Visiting scientist: Should we let rivers run free?

For the past 20 years, researchers have been running studies around an experimental floodplain around California's free-flowing Consumnes River. Photo / Joshua Viers

We dam, divert, drain and contain our rivers to benefit ourselves and our economy: but what if we let these systems do their natural thing?

A visiting US scientist has just shared with New Zealand ecologists the fascinating case of California's free-flowing Consumnes River, where, for the past 20 years, researchers have been running studies around an experimental floodplain.

Running 84km through Northern California, the Consumnes is the only un-dammed river on the western slope of the Sierra Nevada mountain range, where most of the state's water supply is sourced.

Associate Professor Joshua Viers, of the University of California Water Security and Sustainability Research Initiative, said the Consumnes was what might be called the "ugly duckling" of California's infrastructure-laden major rivers.

Because of its low water supply yield, it had never been dammed: something that meant the river flooded regularly and few people but farmers lived around its edges.

That flooded land supported some of the last native riparian forest within the central valley of California, and these areas had been designated as nature reserves so the birds and animals within them could also be protected.

A levee breach in the Consumnes River resulted in eye-opening effects for scientists. Photo / Joshua Viers

But Viers said something yet more interesting happened as a result of the river's flooding: when a stream-side levee failed, a cottonwood forest sprang up from the sand deposits and other sediments that the flooding had left behind.

As farmers were unable to repair the levee, the forest continued to grow over a period of years.

"People then realised that by letting a river do the work, it was much more successful than other attempts at doing forest restoration," Viers said.

"Since then, we've embarked on a number of experiments where we have allowed rivers to flood on to their floodplains."

This had brought about a huge range of ecological benefits, which had "profound consequences" for California's river management.

California's Consumnes River from above. Photo / Joshua Viers

The biggest benefit was the access to habitats and rich buffet of zooplankton that was provided to juvenile salmon that used that river, enabling them to grow larger and survive longer.

Another gain was the groundwater "recharging" effect that the flooding effect had on vital aquifers.

"Previously, these floodwaters from the Sierras would have flooded vast areas, creating wetlands and habitats, and we would have had natural recharge of our aquifers," Viers said.

"Unfortunately, because of the dams and the side-stream levees, that rarely happens now - yet about 40 per cent of the water supply of core California comes from groundwater.

"And after this most recent near-decadal drought, we have been actively trying to figure out ways in which to replenish our groundwater stores, so connecting seasonal floodplains is a win-win for everyone."

California's Consumnes River from above. Photo / Joshua Viers

Viers said the benefits reaped from what was an entirely natural process was an unforeseen event, but in hindsight, one that could have been perfectly predictable.

"In some ways, you could have looked at the levee failure being a 'black swan' event, in that it changed people's thinking.

"So really, it's about encouraging bio-physical processes: if you accept most of the world is a managed ecosystem, and if we are going to achieve any sort of balance, I think we can use nature as a pathway."

He saw no reason why the same concept couldn't be picked up in New Zealand river management.

"This approach to what we call green infrastructure is certainly transferable - it's something you can do no matter where you are."

Viers was a key speaker at this week's inaugural conference of the "New Zealand's Biological Heritage" National Science Challenge at Te Papa in Wellington, where a new strategy for threatened species was launched this morning.