Upper Ruamahanga River
The Ruamahanga River system, from the Waiohine confluence upstream, has a total catchment area of 1560 sq km. This catchment is made up of sub-catchments consisting of the Ruamahanga above Mount Bruce, and the Waipoua and Waingawa rivers which flow from the eastern side of Tararua Range, and the Kopuaranga, Whangaehu and Taueru rivers which are sourced from the eastern Wairarapa hills.
The main river channel from Mount Bruce downstream to the Waiohine is 58 km in length, characterised by a semi-braided form in its upper reaches changing to a single thread in the lower reaches.
The Upper Ruamahanga floodplain soils are formed from alluvial parent materials with two different sources. The rivers from the Tararua Ranges contribute greywacke alluvium, and the rivers sourced from the eastern Wairarapa hills contribute alluvial silts and sands eroded from mudstones, sandstones and limestones.
Different soil types have developed at various locations on the floodplain depending on the rate of flood deposition, the source of material, time since deposition, and natural drainage. The natural fertility and erodibility of these soils is quite variable. Inappropriate land-use and lack of shelter may cause wind erosion.
Ever since human settlement began in the Wairarapa it is likely that there has been a need for people to protect themselves and their assets from the threat of flooding. This need would have grown greatly with increasing European settlement and more intensive land-uses which has meant that protection from the harmful effects of the river has become a necessity.
During the 1930s settlers suffered damage and loss when the Ruamahanga River overflowed its banks, washing shingle onto valuable pastures. The bed of the river had become badly choked with willows, restricting flood flows, and the channel was inadequate and of irregular alignment.
A river control scheme was implemented in 1953 and during the next 20 years achieved success in bank edge protection, river alignment, and reduced the incidence of flooding along many sections of the river.
The Upper Ruamahanga River Control Scheme was established in 1982 and covers the length of the Ruamahanga River from Mount Bruce downstream to the Waiohine confluence. The scheme was designed to protect an area of about 2760ha of rural land and a number of public utilities using stopbanks, heavy bank protection, vegetation buffer zones and the Te Ore Ore grade-control weir. Greater Wellington Regional Council is responsible for the implementation and maintenance of the scheme.