Long term sedimentation in Whaingaroa (Raglan) Harbour

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 (now Waikato Regional Council) commissioned NIWA to collect and analyse sediment cores from Whaingaroa Harbour. The specific objectives of the study were to: - quantify long-term sediment accumulation rates (SAR) under undisturbed native forest land cover before human settlement - quantify changes in SAR associated with large-scale catchment deforestation (~1880-1925 AD) and subsequent conversion to pasture - 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.

Deep sediment cores taken from intertidal and subtidal sites within the Waitetuna and Waingaro arms of the Whaingaroa estuary. Sedimentological, pollen and radiocarbon analysis of core material. Interpretation of data with regard to pre- and post-european settlement rates of sedimentation in the estuary. Discussion of results. Report also contains a description of historical catchment landuse changes.

Whaingaroa (Raglan) Harbour

Field samples were collected in May 2002. Analysis of core material was undertaken between 2003-2004, as funds became available.

Cores were collected at six intertidal sites, and three subtidal sites within the estuary, using a Livingston piston corer. Cores were 50 mm diameter, driven 2-4 m into the sediment. A three point anchor system was used to hold the vessel in place, and positioning undertaken with GPS to +/- 5 m. Two cores were collected from each site and the one with least compression was analysed. Cores were split in half, logged, and samples taken from 1 cm thick slices to undertake pollen analysis. Full details of core analysis techniques are given in the report referenced below. Core material was analysed for particle size, bulk density, pollen and radio isotopes. Frequency of collection: One off data collection

Freely available.