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The Rangitata river flows in braids with a state highway bridge cutting over.

Looking down over the Rangitata riverbed and State Highway 72 bridge at Arundel from an altitude of 120m above ground level, July 2019. Photo: Thomas Kay.

The modification of rivers for flood management, irrigation, and agriculture has resulted in a decline in the geomorphic condition and habitat quality of river systems.

Forest & Bird undertook an HQI assessment of the Rangitata River between Arundel and Ealing in order to quantify any changes in habitat quality from 1937 to 2016-2018. The assessment method allows for a river’s ‘current’ physical condition to be assessed against its historical, or ‘near natural’ condition. It presents a measure of the change in condition from ‘then’ to ‘now’.

The overall changes for sections of the river and for active channel width are cause for concern, indicating a significant reduction in the condition of the river. These changes are likely a result of the reduction in flows that has occurred with increased water takes for irrigation, and the ‘development’ of vegetated bars and the floodplain along the edges of the river by farmers.

Flooding in December 2018 illustrated the impacts of these changes when the river temporarily
reclaimed previously flowing channels and again resembled its condition in 1937. 

Construction of the storage ponds for the South Rangitata irrigation scheme appears to have caused further concerning changes to the floodplain.

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