Carters Reserve: an Investigation of the Water Quantity and Quality of Riverine Swamp in the Wairarapa

Abstract

Carters Reserve is a riverine swamp situated in abandoned channels of the Ruamahunga River near Carterton, Wairarapa. The Reserve encompasses one of the few remaining lowland forest remnants in the Wairarapa. This study outlines an attempt to characterise the hydrology and water quality of the wetland, in response to an application to divert a branch of the East Taratahi water race that discharges into the wetland. The study included establishing a water balance, a level survey, sediment core analysis, flood frequency analysis, and water quality measurements. Over the period of July 2002 to June 2003 inflows to the wetland were predominantly (83% of inflows) surface water from a natural stream that is augmented by the water race. The branch of the water race to be diverted supplied 3% of the total inflow to the wetland. The large residual change in storage (46% of inflows) was indicative of potential losses to groundwater surrounding the wetland and the result of systematic error. Flooding of the Reserve was recorded on six occasions during monitoring, which was consistent with analysis of the long-term flow record. The analysis suggests that the diversion of the water race is unlikely to have an adverse impact on the wetland because the contribution of water from the race is small and of questionable quality. If the water race is not diverted then fencing and planting the channel to help minimise the input of nutrients should improve the water quality. Historical changes have altered the character of the wetland; however, it is likely that the wetland has adjusted to the new hydrological regime therefore the best management option for the hydrology of the wetland is probably the status quo.