow and, with Temuka River flow, would also ‘ameliorate’ the frequency of a closed river mouth condition. This recommendation was partly supported by observations noted in Sagar (19843) that a flow of 2.78 m3/s at Saleyards Bridge (which Sagar reported may equate to 1.4 m3/s at Grassy Banks) did not provide for salmon passage. Sagar (19884) went on to recommend the following minimum flows for fishery purposes in the lower Opihi River:

Saleyards Bridge 3.2 m3/s throughout the year;

Rockwood 3.7 m3/s September to April inclusive;

3.0 m3/s May to August inclusive.

The flow recommendation for the Saleyards Bridge site was made on the basis that this will maintain a continuous flow between the Saleyards Bridge and the Temuka confluence and an open river mouth will be maintained ‘most of the time’. It was also considered that habitat favoured by fish and invertebrates declined rapidly once flow falls below 3.0 m3/s.

2 Palmer, K. & Sagar, P.M. 1990. Sword Feasibility Study - Stage 3: Initial Assessment of implications to benthic and fisheries habitat of the Opihi River. MAF Fisheries. New Zealand Freshwater Fisheries Miscellaneous Report No. 34. 3 Sagar, P.M. 1984. Comments on draft Opihi River Water Management Plan by Fisheries Research Division. Submission by Ministry of Agriculture and Fisheries. 4 Sagar, P.M. 1988. Assessment of flows in the Opihi River necessary for fishery purposes. From Appendix 2 of ‘Opihi River Augmentation Scheme Environmental Impact Assessment’. Prepared by MAFFish, Christchurch.

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Sagar further noted that at Rockwood, optimum river flows for brown trout spawning, feeding and juvenile habitat are 2-3 m3/s, adult brown trout habitat is 5-7 m3/s, and food (invertebrate) production is 7 m3/s. The median flow at Rockwood at the time was deemed to be 3.7 m3/s and Sagar recommended this as the ‘minimum’ flow for fisheries purposes, and it was anticipated that flow augmentation (from Lake Tekapo) would provide this higher flow.

Palmer & Sagar (1990) cited Todd (19835), who suggested 6 m3/s as the minimum flow required to maintain an open mouth. Below this level the mouth was report to be closed 90% of the time. Closures at higher flows occur, but are short duration events (Todd 19856).

I return to the issue of minimum flows in section 8 of my evidence.

6 OVERVIEW OF ECOLOGY AND WATER QUALITY OF THE OPUHA AND LOWER OPIHI RIVERS

In my evidence on behalf of the FAWP, I summarised the general physical character of the Opihi and Opuha river catchments and will not repeat that information here, except to note that these two rivers converge at Raincliff, approximately 25 km downstream of the Opuha Dam tailrace (Figure 1). At this point the Opuha River has a much greater flow on average than the Opihi River (Table 1). Being a regulated river, the Opuha River’s flow is subject to minimum flow requirements provided by releases of water from Lake Opuha.

Opuha River

Two monitoring sites in the Opuha River indicate that DRP concentrations are typically very low (median concentrations of < 0.002 mg/L) and DIN concentrations are relatively low (median concentrations of < 0.3 mg/L).

A key ecological aspect of the Opuha River since the construction of the Opuha Dam has been its effect on the benthic community. Periphyton monitoring and nutrient surveys have been conducted at key sites on the

5 Todd, D.J. 1983. Effect of low flows on river mouth closure in the Opihi River. University of Canterbury, Christchurch (Unpublished M.Sc. Thesis). 6 Todd, D.J. 1985. Opihi River Mouth Behaviour. 1985 Australasian Conference on Coastal and Ocean Engineering. Christchurch N.Z. 2:569-579.

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Opuha and Opihi Rivers since 2013 (Measures et. al 20187).

Figure 1. Opihi River catchment (shaded) showing major rivers and Lake Opuha.

Didymo flourishes in the Opuha River and most likely is adversely affecting the benthic invertebrate community. Didymo does well in low phosphorus systems. Since the arrival of Didymo, Phormidium cover has been generally low in the Opuha River, but was an issue prior to then. Mr Measures in his evidence for the AMWG describes flows required to manage nuisance algae growths in the Opuha and lower Opihi rivers.

Since 2002, OWL has commissioned annual benthic invertebrate surveys of the Opuha River (downstream of the Skipton Bridge) which are required under Resource Consent CRC950577.5. A summary of the results are presented in Table 3. Sevicke-Jones (20208) concluded that the invertebrate

7 Measures, R., Greenwood, M., & Kilroy, C. 2018. Statistical analysis of periphyton and water quality in the Opuha and Opihi Rivers. Prepared for Opuha Water Limited. NIWA Client Report 2018103CH. 8 Sevicke-Jones, G. 2020. Opuha Water Limited Resource Consent Monitoring (CRC950577.5). prepared for Opuha Water Limited.

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fauna is dominated by taxa at the two monitoring sites that is typical of clean water species (mayflies and caddisflies), although both sites also had abundant diptera (small aquatic flies), which I consider is probably associated with the periphyton biomass. Sevicke-Jones (2020) concluded that the invertebrate biological metrics assessed indicated good to moderate and good to clean habitat (i.e., an ecosystem with low organic pollution).

Table 3. Macroinvertebrate community metrics, Opuha River monitoring (2002-2020) (Sevicke-Jones 2020).

Year Downstream 2.5km of Skipton Bridge Downstream 7km near Glenlea # Taxa % EPT MCI SQMCI # Taxa % EPT MCI SQMCI 2002 13 70 118 5.7 16 63 115 5.8 2003 28 54 108 5.4 25 52 111 4.9 2004 22 27 79 3.9 24 21 80 3.4 2005 23 48 82 3.5 22 59 89 4.2 2006 21 57 98 3.3 12 33 83 4.5 2007 19 53 101 5.9 19 53 96 6.7 2008 16 56 99 5.9 20 45 94 5.7 2009 15 60 112 4.9 16 44 106 5.2 2010 17 47 98 4.6 13 46 98 2.9 2011 20 50 105 5.7 19 42 93 5.7 2012 25 56 114 4.8 22 45 104 5.6 2013 18 78 106 4.9 24 42 98 5.3 2014 20 45 97 5.3 23 43 102 6.7 2015 20 45 104 4.4 22 45 107 5.5 2016 12 25 77 4.0 8 13 88 3.9 2017 27 44 91 3.1 30 37 88 2.2 2018 15 47 107 7.0 9 56 102 7.5 2019 19 19 99 4.4 21 48 105 6.6 2020 28 39 108 5.1 27 37 94 5.1 Average 20 48 100 4.8 20 43 98 5.1

The fish community of the Opuha River appears to be dominated by brown trout, but its angling amenity is marred by the presence of Didymo. CSIF&G have previously reported extensive trout spawning in this river. The river has also been an important habitat for salmon spawning, although in recent years the number of redds has reduced significantly compared to numbers reported in the 1990s through to approximately 2012. Other species recorded in the river include Canterbury galaxias, common and upland bully,

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longfin and shortfin eel, lamprey and torrentfish (Table 4). This assemblage of native fish species is similar to that found in the lower Opihi and Te Ana Wai rivers.

Lower Opihi River

As I described in my brief of evidence on behalf of the FAWP, OWL established a water quality monitoring programme that commenced in January 2019 and is still running. The programme includes a site on the Opihi River at the Saleyards bridge. I have assessed this data up to March 2020 to assess ecosystem health and compared the data with attribute states in the draft 2019 NPS-FM document.

Over the monitoring period, DRP concentrations were generally low and often below the laboratory detection limit of 0.004 mg/L, except in the winter months. DIN concentrations were generally always elevated (median of 0.46 mg/L) and exceeded 1 mg/L on three occasions. Compared against the proposed attribute states in the draft 2019 NPS-FM document, the Saleyards bridge site would be in band A for DRP and band C for DIN.

Concentrations of ammoniacal nitrogen and nitrate nitrogen were low with respect to toxicity attribute states, and would place the site in band A for both attributes.

E. coli levels over the monitoring period would place the river at this site in either band A or band B of the draft 2019 NPS-FM.

Separate water quality monitoring described in Measures et. al (2018) for sites at Saleyards Bridge and SH1 also indicate typically low DRP levels and elevated DIN levels (medians of 0.421 and 0.505 mg/L respectively).

Phormidium is a recognised problem in the lower Opihi River. Measures et. al (2018) suggested that higher Phormidium biomass occurred when temperatures were warmer, DIN concentrations were reduced (but higher than in the Opuha River at Gorge and Skipton) and flows were more stable with longer accrual periods and reduced maximum flows.

I have not surveyed the benthic invertebrate community of the lower Opihi River, apart from Saleyards bridge site in January 2019 (SQMCI score of 7.3), however ECan report average QMCI scores of approximately 6.3

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(excellent) and 5.2 (good) for the river at SH1 and Grassy Banks, respectively, for the period 2011 to 2015. More recent monitoring at SH1 by ECan records QMCI scores of 4.3 and 4.8 (both fair) for the years 2017 and 2019. The shingle bed and clear water of the lower Opihi River should support an invertebrate community dominated by insect taxa such as caddisflies and mayflies. However, it is likely that warm summer temperatures, elevated nutrient levels and stable flows influence the community structure directly and indirectly, such that taxa more tolerant of these conditions are likely to dominate when they occur.

The lower Opihi River has a diverse fishery. Our February 2019 fish survey found bluegill bully, common bully, juvenile brown trout, and torrentfish to be abundant under low flow conditions. Longfin and shortfin eel were also present.

Milford Lagoon

ECan monitor the water level of Milford Lagoon along with water temperature and occasionally other water quality parameters.

E. coli levels are typically low, although there are occasional high spikes. DRP are relatively low, however DIN are elevated and often exceed 1 mg/L.

Lagoon temperature appears to be driven primary by the temperatures of incoming water from the lower Opihi River, which in turn is determined largely by climate, and to some extent by the degree of mouth closure. High flow events appear to temporarily reduce lagoon temperatures, but this is not unexpected as such events are usually associated with rain fronts which are typically associated with cooler weather.

There is less documented information on the ecology of the lagoon. It is known as a whitebait spawning area and supports an eel fishery. Given these are migratory species with a marine life stage, access to and from the sea is crucial. Conclusion

In broad terms, the Opuha and lower Opihi rivers are valued for their diverse native and salmonid fisheries. These are regarded as important for cultural and recreational interests. As with some other rivers in the Opihi river

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catchment, elevated nitrate levels are an issue in the lower Opihi River and coupled with warm temperatures and stable flows, can result in nuisance periphyton growths including Phormidium. A gradual increase in DRP in the lower river is most likely also contributing to high periphyton biomass. The managed flow regime of the Opuha River has resulted in issues associated with nuisance periphyton growths, firstly with Phormidium and in more recent years with the introduced stalked-diatom Didymo. Management of these effects is addressed further below.

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Table 4. Freshwater fish distribution and migration calendar for key species in the Opuha and lower Opihi rivers. The table shows migration timing, peak periods, migration direction and life stage at the time of migration.

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7 MINIMUM FLOWS

I assessed the effects of monthly minimum flows on the lower Opihi River instream ecology using instream habitat data and associated modelling undertaken by NIWA on behalf of ECan (Jellyman 20189). The site used was Kerrytown, located approximately 4.5 km downstream of the Saleyards bridge and 5.7 km upstream of the SH1 bridge near Temuka. The modelling files and output data were made available to me courtesy of NIWA.

In my assessment of habitat retention that I presented in evidence on behalf of the FAWP, I assessed monthly minimum flows in relation to the naturalised 7D-MALF calculated for each river. The lower Opihi River has a highly modified flow regime due to the presence of the Opuha Dam and the flow augmentation it provides to the Opihi River downstream of Raincliff. Consequently, I consider assessing against a naturalised MALF for this section of the river is not appropriate and I note that ECan also does not consider the naturalised 7D-MALF in its assessments of instream habitat for this section of the river.

The approach I have adopted for the lower Opihi River is to compare habitat retention under the monthly minimum flows in Table 14(v) and Table 14(w) of PC7 and compared them against the monthly minimum flows proposed by the AMWG (Table 2). I then compared difference in habitat retention provided by the minimum flows in Table 14(v) with those in Table 14(w). All of this information is summarised in Table 5.

I have presented some of the graphical outputs of the instream habitat modelling for the lower Opihi River in Figures 2a through to 2e.

Figure 2a shows that habitat generally increases rapidly with flows up to about 3 cumecs for the four invertebrate taxa assessed and food producing water, then more gradually beyond about 3-4 cumecs depending on the taxa. The modelling indicates that the river provides a lot of habitat for the Deleatidium mayfly, which is typically a dominant food item in the diets of both native and salmonid fish, and its abundance is an indicator of good water quality and habitat conditions in shingle river systems. The AMWG’s

9 Jellyman, P. 2019. Lower Opihi River ecological flow assessment. Prepared for Environment Canterbury. NIWA Client report 2019231CH.

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proposed minimum flows for January and February result in a >6% increase in Deleatidium habitat relative to that in PC7’s Table 14(v) flows for the same months, and a 4.5% increase in habitat relative to PC7’s Table 14(w) flows. All invertebrate taxa assessed have some gain in habitat. Table 5 shows that there are minor gains in habitat for invertebrate taxa for Table 14(w) summer flows relative to Table 14(v) flows.

Figure 2b shows that habitat for most native fish generally declines beyond flows of between 1 and 3 cumecs. The AMWG’s proposed minimum flows for January and February result in decreases in habitat of between 1-19% for native fish species (except torrentfish) relative to that in PC7’s Table 14(v) flows for the same months, and lesser decreases relative to Table 14(w) flows. There is a mix of habitat gains and losses when comparing minimum flows for March and April under the AMWG minimum flow of 7 cumecs relative to Table 14(v) flows of 7.5-8 cumecs for the same months.

Figure 2c shows habitat availability versus flow for adult and juvenile brown trout and juvenile Chinook salmon. Adult brown trout show significant gains in habitat for the months of January and February under the AMWG’s minimum flows (24% gain relative to PC7’s 2025 flows and 18% gain relative to PC7’s 2030 flows). Mr Webb in his evidence for the AMWG notes that these gains represent better conditions for trout fishing and that higher minimum flows in the Opihi River mainstem will also increase the time the river mouth is open to the benefit of salmon angling.

There are some predicted losses in habitat for juvenile brown trout and juvenile salmon under the AMWG’s January and February minimum flows relative to PC7 flows, and gains in juvenile brown trout habitat in the months of March, April and October. There is no evidence that the lower Opihi River trout fishery is limited by recruitment.

Potential salmonid spawning habitat is presented in Figure 2d. Salmon spawn in autumn (Figure 2) and the AMWG minimum flows over this period provide more spawning habitat than is the case under PC7. Brown trout spawning tends to peak in winter (Figure 2) and the AMWG’s minimum flows over this period provide better conditions than do those in PC7’s Table 14(w).

All minimum flow regimes considered provide similar habitat availability for algae species (Figure 2e). Because the modelling indicates that habitat for

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all algae types generally increases with increasing flow around the minimum flow range considered, somewhat ironically, habitat for most algae groups increase as the minimum flow increases.

As I stated in my FAWP brief of evidence, it is my opinion that the accrual period (time between freshes) is the main factor controlling nuisance periphyton growths. Mr Measures, in his brief of evidence for the AMWG, provides details on flows required to remove nuisance algae growths in the Opuha and lower Opihi rivers. He notes that a Canterbury-wide study that included data from the Opihi River at SH1 found that freshes exceeding 45 cumecs were the flows most strongly correlated with a reduction in periphyton biomass, and freshes exceeding 90 cumecs were the flows most strongly correlated with a reduction in the cover of Phormidium (now renamed as Microcoleus) (Kilroy et al. 201710). These are an order of magnitude greater than the minimum flows considered here.

Table 5 also provides an assessment of habitat gains and losses for various freshwater species and life stages under the monthly minimum flows of Table 14(v) and Table 14(w) relative to habitat under the AMWG’s flows. I have colour-coded cells to reflect habitat gains and losses. For the first two sections of the table, cells shaded blue indicate the AMWG’s minimum flow provides more habitat (or no difference) than flow for the equivalent month in the PC7 tables. Cells shaded red indicate the opposite. An exception to these rules applies to the ‘Bad algae’ columns, where blue shaded cells indicate a reduction in habitat and red shaded cells indicate an increase in habitat. For the bottom third section of Table 5, where the two PC7 tables are compared to each other, cells shaded blue indicate the 2025 Table 14(w) minimum flow provides more habitat than flow for the equivalent month in the 2030 Table 14(v). Cells shaded red indicate the opposite.

Over the summer months, there are minor gains in invertebrate habitat under the 2030 Table 14(w) flows, and minor losses in habitat for native fish (except torrentfish), juvenile brown trout and juvenile salmon, and an 8% gain in adult brown trout habitat. There are significant losses in salmonid spawning habitat under the 2030 flows, and losses in many native fish life stages over the winter months. However, I do not consider these to be overly important

10 Kilroy C., Wech J., Kelly D., Clarke G. (2017) Analysis of a three-year dataset of periphyton biomass and cover in Canterbury Rivers (Draft), NIWA Client Report 2017085CH for Environment Canterbury.

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given fish tend to have lower metabolic activity and food requirements under cold water conditions. I note that Mr Webb makes a similar point in his evidence for the AMWG.

Conclusion

I consider that the AMWG’s monthly minimum flow regime provides adequate instream habitat in the Opuha and lower Opihi rivers for key species and life stages under low flow conditions. In my opinion, minimum flows have relatively little effect on the extent and duration of nuisance algae growths in the Opuha and lower Opihi rivers. Mr Measures notes that, currently, Didymo is the main nuisance periphyton in the Opuha River, but Microcoleus (formerly Phormidium) is the main issue in the Opihi River, and that floods and freshes are the primary mechanism for their removal through increasing the sheer stress on the bed and increasing the abrasive force of sediment movement along the bed. That assessment is consistent with my own views on the management of nuisance periphyton growths, as expressed in my brief of evidence presented on behalf of the FAWP. Notwithstanding that view, I am also of the opinion that nutrient management is required to further reduce the risk of blooms developing, and for maintaining general ecosystem health.

OWL has the ability to provide flushing flows down the Opuha River as discussed in Mr Measure’s evidence. He notes that the Opihi River mouth is open more often and for longer periods of time than it did prior to the operation of the Opuha Dam, a point also noted in Mr Webb’s evidence. Most likely, this has been beneficial for migratory fish species with a marine phase, including diadromous native fish and salmon. There is clearly a balancing act between flow releases from Lake Opuha for downstream ecological benefits and maintaining sufficient storage in the lake for downstream abstraction, and minimum flows for ecological purposes over the short to medium term. The flows required to remove nuisance periphyton are large and to provide these following a sustained period of low flows carries with it some risk to other aspects of the river’s ecology. For example, there is risk of flushing fish downstream if waters rise quickly and with force, and a possible risk of stranding fish if waters recede too quickly. Although I am not an expert in riverine birds, I am aware of concerns raised elsewhere about sudden rises in water levels washing out nesting sites on braided rivers.

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Table 5. Analysis of potential changes in physical habitat in the Lower Opihi River (between Saleyards bridge and SH1 bridge) using NIWA's instream habitat model outputs and proposed PC7 (Tables 14(v) and 14(w)) and AMWG monthly minimum flows at Saleyards Bridge. Cells shaded blue indicate the AMWG’s minimum flow provides more habitat (or no difference) than flow for the equivalent month in the PC7 tables. Cells shaded red indicate the opposite. For the bottom third section, cells shaded blue indicate the 2025 Table 14(w) minimum flow provides more habitat than flow for the equivalent month in the 2030 Table 14(v). Cells shaded red indicate the opposite. (note: for ‘Bad algae’ columns, blue shaded cells indicate a reduction in habitat and red shaded cells indicate an increase in habitat)

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Alternative Management rules have been assessed by Mr Kerr on behalf of the AMWG. His modelling has predicted that, under PC7’s proposed Alternative Management rules, the lower Opihi River would not be able to maintain the plan’s minimum flow regime for the February to April 2015 and March to April in 2016 period when the catchment faced drought conditions. He also determined that a flow of 3 cumecs at Saleyards Bridge (in February 2015) would not be able to be maintained. As I noted in section 5, flows of around 3 cumecs have been considered necessary to avoid significant adverse effects on river ecology including surface connectivity downstream of the Saleyards bridge.

Mr Kerr’s modelling found that the AMWG regime maintained ecological flows of 3 cumecs more often than under PC7, and has the least amount of time with flows at Saleyards bridge below the regime’s minimum flows.

In terms of ecological effects of Alternative Management rules, I first consider that if a situation is reached that requires the rules to be implemented, then it is highly likely that the downstream river system will already be under significant stress associated with prolonged low flows under warm weather conditions. This will mean daily maximum water temperatures will be high and particularly stressful for salmonid fish, daily swings in dissolved oxygen and pH will be large, nuisance algae growths will be common, fine sediment deposits will have built up on stones surfaces and wetted width of rivers will have receded potentially isolating backwaters and stranding fish and benthic invertebrates. Therefore, any measures to avoid or reduce these effects will provide some relief to the river ecosystem. Mr Kerr’s modelling indicates that the extent of low flows under drought conditions are less pronounced under the AMWG’s rules relative to rules proposed under PC7. Further, the AMWG’s rules avoid Lake Opuha from being emptied, which must already be regarded as a positive ecological effect relative to the situation under PC7 rules.

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Figure 2a. Relationships between potential habitat and flow for invertebrate taxa and food producing water modelled by NIWA for the lower Opihi

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River at Kerrytown (downstream of the Saleyards bridge). Vertical arrowed lines indicate January-February minimum flows in tables 14(v) and 14(w) and that proposed by the AMWG. Raw data supplied by NIWA.

Figure 2b. Relationships between potential habitat and flow for native fish species/life stages modelled by NIWA for the lower Opihi River at Kerrytown (downstream of the Saleyards bridge). Vertical arrowed lines indicate January-February minimum flows in tables 14(v) and 14(w) and that proposed by the AMWG. Raw data supplied by NIWA.

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Figure 2c. Relationships between potential habitat and flow for salmonid species/life stages modelled by NIWA for the lower Opihi River at Kerrytown (downstream of the Saleyards bridge). Vertical arrowed lines indicate January-February minimum flows in tables 14(v) and 14(w) and that proposed by the AMWG. Raw data supplied by NIWA.

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Figure 2d. Relationships between potential habitat and flow for salmonid spawning modelled by NIWA for the lower Opihi River at Kerrytown (downstream of the Saleyards bridge). Vertical arrowed lines indicate January-February minimum flows in tables 14(v) and 14(w) and that proposed by the AMWG. Raw data supplied by NIWA.

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Figure 2e. Relationships between potential habitat and flow periphyton modelled by NIWA for the lower Opihi River at Kerrytown (downstream of the Saleyards bridge). Vertical arrowed lines indicate January-February minimum flows in tables 14(v) and 14(w) and that proposed by the AMWG. Raw data supplied by NIWA.

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8 LAKE OPUHA

Lake Opuha is well regarded as a recreational asset and is used for bathing, fishing and general boating activities. Mr Measures has discussed in his brief of evidence the continuous monitoring of dissolved oxygen and temperature through the water column. OWL also monitors surface water quality at the dam’s boat ramp and has this assessed for nutrient and chlorophyll-a concentrations. This information is used to calculate the trophic lake index (TLI). TLI outcomes are included in Table 1b of PC7 (Freshwater Outcomes for Canterbury Lakes). Under PC7, Lake Opuha is classed as an ‘Artificial lakes – on river’ which have a TLI outcome of 3 (maximum score).

Figure 3 displays the nutrient and chlorophyll-a concentrations from data collected by OWL over time. TLI scores are also presented. The data show that the lake’s TLI scores typically range between 3 and 4 (average over time of 3.7 or mesotrophic), but have exceeded 4 on occasions. Based on this assessment, it is my understanding that Lake Opuha would not meet PC7’s Table 1b outcome for TLI.

Analysis conducted on the data presented in Figure 3 suggests that chlorophyll-a concentrations are trending up over time (statistically significant), as is total nitrogen, but to a lesser extent. However, total phosphorus, if anything, appears to be decreasing, and the TLI index has remained reasonably static with no clear trend.

Table 14(e) of PC7 (Water Quality Limits for Orari-Temuka-Orari-Pareora Lakes) has a TP annual average limit of 0.011 mg/L for the lake. My assessment of the OWL monitoring data for Lake Opuha is that this limit would not be achieved in three out of the last six years, although there is limited data for some of those years. Table 14(f) of PC7 (Water Quality Targets for Orari-Temuka-Opihi-Pareora Lakes) has a TN annual average target of 0.75 mg/L for the lake. My assessment of the OWL monitoring data for Lake Opuha is that this target would not be achieved in two out of the last six years.

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Figure 3. Chlorophyll-a, TN and TN concentration data collected from Lake Opuha (boat ramp) on behalf of OWL, along with calculated TLI scores, August 2005 to May 2020.

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In general, the monitoring data in Figure 3 is consistent with the observations I noted in my brief of evidence for the FAWP regarding increasing nitrate concentrations in the lower North Opuha River, but that phosphorus concentrations were low and show no signs of increasing (as is the case for the South Opuha River also).

In relation to the 2019 draft NPS-FM lake attributes for TN, TP and chlorophyll-a, in recent years the lake appears to range between bands B and C for TN, bands A and B for TP, and bands B and C for chlorophyll-a.

Gregory Ian Ryder

17 July 2020

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