Report: TR05/36

Authors: A Swales, R Ovenden, R Budd, J Hawken (NIWA), M S McGlone (Landcare Research Ltd, Lincoln), N Hermanspahn, M J Okey (National Radiation Laboratory, Christchurch)

Abstract

Whaingaroa (Raglan) Harbour is an important coastal resource for the Waikato Region. The community has expressed concerns regarding perceived environmental degradation of the harbour and its catchment.

Environment Waikato commissioned NIWA to collect and analyse sediment cores from Whaingaroa Harbour. The specific objectives of the study were to:

1. quantify long-term sediment accumulation rates (SAR) under undisturbed native forest land cover before human settlement
2. quantify changes in SAR associated with large-scale catchment deforestation (~1880-1925 AD) and subsequent conversion to pasture
3. determine differences in SAR and sediment profiles in the Waingaro and Waitetuna arms of the harbour.

Results from radio-isotope and pollen analysis indicate that Whaingaroa Harbour had largely filled with sediment prior to human settlement. Consequently, all but the upper two metres of the present day sediment column was deposited before 6000 years BP and thousands of years before the arrival of Maori some 700 years ago.

The effects of large-scale catchment deforestation (1890-1920s), conversion to pasture and plantation forestry on sedimentation have occurred very differently in the Waingaro and Waitetuna arms of the harbour. Results from the Waitetuna arm indicate that pre-human sediment accumulation rates have increased threefold following deforestation since 1890. In contrast, results from the larger Waingaro arm indicate that long-term sedimentation has not occurred in this part of the harbour for at least the last 150 years and probably much longer.

Whaingaroa (Raglan) Harbour : Sedimentation and the Effects of Historical Catchment Landcover Changes [PDF, 1 MB]