Published in the Gisbourne herald by Michael Nielson:

http://gisborneherald.co.nz/environment/2513506-135/doc-calls-to-protect-river-flows

“…natural “flow variability” … is… essential to the life-supporting capacity and ecosystems in freshwater, yet it was increasingly disrupted by human activity, largely via water abstraction, barriers, deforestation and climate change.

Certain species live between the rocks, yet during low flow sediment can build up in these areas and they need larger-than-normal flows to flush it out.

Any measures that would steady the flows of rivers, especially those with slow flow, could prevent sediment build up from flushing away and affect the ecosystems.

“Flushes and floods are essential for maintaining habitat in rivers, especially slow flowing rivers,” said Mr Canning.

Many native fish species, including eels and five whitebait species, rely on river flows for migration and reproduction. The flow needs to be continuous and connect with the sea for them to complete their lifecycles.

High flow increases dissolved oxygen in the river and decreases rock slime (periphyton) build-up.”

Their comment on high flows increasing dissolved oxygen isn’t quite right. Rivers naturally experience a certain level of diurnal dissolved oxygen fluctuation as algae becomes a net producer of oxygen during the day and net user of oxygen at night. However, when algae growth becomes prolific the lower layers begin to rot and the bacterial decomposition uses high volumes of oxygen; at night time this oxygen can cause dissolved oxygen to plummet to stressful or even fatal levels. During the day time, excessive oxygen fixation may supersaturate water resulting in fish and invertebrates developing air embolisms which may be stressful for aquatic life. Regular floods scour algae/periphyton/slime from rocks and prevent it from building up to stressful levels.

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