Publications: Research reports and publications

A framework for flow management in the Takaka River catchment

1 December, 2006
Cawthron Report 1172. Prepared for Tasman District Council.

EXECUTIVE SUMMARY

Until recently the demand for water in the Takaka Catchment has been low, especially when compared to the drier eastern parts of the Tasman District. However, demand for water has increased substantially over the last few years resulting in the need for an appropriate framework for managing instream flows and water allocation. This report identifies a framework for flow management that should be applicable and practical for the Takaka Catchment.

Instream values vary considerably in different parts of the Takaka Catchment and it is sensible to group waterways with similar values together so they can be managed in a similar fashion. Six groups of waterways are proposed based on a combination of local knowledge, source of flow, waterway size and freshwater fish distribution. These groups are the Takaka North rivers, Spring-fed rivers, East Takaka streams, Pohara Flats streams, Small Headwater streams, and Major rivers and their tributaries. Instream values and management objectives have been suggested for each of these groups, along with critical values that, if protected, should sustain the other significant values. Protection levels are suggested for each group of streams based on whether the values are considered to be high, medium or low. Consultation with the local community and other stakeholders is required to finalise these suggestions. Further sampling of some waterways would also be helpful in accurately determining appropriate values upon which to base instream management objectives.

Hydrological analyses would be appropriate for setting flow regimes for the East Takaka streams, while generalised habitat models would be appropriate for the Pohara Flats and Small Headwater Streams. A water quality model such as WAIORA would be appropriate to manage flows in the Spring-fed streams, on the basis of the relationship between flow and dissolved oxygen concentration, although the low levels of dissolved oxygen in groundwater may make this problematic. Detailed habitat analyses and modelling is required to determine appropriate minimum flows for the Takaka North rivers and Major Rivers and their tributaries. If the expense of detailed habitat analyses cannot be justified for these rivers then a conservative approach would be to set the minimum flow at the MALF.

Sensible allocation limits are required to maintain the security of supply for water users and avoid flows being held at the minimum flow for prolonged periods (i.e. flat-lining). The difference between the minimum flow and the summer 7-day Q95 (flow exceeded 95% of the time over summer) could beused as an allocation limit because it gives users a clear expectation of the security of their supply. Other allocation options such as flow sharing, flow rostering and primary/secondary allocation limits could also be used to maximise the effectiveness of water use without compromising minimum flows.