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Minimum Flows for Ecosystem Health in Selected Coromandel Streams: Awaroa, Tapu, Waiomu and Wharekawa

Report: TR 2007/31
Author: Thomas K. Wilding (NIWA)

Abstract

Managing the water resources of the Coromandel Peninsula requires information on the flow requirements of aquatic ecosystems. Four streams were surveyed in 2006; the Wharekawa, Awaroa, Tapu and Waiomu. These streams were proposed by Environment Waikato because they are considered representative of Coromandel streams and have good flow information. This study builds on data collected by NIWA and Environment Waikato in previous years that looked at other catchments in the Coromandel.

Potential instream ecological issues relating to flow include fish and invertebrate habitat, water temperature and dissolved oxygen. Environment Waikato asked NIWA to investigate the flow requirements for these specific issues. The relative importance of each of these issues is expected to vary between catchments and between reaches within a catchment. The selected catchments were divided into hydraulically similar sections with common issues, distinguishing upland reaches from lowland reaches. The methods chosen to investigate each issue were WAIORA for oxygen modelling and RHYHABSIM for habitat and temperature modelling.

The recommended minimum flows for the assessed reaches are summarised in Table 1. Fish habitat is considered the critical issue in recommending minimum flows for the Awaroa Stream, Tapu River and Waiomu Stream. Further oxygen modelling is required to confirm preliminary results that oxygen is the critical issue for the Wharekawa River. The change in water temperature with flow was modelled for the Tapu River, Waiomu Stream and the Wharekawa River. The predicted change in temperature with flow is expected to be small (fraction of a degree), especially when compared to daily temperature variation or riparian shade effects, so is not considered a critical issue.

A predictive equation was developed for estimating the minimum flow requirements of aquatic ecosystems of upland streams in the Coromandel area. This is intended for application to streams where habitat surveys have not already been carried out. For lowland and midland streams of the Coromandel, further work is recommended to develop better predictive equations for minimum flow requirements.

Minimum Flows for Ecosystem Health in Selected Coromandel Streams: Awaroa, Tapu, Waiomu and Wharekawa
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Table of contents

  Executive Summary iv
1. Introduction 1
1.1 Study brief and background 1
1.2 Framework for determining minimum flow requirements 1
1.3 Introduction to Instream Habitat Modelling 3
1.3.1 Flow Assessment Methods 3
1.3.2 Habitat preferences and suitability curves 4
1.3.3 Habitat Mapping, Instream Habitat Modelling, and Prediction of Habitat Suitability 5
1.3.4 Procedure for Calculating Instream Habitat 6
1.3.5 Assessing Minimum Flow Requirements 6
2. Sites and Methods 9
2.1 Study Sites 9
2.1.1 Awaroa Stream 11
2.1.2 Tapu River 15
2.1.3 Waiomu Stream 16
2.1.4 Wharekawa River 18
2.2 Fish Community 22
2.3 Instream Habitat 22
2.4 Dissolved oxygen 24
2.5 Water temperature 27
2.6 Tide and aquatic plant survey 27
3. Results 29
3.1 Natural flow estimates 29
3.2 Fish Community 30
3.3 Instream Habitat 32
3.4 Dissolved oxygen 47
3.5 Water Temperature 50
3.6 Tide and aquatic plant survey of the Wharekawa River 55
3.7 Applying results to other Coromandel streams 56
3.7.1 Habitat 56
3.7.2 Oxygen 59
4. Discussion 62
4.1 Minimum flow recommendations 62
4.2 Applying results elsewhere 65
5. Acknowledgements 69
6. References 70
7. Appendix 1: Environment Bay of Plenty Instream Management Objectives
(reproduced from Wilding 2003)
76
8. Appendix 2: GPS locations for survey sites. 85
9. Appendix 3: Habitat suitability curves. 86
10. Appendix 4: Rating curve changes. 92
11. Appendix 5: Physical habitat data. 93
12. Appendix 6: Temperature modelling parameters. 98
12.1 Waiomu Stream 98
12.2 Tapu River 99
12.3 Wharekawa River 101