Northland is characterised by a large number of small river catchments and short, meandering streams. Most of the major rivers flow into estuaries rather than directly to the open coast. These rivers and streams play an important role in Northland, often providing water for stock, industry and domestic use.
Climate and geology influence flow within Northland's rivers. Northland's marked seasonal variation in rainfall is reflected in the pattern of higher flows during the winter months and lower flows during the summer months. Most rivers flow at only 10 - 20% of their yearly average flow during the summer.
Droughts occur as a result of lower than usual rainfall, which causes prolonged periods of low river-flow. During drought months (most typically January to March), more accurate monitoring of stream flow is undertaken by NRC to establish levels at which water use restrictions may need to be implemented.
During 2007-08, rivers in Northland were characterised by above average monthly flows in July, August, December, February and May. River flows in the far north were also above average for June. Low flows were recorded for September through to November and January, although some rivers in western areas recorded below their average monthly flows during March and May as well. Near average to above average flows were recorded in July.
During 2007-08, a total of 245 flow measurements were taken on the region's rivers, of which 45 were taken during flood events. Over the past two years, there has been a significant increase in the number of measurements taken due to the use of a new Accoustic Doppler Current Profiler (ADCP). This equipment enables flows to be measured more accurately and efficiently compared to traditional methods.
Photo: ADCP flow Gauging - Waipoua River Photo: Conventional flow gauging Kaihu River
The figures below show the mean monthly flows in 2007-08 compared to the historical mean monthly flows for various rivers in Northland.
Chart: Monthly flows from July 2007 – June 2008, compared to monthly average flows