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 [PDF, 1.3 MB]

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