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.