Late Holocene Sediment Deposition in Lake Wairarapa

Abstract

Lake Wairarapa is a highly modified lacustrine system at the southern end of the North Island, New Zealand. Not only is it situated in a region that is affected by catchment altering natural phenomena such as earthquakes, storms and fire, but both the catchment and hydrology of the lake have also been significantly altered by humans. Polynesian settlers arrived in the area approximately 700BP and proceeded to deforest the lowlands. European settlers began arriving from 1844AD onwards, completing deforestation of the lowlands and Eastern Uplands. In 1964 the Lower Wairarapa Valley Development Scheme was commissioned in an effort to alleviate flooding. This scheme significantly altered the hydrological regime of the lake. Interest in the condition of the lake and associated wetlands, and the realization that it has important recreational, cultural and ecological value, began to develop in the 1990's. This has led to a desire to see the lake restored to a more natural condition while still maintaining its flood protection capabilities. However, the lake has only been monitored over the last several decades. Any evidence of the lakes condition prior to this time is anecdotal and little is known of its natural tendencies and functions. This research has investigated and quantified morphological changes to Lake Wairarapa at the decadal and millenial scale using a combination of aerial photograph analysis, bathymetric survey comparison and lakebed core analysis. Study at these diverse scales has allowed the observed changes to be related to human environmental modification, while also being juxtaposed against natural trajectories of change. It is hoped that this can inform lake management and restoration efforts and provide a benchmark for measuring future changes to the lake, while also addressing wider issues concerning natural versus anthropogenic landscape change at the local and regional scale. The results of this project suggest that the lake has been steadily infilling over the last 6000BP – particularly along the eastern shore. For the two decades after significant hydrological changes to the lake associated with the Lower Wairarapa Valley Development Scheme, the rate of infilling on the eastern shore increased more than tenfold. However, this was accompanied by deepening in other parts of the lake. Today infilling along the eastern shore appears to have returned to natural rates and overall the lake in 2010 is only slightly smaller in volume than in 1975. Longer term anthropogenic influence on the lake and catchment was also evident. In particular Polynesian settlement and subsequent deforestation by fire was apparent in the lakebed cores. This result not only addresses the immediate issue of anthropogenic influence on this particular lacustrine system, but also informs the debate surrounding the dating of Polynesian arrival in New Zealand.