In part one and part two of the Tony Wright/Newshub special investigation into the health of New Zealand's rivers, we examined what exactly is polluting our waterways, and what is being done to try to protect and save them.
In part three we look at what effects climate change could have on the health of our rivers, and analyse if we've already reached the tipping point for overall decline in waterway health.
One of the biggest problems facing our rivers, especially those in areas of New Zealand facing drought such as the Canterbury plains, or a noticeable rise in temperature such as Waikato and Northland, is keeping the water flow cool.
As we examined in part two , Kiwi dairy farmers are undertaking a massive planting operation alongside the area of land immediately adjacent to waterways on their farms, known as the riparian area - and one of the many reasons these plants are important is because they help shade the water, keeping them cool.
NIWA has worked with the dairy industry on best practice to protect rivers from getting warmer, and chief scientist of freshwater and estuaries Dr John Quinn says riparian planting is a key weapon in fighting to keep our rivers from heating up.
"If we get riparian shade around small streams now it's one thing we can do to mitigate some of that impact by reducing the heating that's going to occur.
"But that's just the reality that if we carry on the way we are with climate change we're going to see less water in those areas that will have big impacts in both the agricultural system and on the stream system, it certainly exacerbates the problems we have with purification in both lakes and in rivers."
NIWA has predicted what river flow in New Zealand could look like by the end of this century, using data from current climate change projections.
As you can see, rivers on the east coast of the South Island, Wairarapa, Hawke's Bay, Waikato, Auckland and Northland all face the possibility of having a 10 - 100 percent reduction in river flow by 2090.
Alpine rivers however, especially those in the Southern Alps of the South Island, are projected to have a drastic increase in flow.
On the projection, NIWA says: "In a warmer world New Zealand is exposed to more westerly airflow from the Tasman, and few easterlies. This would bring more precipitation to the Southern Alps in particular, resulting in higher river flows in rivers with alpine sources. This is seen quite vividly as Canterbury's major rivers become wetter on average while their shorter nearby rivers tend to dry. The largest decline, in percentage terms, centres on parts of Hawke's Bay."
There are obviously thousands of agriculture businesses in those areas expected to see the most reduction in river flow - not just dairy farming, but beef, lamb and a myriad of other land based industries.
Warmer temperatures will kill off vital insect populations
DairyNZ water scientist Dr Tom Stephens told Newshub he believes one of the biggest threats to our waterways is rising temperatures.
"The biggest way of removing oxygen from our waterways is simply by heating them up. And if you raise the water temperature of a waterway on top of removing the oxygen carrying capacity, it actually directly impacts on our high value insect species, the stonefly and the mayfly.
"If a stream rises above 20 degrees, those species will disappear. They undergo mortality, what we call extinction, and yet those species are critical sources of food for our fish, and they're also critical to grazing algae and keeping that under control."
The larger the waterway - the harder it is to cool
NIWA's Dr John Quinn says that while riparian planting can help protect smaller streams from heating up, the problem becomes more complicated the larger the waterway gets.
"So clearly, if you can shade out the stream you reduce the temperature which is one of the things that is making algal blooms grow more quickly, and the light which makes them grow.
"We've done experiments on different scales that show that that really does work, but once streams get bigger - beyond 10 to 15 metres - it becomes more and more challenging to shade, to use shade to control."
Dr Stephens agrees that protecting these larger waterways from the heat of the sun is incredibly important. New Zealand's longest river, the Waikato, is also one our widest, and in certain areas has seen a stark decline in health. He puts this down to insect populations, which are incredibly important to river health.
"One of the biggest reasons why we don't have the insect diversity we need and we want in our community in the Waikato waterways is that they're too hot.
"Algal growth can be important to insect health, particularly in shallow streams, when we're talking about a deep river or a hydro lake it's slightly different, because the algal growth isn't attached to the bed, it's the free floating stuff, it's what we call phytoplankton.
"Because you can't shade the lake, or a river that's larger than five metres wide, you just can't get enough shade on it to control for that algae. "
So climate change looks set to have a drastic impact on the health of our waterways, and it would appear science can only do so much to protect them. Discussions need to be happening now about how best to tackle this problem before it spirals beyond control and the rivers on the Canterbury Plains and Hawke's Bay dry up for good.
The Selwyn River in Canterbury looks to have almost dried up already - and a major investigation needs to be done to find out why. If too much water has been taken out of the Selwyn to irrigate agricultural-based industries, which has been suggested, then surely the Government must get involved to stop this from happening to other rivers.
So have we reached a tipping point for overall river decline in New Zealand?
Dr Mike Joy teaches environmental sustainability at Massey University and says the shocking state of the Selwyn River in particular should be a wake-up call for all New Zealanders.
"There's certainly some basket cases like the Selwyn. It's a bad example of just decades of putting the wrong things in the wrong place and now, it's coming home to haunt us.
"If you take the whole country then nearly all of our rivers are in perfect condition where they start off, in a conservation state they're fantastic pristine rivers as good as anywhere in the world, but it's the bottom ends that are the problem."
It could take generations for our rivers to recover
Dr Kevin Simon teaches freshwater ecology at Auckland University. He believes we've not reached the tipping point yet, but we are very close.
"Could it be saved? Sure. It certainly could be made better. It will take a long time and a very large amount of effort.
"So I think that one of the big questions is how much effort are we going to require to repair some of these systems to make them better and how long should we be thinking about in terms of timeframe for recovery.
"It could be very well in order of talking about decades or generations."
Is 'polluter pays' part of the answer?
Dr Joy shares Dr Simon's views but is perhaps less optimistic of a positive outcome for our rivers.
"The majority of our rivers are in decline, they're getting worse, they're not even stable.
"They're all of the rivers that are in pastoral catchment, nearly all of them are getting worse and so there's no sign, we're doing nothing that's going to reduce it unless we get some kind of a handle on reducing intensification and we have to do something about polluter pays.
"So until we do that, until we actually start making some change and not just talking about it then we're just going to have continuing decline of our water quality.
"We have to face up to the fact that we've made a mess and actually do something about it."
Dr Stephens paints a far more positive picture.
"I wouldn't say that the rivers are getting worse as such. If you look at the latest trend reporting by NIWA and the Waikato Regional Council, indicators for sediment are improving, indicators for phosphorus are improving, indicators for algae in our hydro lakes are improving.
"There isn't a hydro lake where the algal growth is worsening, not in the last 10 to 20 years."
LINK: Part one of our special report into river health
LINK: Part two of our special report into river health
Tomorrow, in part four of Newshub's special investigation into the health of New Zealand's waterways, we'll examine the blame game and finger-pointing that has ramped up over the decline of our river health - and ask whether it is time for all parties, organisations and industries to have a frank and urgent discussion about how they can best work together to save them.
THANKS TO TONY WRIGHT AND NEWSHUB