Waikato · New Zealand
The Waikato River
New Zealand’s longest river, from the ice on Ruapehu to the Tasman. Where it leaves Lake Taupō you can see 7.5 m into it. By Hamilton, home to 71% of the people who live on it, 1.65 m; by Tuakau, 0.65 m. Over the same water, nitrogen rises 10-fold and phosphorus 14-fold.
Who lives on it
Hamilton holds 71% of the people living within 3 km of the river. From Wairākei to Ātiamuri no town stands on its banks at all. It is dairy country, and north of Taupō tens of thousands of hectares of pine forest have been turned into dairy farms.
What is in it
Nitrogen and phosphorus feed algae. Down to Ngāruawāhia algae take about half the lost clarity; below, silt takes twice as much, and the water turns from blue to green to brown. Of its nitrogen, 61% comes from land use, mainly pastoral farming; 6% from towns and industry.
Who drinks it
Up to 225 million litres a day are drawn near Tuakau for Auckland’s taps, a fifth of the city’s water, from where nitrogen and phosphorus are highest and the water least clear. Hamilton, Ngāruawāhia, Huntly, Te Kauwhata and towns in Waipā drink it too.