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Many rivers are low and could go lower

Many rivers are low and could go lower

NIWA’s National Centre for Water Resources is warning water managers and users to be prepared for low water levels in rivers, hydro lakes, and groundwater over the summer.

‘Mean river flows for spring were the lowest on record for large parts of the South Island, and for parts of the North Island,’ says NIWA hydrologist Charles Pearson. ‘Last month, we saw a sharp contrast in the North Island between record high river flows in Gisborne, and record low river flows in parts of Taranaki, the central plateau, Wellington, and the Wairarapa. In the South Island, river flows were the lowest on record for November in much of Marlborough and Nelson, and in parts of Canterbury and Otago.’

Over the next three months, the water resources outlook is for normal or near-normal soil moisture and river flows in the north and east of the North Island but generally dry conditions for the rest of the North Island and all the South Island.

‘There’s a high probability of below normal river flows and soil moisture in coastal Canterbury and east Otago. This is likely to exacerbate the low groundwater levels already reported by Environment Canterbury. Irrigation may well continue to be under pressure in the east,’ says Charles Pearson.

The snowpack in the Southern Alps is below normal levels due to poor snow in winter, which is contributing to the below-normal hydro lakes.

Water quality is likely to be poorer than normal in unusually low flowing rivers and in the lakes and coastal environments they feed. This increases the risk of algal blooms.

‘During low flows, the river is not as effective at scouring algae off the riverbed. This is exacerbated by the reduced dilution of nutrient-rich groundwater getting into rivers, and by increased water temperatures. Filamentous green algae love these conditions,’ says NIWA river ecologist Dr Barry Biggs.

‘The algae smother bed sediments needed for insect and streambed life. We’ll also see low dissolved oxygen levels, leading to stress or death in invertebrates on which adult fish feed. As rivers dry up, fish become confined to deeper pools and experience stress from high temperatures and low dissolved oxygen levels,’ says Dr Biggs.

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