Ten biggest rivers
New Zealand has around 426,000 kilometres of rivers. In terms of the quantity of water discharged, South Island rivers hold the top five positions, and only two North Island rivers make the top 10 (Waikato and Whanganui). Four drain to the Tasman Sea (Buller, Grey, Taramakau and Haast) – these rivers are relatively short but carry enormous volumes of water from the heavy rain band along the Southern Alps.
The flow regime of a river is the way its flow changes from day to day, season to season and year to year. Periods of low flow are interspersed with floods, caused by storm rainfall upstream in the river catchment.
In New Zealand, low flows tend to occur in summer when rainfall is usually lower, water loss from plants and soil is higher due to evaporation, and soil moisture and groundwater levels decrease.
The amount of rainfall required to produce a flood in summer is usually greater than in winter, when the catchment tends to be wetter and so is more strongly affected by storm rainfalls.
In alpine areas, low-flow periods occur more often in winter, because precipitation is stored in the catchment as snow and ice, which is released during spring and summer thaws.
The rivers run dry
Taking water for domestic and agricultural uses has led to many small rivers in drier eastern parts of New Zealand – such as the Pareora in South Canterbury – becoming shadows of their former selves. Communities need to weigh up the economic gains from taking water out of natural systems against the ecological and recreational costs – such as not having swimming holes and trout to catch.
New Zealand has a huge variety of river types, including boulder-filled mountain torrents, braided rivers on coastal plains, meandering lowland spring-fed channels and concrete-lined urban waterways. Each river’s character is a product of climate, catchment geology, water source and vegetation characteristics.
Monitoring and managing rivers
The Ministry for the Environment and the National Institute of Water and Atmospheric Research developed the River Environments Classification system, to aid the monitoring and management of rivers. The system groups rivers of similar physical characteristics, so they can be compared with one another.
Rivers can be immensely powerful agents of change in the landscape, through flooding and changing path. To protect against this, river engineers have, in many places, modified riverbeds and banks to limit potential floods and control the river’s path. For example, around 1,000 years ago the Waimakariri River flowed through the current location of Christchurch. Without extensive stopbanks on the lower Waimakariri and gravel extraction on the riverbed, changes in the course of the river during floods might have catastrophic consequences for Christchurch.