Wairoa River Bed Sediment

The Wairoa River carries large quantities of fine sediment (clays, silts and sands). This fine sediment cloaks both the bed and the banks of the river and the river appears to be mainly soft‐bottomed downstream from Frasertown. The question this investigation seeksto begin to answer is whether the river has always been characterised by such large amounts of fine sediment, or whether changes in land‐use in the catchment may have altered the condition of the river.

This investigation used the sediments exposed in the riverbanks as a window back in time. Gravels were seen in the riverbanks as far down the river as immediately below the old river channel at Awamate (now the Awamate Lagoon), but gravels were not seen in the bank where the old Awamate channel has been plugged. Estimates of the age of these riverbank deposits was provided using radiocarbon dating of wood recovered from the base of banks at four locations along the lower Wairoa River.

Preliminary findings suggest that the Wairoa River as far as Awamate was gravel‐bedded until at least 3000 years ago. Wood underneath thick fines infilling the Awamate channel indicates these deposits are younger than 1365 years old. More work is needed to understand the details of these changes to the sediments in the Wairoa River, but these results are not inconsistent with the idea that land clearance and subsequent escalation of catchment erosion has contributed to the condition of the river as we see it today.