Sediment (particularly muddy sediment) discharged into rivers, streams, and harbours can negatively impact a range of values, including ecosystem health and the way people use water for recreational, cultural and spiritual purposes. Long-term or event-related shifts from sandy to muddy sediment can lead to associated shifts in ecological communities. The rate of sediment deposition, and grainsize of the sediment deposited, affects the ability of estuary biota to cope with the influx of catchment derived material.

The depth of sediment overlying concrete pavers buried at discrete sites provides an indicator of local sedimentation. The map shows annual sedimentation at monitoring sites (circles). Positive values indicate where there has been sediment deposition (accumulation) and negative values indicate erosion.

Rates of sediment accumulation have exceeded the 2mm/yr national guideline every year since monitoring began. However, the drivers of deposition are different at the two sites. At Site A, in the lower estuary, the movement of mobile sands and bioturbation by sediment infauna explain increases at that site. Site B, however, appears to reflect the deposition of fine sediment (i.e., >25% mud content) derived from catchment inputs (see “Land cover 2018⁺” toggle of the map).

At discrete fine scale and sedimentation monitoring sites, mud content is determined from laboratory analysis of surface (0-2cm) sediment samples. Increasing mud contents lead to changes in the macroinvertebrate community living on or within the sediment. Some highly sensitive species are displaced at mud contents as low as ~5%, although the largest ecological shift appears to occur when mud contents exceed >25%. Above this threshold, macroinvertebrate communities are often characterised by a relatively small number of hardy or disturbance tolerant species.

Site A1 in the lower estuary comprises mobile sands (see the “Substrate type” tab on the Habitats page) and sediments have remained sandy (rated ‘Very good’) over the monitoring period. Site B, in a deposition zone in the upper Catlins Lake, has consistently exceeded the biologically relevant threshold of 25% mud content (rated ‘Poor’). Mud content increased from 2017 to 2019, but has subsequently trended back toward the 2017 baseline. A concurrent increase in sediment accrual from 2017 to 2019 suggests a deposition event followed by erosion of finer sediments.

In 2023, mud content was measured at additional sites within different habitat types as part of a synoptic survey that assessed sediment quality and biota. Results are presented in Roberts et al. (2024).