Science & Research Series N0.16 Flow Management in the Tongariro River

The Tongariro River supports a world renowned trout fishery and a nationally important hydroelectric power scheme which has caused profound changes to the river's flow regime and some minor changes to its fishery. Specifications for the regulated compensation flow in the lower river, whilst originally intended to protect the fishery, constrain electricity generation and fail to address all requirements of the fishery. There is opportunity to reform the rules for flow management, both to enhance the fishery and to increase electricity generation. The number of trout running into the Tongariro River is greatly affected by the number of juveniles which grow longer than 9cm in the river and its tributaries. Whilst diversion for power generation has increased the amount of fingerling habitat, certain artefacts in the regulated flow regime appear to prevent a corresponding increase in juvenile production. Undesirable features of the present flow regime include: i) Artificially-induced surges. ii) Abrupt, truncated flood recessions. iii) Minimum flows soon after rainfall, higher flows in droughts. iv) Absence of seasonal variation in the base flow. v) Sandy bedloadaccumulation. The effects of summer floods were far more damaging than undesirable features of the present flow regime. Nevertheless, numbers of trout running into the Tongariro River could be increased, particularly in years in which significant summer floods do not occur, if: 1. The minimum flow requirement at Turangi were abandoned and all flow specifications applied to the recorder site below Poutu intake. 2. A maximum surge rule were implemented such that artificially-induced water level changes never exceed 3. Conditional on "2" above, the restriction on the period for adjustment of gate and valve settings were lifted. 4. Ministry of Works and Development guidelines for scouring sediment were adopted in full.