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Minimum Flows for Ecosystem Health in Lowland Streams of the Coromandel: Wentworth, Stony, Whareroa, Waikawau and Waikanae

TR 2007/29
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Report: TR 2007/29

Author: Thomas K. Wilding (NIWA)

Abstract

Managing the water resources of the Coromandel Peninsula requires information on the flow requirements of aquatic ecosystems. The purpose of this report is to assess the minimum flow requirements for aquatic ecosystems inhabiting Coromandel streams. Site specific minimum flows were determined for five streams (Stony, Waikanae, Waikawau, Wentworth and Whareroa), and methods were investigated for predicting the flow requirements of lowland streams not directly surveyed. This research is intended as the third and last year of minimum flow investigations for the Coromandel area. It is complimentary to previous work, and focuses on lowland streams.

Potential instream ecological issues relating to flow include fish and invertebrate habitat, water temperature and dissolved oxygen. From previous research (Whenuakite, Wharekawa, Opitonui), oxygen and fish habitat are expected to be critical issues for lowland streams of the Coromandel area. The methods chosen to investigate these issues were WAIORA for oxygen modelling and RHYHABSIM for habitat 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 Stony Stream, Waikanae Stream, Waikawau River and Whareroa Stream. Maintaining adequate dissolved oxygen concentrations was the critical issue for the Wentworth River. A repeat of oxygen modelling for the Wharekawa River determined that oxygen is not a critical issue for this river, so minimum flows determined in previous reports for fish habitat should be adopted as minimum flows for this river.

Predicting the flow requirements for lowland streams of the Coromandel area requires the consideration of both oxygen and habitat. For dissolved oxygen, a risk assessment key is provided for determining the need for more detailed investigations before recommending a minimum flow. This risk assessment procedure was proposed in previous reports was supported by further testing for this report. Flow requirements for fish habitat can be adequately predicted using equations developed based on the five-year low flow.

Minimum Flows for Ecosystem Health in Lowland Streams of the Coromandel: Wentworth, Stony, Whareroa, Waikawau and Waikanae [PDF, 1.2 MB]