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Are we serious about stopping floods?

Driving across Hawke’s Bay today, you can see a quiet but important transformation underway. Streams are being fenced off and replanted with natives. Hillsides are being stabilised with poplars and other erosion-control trees. Pastures are being managed more carefully. All of this is aimed at one thing: slowing water down, keeping soil where it belongs, and reducing the risk of floods and sediment choking our rivers.

It is hard to argue with the logic. Vegetation soaks up rainfall, anchors soil, and slows the speed at which water moves downhill. The more plant cover we have, the more resilient the landscape becomes. It is why councils, farmers and ratepayers are spending millions on stopbanks, riparian planting and land retirement.

But while all this effort is happening in the lowlands and hill country, a very different story is unfolding in the high country.

Drive past the Kaweka Ranges on the way to Taupō and you see something unsettling: large areas of steep hillside that are still bare decades after being damaged. These slopes were first degraded by overgrazing with merino sheep in the late 1800s. More recently they have been further stripped by deer and then hammered by storms that trigger landslides. The result is a landscape with less vegetation, more exposed soil, and faster runoff when it rains.

That runoff does not disappear. It flows down into the river systems and eventually onto the Heretaunga Plains as floodwater carrying silt and pumice. West of the Kawekas, across to the Kaimanawas, the geology makes matters worse: greywacke hills dusted with Taupō ash and valleys filled with loose pumice that can literally float away in floodwaters. In this mountain environment regeneration is painfully slow. Once plant cover is lost, it can take generations to return.

This is not a new problem. In the 1960s the New Zealand Forest Service established a research station in the Kawekas specifically because of the flood risk to Hawke’s Bay. Its task was to develop erosion-control techniques to stabilise the ranges while catchment boards worked on the rivers below. At that time, deer numbers were actively controlled by professional cullers. Native regeneration began — slowly, but visibly.

Then the system changed. With the creation of the Department of Conservation and regional councils in the early 1990s, deer control stopped being a core task. DOC did not prioritise erosion control in the mountains, and regional councils regarded it as DOC’s problem. The predictable result has been an explosion in deer numbers and their spread into adjacent pastoral land.

This matters because deer are not just another animal in the bush. They are browsers. They eat seedlings, saplings and understorey plants. Over time, forests become thin, open and easy to see through — the opposite of what a healthy, resilient forest should look like. Anyone who has seen fenced-off vegetation plots in the hills knows the difference immediately: inside the fence, dense and layered growth; outside, sparse and struggling.

[Editor: See photos illustrating the effects in this prior BayBuzz article by Marie.]

There will always be erosion in New Zealand. We are a young country geologically and some slips are too deep-seated to be fixed with plants alone. But in general, vegetation makes a huge difference. It intercepts rainfall, slows its movement, allows more water to soak into the ground and recharge groundwater, and holds soil in place.

Around the world, this principle is being rediscovered. In Europe and North America, beavers are being reintroduced because their dams slow water, spread it across floodplains and reduce peak flood flows — by around 27 per cent in some catchments. Where beavers are absent, people are building “beaver analogue dams” to mimic their effect. In cities, planners talk about “sponge cities” that absorb rainfall rather than shunting it rapidly into drains.

All these ideas point in the same direction: slow the water down at the top of the catchment, and you reduce damage at the bottom.

Which makes our treatment of the Kaweka and Kaimanawa ranges so puzzling.

Recently, the Government designated a Sika Deer Herd of Special Interest in these ranges. Submissions on the first management plan have closed, largely unnoticed by most Hawke’s Bay residents. The stated purpose of the herd is to breed world-class trophy stags for international hunters. To achieve this, the deer will effectively be farmed: hinds selectively bred and the population managed to maximise antler quality.

This would be controversial enough on its own. But the bigger issue is where this “farming” is proposed to occur: on Land Use Capability Class 7 and 8 land — land classified as having severe to extreme limitations for use because of erosion risk and fragility.

In other words, the most environmentally vulnerable land in the region.

Yet the draft management plan contains remarkably little about how that risk will be managed. There is no clear system of proven environmental controls. Monitoring and mitigation appear to be things that will be worked out later, rather than being prerequisites. Minimal research is cited. One study notes that it takes around 40 years for a beech tree to grow from a seedling into a closed canopy — a stark reminder of how marginal this environment is.

Compare this with what is required of farmers on the plains and hill country. They must fence waterways, retire steep land, plant natives, maintain erosion-control trees and minimise off-site effects. The standards are high and becoming higher.

Yet under the sika deer plan, DOC — the landowner — would be “farming” deer in an extreme mountain environment with fewer environmental controls than a farmer on the Heretaunga Plains.

That is backwards.

It is also inconsistent. While we pour money into stopbanks and urban flood protection, the funding to keep hillsides intact has been cut back. The Hill Country Erosion Control Scheme is gone. The very places where floodwaters originate are receiving less attention, not more.

Cyclone Gabrielle gave us a brutal real-world experiment. Slips were far more common on pastoral land than in forested or bush-covered areas, or on slopes planted with erosion-control trees. There is work documenting this, but anyone who drove the back roads afterward could see it with their own eyes.

Revegetation does more than hold soil. Over time it changes the hydrology of the land. Springs reappear. Streams run longer into dry periods. The land becomes a buffer rather than a chute.

Deer work in the opposite direction. By suppressing regeneration, they keep the landscape locked in a degraded state, especially on our fragile upland soils. Bare ground sheds water. Open forests let rain hammer the soil. Slips become more likely. Flood peaks become higher.

Which brings us to the uncomfortable but unavoidable conclusion: if we are serious about reducing erosion and flooding, we must be serious about controlling deer numbers.

It also means recognising that deer impose real economic costs. Not just through environmental damage, but through losses to pasture, crops, plantation forestry and biodiversity. A proper accounting of those costs, compared with the price of fencing and control, is long overdue.

We already accept that cows should not be in rivers, that steep slopes should be planted, and that land must sometimes be retired for the common good. It is incoherent to apply those principles everywhere except the place where water actually begins its journey.

If the sika deer management plan proceeds as drafted, farmers and ratepayers alike should ask hard questions about the Government’s commitment to environmental protection. Why should private landowners be held to stricter standards than the Crown? Why are we willing to risk downstream communities for the sake of trophy antlers?

We cannot build our way out of floods with stopbanks alone. The real work begins on the hills. Dense vegetation, intact forest, and slow water are our best defence.

And that means fewer deer.

If we truly want safer rivers, cleaner water and fewer disasters, then deer culling is not a side issue. It is part of flood control.