Martin Williams: How will dam harm Tukituki?

The Tukituki River looking towards the Waipukurau bridge.

There is no doubt that "the Dam" is shaping up as a key election issue. To help you make up your mind on the issue, this piece tackles the all-important question of concern to everyone passionate about our region - "Will the dam harm the Tukituki?"

During the Board of Inquiry process in 2013, there were widely reported claims in the media that the dam would turn the Tukituki River "toxic" or even kill the river. I don't think anyone could honestly say that intensified farming using irrigation water from the Ruataniwha Water Storage Scheme would have no impact.

But from everything I have read, seen and heard, in my opinion there is no likelihood of anything as serious as claimed in these media reports.

The first thing to get straight is exactly what "harm" is at stake here. Fish and Game experts gave evidence at the Board of Inquiry hearing that excessive periphyton (or slime) caused by nutrient runoff from farming can change the insect communities living on the bottom of rivers from preferred mayflies, stoneflies and caddisflies (which trout eat, for example) to worms, snails and midges. There is also the issue of concern to dog owners, about undesirable species of periphyton such as phormidium forming mats on the river bed.

Excessive slime reduces the amenity of a river for trout fishers, swimmers or anyone using the river for recreation.

So that then defines the problem; the question is, what is being done about it, and will the Ruataniwha Water Storage Scheme help with this or make the problem worse?

The fact is the Tukituki suffers from long "accrual" periods, ie, periods of low river flows between floods especially over summer, when excessive slime can build up even at low nutrient levels, as it has for many years if not decades.

At present, we have to rely on Mother Nature to deliver floods within the Tukituki river system to deal with these existing build-ups of nutrients and slime. This year's late Indian summer shows how unreliable that prospect can be, and the frequency and severity of droughts is only going to increase with climate change.

An advantage of the dam is that there is a guaranteed four flushing flows available per summer, each releasing 1 million cubic metres of water harvested in winter, and timed to align with "summer freshes" to make them most effective in removing slime. Not all experts agreed that these flushing flows would work for the lower Tukituki River, and unlike a natural flood, they would not flush tributaries.

Personally, though, I cannot see how having this additional management option is worse than the current situation. At present, none of the Tukituki River or any of its tributaries might be flushed by natural floods in a given summer, and we clearly currently have that very problem (the long accrual period when excessive slime builds up).

There is also then an ability to use the additional "environmental flow" water allocation that has attracted media attention recently, for the way the agreement to buy it may be structured. This water could be used to supplement flows in streams across the catchment during summer periods, for example flushing the Mangatarata stream to benefit Lake Whatuma. Again, this is an option which was not available under the status quo, and something to bear in mind in looking at the overall picture.

Change 6 to the Regional Plan sets minimum flows that are critical to sustaining habitats for fish and other species. The scheme will help the region achieve these limits by releasing water that farmers within the Ruataniwha basin currently depend on for their livelihoods back into the system, when it is most needed by that system.

Change 6 also includes requirements to fence stock out of rivers, and the scheme will fund a number of significant restoration projects including planting of riparian areas alongside streams. Any effect of more nutrients in the river from additional or more intensive farming needs to be seen in that context, and remembering that the Board of Inquiry set very strict nutrient limits on the river anyway, which it found would safeguard ecological health.

To conclude then, as I see it, there is no scenario here of a permanent and irreversible impact for all time, with no way back, as some may be fearing.

The issue for the Tukituki is about what happens in summer in terms of the build-up of slime in the river (how often, and how severely). I believe the scheme will help the council tackle that issue, not harm the river.

- Martin Williams was part of the legal team advising HBRIC Ltd though the Board of Inquiry process. He has a first-class honours degree in zoology, completing a thesis in river ecology (dietary competition between blue duck and trout) at Massey University. Martin is standing for the regional council in this year's local body elections in October.

- Views expressed here are the writer's opinion and not the newspaper's. Email: editor@hbtoday.co.nz