Victoria University’s Dr Mike Joy argues that large-scale dams lock us all into a high-risk, high-cost, high-impact water storage system

Big irrigation dams like the one proposed for Waimea River in Nelson sound like a great idea, especially when you hear Regional Economic Development Minister Shane Jones talking about the economic gains to be expected. Dam proponents love to make these claims about the wonderful gains to be had for all, and the claims become more extravagant when taxpayers become likely to foot the bill.

Economist Peter Fraser has made a strong critique of the economics of all this. As we’ve seen with the Ruataniwha dam in Hawke’s Bay, the process can go badly wrong: millions of dollars spent over many years and nothing to show for it but a divided council and a divided local community. Any claim of a net environmental win for rivers from a dam is 100% pure fabrication.

Large-scale dams make farms less resilient. In order to fund dam construction and ongoing maintenance – neither of which is cheap – a high price gets put on water for irrigation. To pay this added cost, farmers intensify. In most cases this means converting to dairy farming.

This means greater dependency on water. If water becomes scarce, farmers are more at-risk, because they have more animals and more crops. Inevitably they become less resilient.

We saw this with the Opihi irrigation dam in South Canterbury. To pay for its water, farmers intensified; therefore water consumption went up. Then we hit some dry years. In the last few years, there has not been enough water to fill the dam. Water quality in rivers downstream has declined dramatically and the community and farmers have lost out.

Dam proponents like to claim there are ecological gains to be had from dams. These claims tend to avoid specificity, but they seem to be around the vague notion of ‘flushing flows’: sudden releases of water from dams intended to dislodge algal build-ups downstream. In reality the energy of the release is soon dissipated downstream, and of course only the main channel is affected, not tributaries. And, like the toilet, you only flush when you need to. Intensifying farming means more algal blooms. Any need to flush is driven by intensification. Dams drive intensification. So it’s a completely circular argument.

Damming a river also removes any natural flushing flows, so the idea that it is a gain to take away the natural flow variability and replace it with an artificial one is just daft. New Zealand’s native fish, already in trouble with only one quarter of the species not listed as threatened, depend on free passage up and down rivers. Another dam would be very bad news for them.

Thousands of words would be required to detail the long list of negative ecological impacts from dams, so let me point to a recent peer-reviewed scientific paper. The authors reviewed 165 papers on dam impacts and found that 92 percent of them reported declining or negative ecological measures as a result of dams.

Other countries are recognising these negative effects. The United States, for example, has stopped building new dams and now dams are being removed from many rivers. Large-scale dams lock us all into a high-risk, high-cost, high-impact water storage system. They’re a folly.

Dr Mike Joy is a freshwater ecologist. He has researched New Zealand rivers for more than 20 years, has published dozens of peer-reviewed articles in this field and has written more than 100 commissioned reports on river management. This is an adaptation of an article in the latest fortnightly newsletter from Victoria University of Wellington’s Institute for Governance and Policy Studies (for which free subscriptions are available from with “Subscribe to newsletter, please” in the subject line).

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