In the Lower Wairarapa Valley the dominant river is the Ruamahanga. It travels from the northern central Tararua Range through open country across the eastern side of the Wairarapa plains to Lake Wairarapa and Lake Onoke. The area also includes the Waiohine confluence to the Lake Onoke outlet to Palliser Bay. Other major tributaries draining from the eastern forested Tararua Ranges include the Waingawa, Waiohine and Tauherenikau Rivers. These rivers all flow through vast areas of farmland and are therefore susceptible to run-off particularly from dairy farms and disturbances from stock entering the river channel.
The Lower Wairarapa Valley system also includes two lakes - Lake Onoke and Lake Wairarapa. The lakes are situated in the lower portion of the catchment and the majority of severe flooding occurs in adjacent areas.
Before the Lower Wairarapa Valley Scheme, a major flood could affect up to 20,000ha of land, with some areas being flooded eight times a year. Roads were blocked for days on end, communication lines damaged, and fences destroyed. And despite an efficient flood warning service which began in the 1950s, stock losses at times were severe.
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