Report: TR 2008/07

Author: Max Gibbs, David Bremner (NIWA)

Abstract

Sedimentation rates in the Wharekawa Estuary are higher now than prior to human land use changes in the catchment. As part of forestry consent conditions, Rayonier (and, previously, Carter Holt Harvey) is required to carry out sediment and biological monitoring of the Wharekawa Estuary. This monitoring has shown a decline in invertebrate species sensitive to sedimentation at the monitored sites within the estuary. The source of sediments causing these ecological effects was unknown however. This report presents the results from a study of the Wharekawa Estuary, using a forensic stable isotope technique, to identify and apportion the sources of soil, by land use and by sub-catchment.

Results indicate that terrigenous soil contributions were present at all estuarine sites from pine (1-23%), pasture (<1-10%), native forest (<1-3%) and slip (<1-13%) land use sources. It was found that flood-plain material contributed high proportions (29-95%) of the soil in the sediments across the estuary and further analysis indicated that slip and flood-plain silt made up about a quarter of the soil transported in the Wharekawa River, suggesting that there was a continuing supply of the flood material being washed out of that river. As only about 25% of the soil transported in the Wharekawa River is flood-plain silts, the amount of flood-plain silt in the estuary appears to be disproportionately high. This apparent inconsistency is explained by examining the bulk density of the estuarine sediments, which were higher than the terrigenous source soils, indicating a sorting mechanism whereby coarser materials are deposited on the intertidal zones while the finer materials are carried out of the estuary.

When the data were modelled to evaluate the soil contributions by sub-catchment, the results showed that the major sediment contributions in the mid-to-upper estuary came from the Wharekawa River sub-catchment (20-60%) with the Kapakapa sub-catchment producing 7-50% and the Tawatawa and Wahitapu sub-catchments each producing about 1-10%. The study results indicated that only the inshore parts of the large pine-slash debris field deposited in the estuary near the Kapakapa Stream mouth during the July 2005 storm are still producing fine silt which is accumulating along the upper tide level downstream of the Kapakapa Stream. The significance of this is that it indicates that the sediment load from the Wharekawa River sub-catchment, which has a high proportion of the flood-plain soil, is gradually burying the older sediments and seagrass beds in the mid-to-upper estuary. It is possible that the accumulation of this mainly heavy sand is altering the alignment of the channels in the estuary allowing sedimentation of new and redistributed sediments to occur in the areas showing an ecological impact in the biological monitoring.

The results of this study highlight the effects of changing weather patterns and the impacts of extreme weather conditions, the extremity and frequency of which have been increasing in recent years. Land use practices which remove the protective cover of plants on steep land will exacerbate the production of sediment during extreme events. Furthermore, flood material deposited in the river and stream channels during extreme events may continue to be discharged into the estuary over extended periods as chronic loads long after the extreme event has passed. This chronic sediment load may adversely affect some invertebrate species such as the cockle, Austrovenus stutchburyi, and the mud snail Amphibola crenata.

Wharekawa Estuary Sediment Sources [PDF, 2.9 MB]