The Ruamahanga River is the largest river in the Wairarapa, about 130 km in length. It rises in native forest in Tararua Range near Mount Bruce, and then runs through the Wairarapa Valley, passing through or near several townships and extensive farmland before discharging into Lake Onoke which opens into the sea at Palliser Bay.
Agricultural land use dominates the catchment and is the main pressure on catchment water quality. Treated sewage discharges from the five main Wairarapa townships also impact on water quality.
Water quality is monitored at four locations along the Ruamahanga River and shows a marked decline with distance down the valley. Water quality is also monitored on many of the main tributaries, including the Kopuaranga, Waipoua, Whangaehu, Waingawa, Waiohine, Taueru, and Huangarua rivers. Generally, water quality is excellent in the western tributaries that have their headwaters in the bushclad Tararua Range and poorer in the eastern tributaries.
Lake Wairarapa is also located within the lower catchment. The Tauherenikau and Waiorongomai rivers discharge into this lake – both of these rivers have excellent water quality.