Waitaha Hydro Scheme
Comprehensive freshwater ecology assessment, effects management, and expert evidence for a run-of-river hydro scheme on the West Coast’s Waitaha River – spanning 17 years of investigation from initial surveys through to Fast-track Approvals.
2007–present (surveys 2007–2024, AEE 2025, Fast-track 2026)
context
The Waitaha River is a West Coast river with a 315 km2 glacial-fed catchment, with one of the highest natural flood flow frequencies in the country and annual rainfall reaching 12–14 m. The freshwater fauna that make this river their home have been honed down to those that can survive these natural challenges, whilst distinctive biodiversity hotspots exist within some of the catchment’s hidden stable tributaries. Westpower proposes a run-of-river hydro scheme diverting up to 23 m3/s via a weir at Morgan Gorge through a pressurised tunnel to a power station 2.5 km downstream. The Scheme is being assessed under the Fast-track Approvals Act. Poutini Ngāi Tahu are project partners and kaitiaki of the Waitaha River.
challenge
- Characterising ecological values across a remote, high-disturbance glacial-fed non-wadeable river and wider catchment that presented substantial physical and logistical challenges to sampling. To build a comprehensive and defensible evidence base against which to assess potential effects, sampling spanned multiple survey campaigns from 2007 to 2024, encompassed mainstem and tributary habitats within and outside of the Scheme footprint, and integrated conventional freshwater ecology sampling and eDNA sampling methods, as well as the development of site-specific IFIM outputs based on detailed invertebrate and habitat sampling throughout the river mainstem.
- Morgan Gorge acts as a natural barrier to all fish except kōaro – the intake weir design must preserve kōaro passage upstream while maintaining the exclusion of other fish species.
- Protecting the ‘Stable Trib’ – which we identified as a biodiversity hotspot with distinct invertebrate communities, significant lamprey rearing habitat, and the only site where freshwater crayfish were recorded – from construction and operational effects.
our role
EOS Ecology has been freshwater ecology lead since 2007 – from the initial scheme optioneering and the earliest freshwater ecology surveys through to the AEE, Freshwater Ecology Management Plan, and expert evidence for the 2026 Fast-track hearing. Our role spans the full ecological evidence base: comprehensive surveys across the catchment, significance assessment against regional and national policy, effects assessment for construction and operations, freshwater management plan development, and response to independent technical review via the Fast-track process.
how we approached it
- Built a 17-year evidence base through fish surveys at 48 sites (electrofishing, fyke netting, spotlighting), invertebrate, periphyton and water quality sampling at 31 sites (including 308 quantitative invertebrate samples), and supplementary eDNA sampling at 20 sites using technology unavailable during the original surveys. Critical interpretation of eDNA data was essential — expert site knowledge identified a weak salmon signature recorded in a seepage habitat with no surface water connection as a false positive, demonstrating that eDNA results require ecological context to be defensible. With such a comprehensive dataset we were able to compare the ecology of this catchment against national datasets, and were also able to confirm the that Morgan Gorge is a barrier to the upstream movement of all fish bar kōaro.
- Implemented a habitat and invertebrate sampling programme that collected data on the river mainstem that could be used to develop site-specific habitat suitability indices that allowed for more relevant IFIM (Instream Flow Incremental Methodology) outputs. This required ‘out of the box’ thinking for how to implement a transect-based survey across a deep, high velocity environment with substrate that varied from bedrock to boulder to gravel.
- Assessed the Waitaha’s ecological significance against ten policy criteria, and evaluated construction and operational effects across 16 categories.
- Authored the Freshwater Ecology Management Plan (FEMP) setting out construction protocols, operational procedures for residual flow and shutdowns, compliance monitoring, and adaptive management provisions for supporting the freshwater values of the Scheme.
- Responded to independent peer reviews as part of the Fast-track process and presented expert evidence at the Fast-track empanelment, resulting in general agreement from expert reviewers around matters pertaining to fish ecology, fish passage and fish passage design criteria for the scheme’s main infrastructure (intake weir and tail race), and consent condition adequacy to maintain ecological values. Also contributed to a series of community engagement evenings across the West Coast, presenting the ecological aspects of the scheme to the wider community.
outcome
The projects demonstrates the value of a team with technical knowledge of naturally high flow disturbance river systems, who were able to develop and implement a comprehensive survey programme that would provide the data needed to more accurately predict likely outcomes and make informed decisions. The 17-year evidence base provided a robust foundation for the Fast-track Approvals Act hearing. Operational effects are concentrated on the 2.5 km abstraction reach and the weir intake structure, with the river’s naturally harsh conditions – extreme flood frequency, high sediment load, low water temperatures and extremely low nutrients – moderating the effects of the run-of-river scheme. The most sensitive habitat – the ‘Stable Trib’ downstream of the power station has been protected through keeping all scheme infrastructure away from it. Maintaining fish passage of kōaro but no other fish species upstream of the gorge is managed through the design of a fish passage structure that addresses the unique climbing ability of kōaro over other fish species. A long-term monitoring programme and adaptive management programme provides the mechanism for effective ongoing management.
wider impact
- Demonstrates how long-term ecological investigation builds the defensible evidence base required for complex consenting under New Zealand’s evolving regulatory framework, including the Fast-track Approvals Act.
- The kōaro passage design challenge is nationally significant – integrating climbing-specialist fish ecology with hydraulic engineering at an intake structure, with few precedents in NZ.
Freshwater ecology for energy infrastructure
Hydro schemes, wind farms, and transmission corridors all interact with freshwater environments in ways that require rigorous assessment and practical management plans. EOS brings nearly two decades of experience — from baseline investigation through consenting, construction oversight, and long-term monitoring. Talk to us early.