Assessment of sediment load reductions to achieve target attribute states in the streams and rivers of the Auckland region

Extract from the Executive summary:

The National Policy Statement for Freshwater Management 2020 (NPS-FM) requires Auckland Council (AC) set a target attribute state (TAS) for fine suspended sediment for all rivers and streams in the region and prescribe limits on resource use that will achieve these targets. As a first step in setting TASs and limits, this study has assessed sediment load reductions required to achieve options for several TASs in streams and rivers of the Auckland region.

Fine suspended sediment is a contaminant that affects ecosystem health by changing the optical characteristics of water (visual clarity and light penetration) as well as physical effects on aquatic animals such as gill clogging and abrasion. The NPS-FM fine suspended sediment attribute is quantified by observations of visual clarity based on measurements taken in the field as the horizontal sighting distance of a black disc, which therefore has units of metres (m). The fine suspended sediment attribute is defined (i.e., both the baseline state and the TASs) by the median value of monthly visual clarity observations in streams and rivers.

This study does not consider how the sediment load reductions would be achieved or whether the TASs are reasonable. Rather. it aims to inform AC about the magnitude of the load reductions needed for each option, how these vary across the region, and the uncertainty inherent in the assessments. In addition, this study does not consider sediment objectives for downstream estuarine receiving environments. Sediment load reductions required to meet any (yet to be identified) sediment-related outcomes in estuaries could be greater than the load reductions estimated in this report to achieve TASs in the region’s streams and rivers. ...

LWP Land Water People for Auckland Council, October 2023