Publications: Research reports and publications

Instream habitat flow analysis for the Waimea River and provisional minimum flows for proposed dam sites in the Upper Wairoa and Lee catchments

1 November, 2005
Cawthron Report 1061. Prepared for Waimea Water Augmentation Committee.

EXECUTIVE SUMMARY

This report assessed the minimum flows required to provide instream habitat in the Waimea River and at two potential alternative dam sites as part of a feasibility study into the potential to augment flow in the Waimea from a water storage reservoir in its catchment. In each case we provide three alternative minimum flows to span a range from an environmental benchmark minimum flow that would be conservative in terms of environmental protection, to a minimum flow that would be weighted towards out-of-stream values.

The proposed water augmentation can be seen as an opportunity to redress the balance between instream and out-of-stream water uses. Current water allocation in the catchment is heavily biased toward out-of-stream users, and the instream values have suffered as a result.

Instream flow requirements for the Waimea River in the section upstream of the Appleby Bridge were assessed using physical habitat modelling within the Instream Flow Incremental Methodology (IFIM). These analyses update earlier habitat modelling undertaken in this reach by Hayes (1998).

The proposed minimum flows provided for this reach were based on maintenance of adult brown trout habitat. Brown trout attract relatively high angler use of the Waimea River and have the potential to support a valued fishery, given sufficient maintenance flows. Brown trout are also among the most flow demanding freshwater fish in New Zealand rivers, and so providing adequate flow for them should also provide for the flow needs of other species, including most native fishes.

The estimated natural MALF of 1.3 m3s-1 is proposed as the environmental benchmark minimum flow for the Waimea River immediately upstream of the Appleby Bridge. A minimum flow of 0.5 m3s-1 would retain 70 % of the adult brown trout habitat available at the natural MALF, while a minimum flow of 0.8 m3s-1 would retain 80 % of the habitat available at the natural MALF for adult brown trout. Any of these proposed minimum flows would be an improvement over the present situation. Historical flow statistics were used to provide a range of potential minimum flows at the potential dam sites that can be used to assess the feasibility of these sites. Once a site is chosen a more detailed analysis (

e.g. IFIM habitat modelling) could be undertaken to give a more robust indication of the instream habitat requirements at the chosen site, and support a more scientifically defensible minimum flow decision for that site. Maintenance of the existing MALF is suggested as the environmental benchmark minimum flow situation immediately below the potential dam sites, while the 1 in 10 year low flow and the 1 in 5 year low flow would be less conservative options.

When considering minimum flow regimes for the Waimea and the potential location of dams, the need to maintain flow variability, in particular flows capable of flushing out nuisance algal growths, must also be considered.