Most of the major rivers flow into harbours, rather than discharging directly to the open coast which means contaminants tend to take longer to disperse from these sheltered environments.
Rivers are channels for floodwaters, a function that is much needed because of Northland's relatively high rainfall, while prolonged dry spells also lead to very low flows in many smaller catchments. Northland's rivers are generally characterised as being slow flowing and muddy because the land is dominated by deeply weathered geology and fine clay soils. Northland's rivers and streams provide habitat for a range of indigenous flora and fauna, as well as being an important water supply for rural communities, horticulture and agricultural demands.
The Northern Wairoa River is the region's largest river, draining a catchment area of 3,650 square kilometres, or 29 per cent of Northland's land area.