Sediment Research Project
Sediment is one of the major threats to the Pūharakekenui. The Upper Styx River and Smacks Creek have the highest quality of instream biodiversity of Christchurch city’s three waterways. Sediment was identified as a risk to the health of the river circa 2000 in the Styx Vision Planning Process, and this is now coming to pass. Despite improvements in the management and enforcement of sediment discharges in the last 20 years, sediment is still being piped into the Styx River through the stormwater network, accumulating in the Upper Styx River and smothering the macroinvertebrates and aquatic life.
It is only a matter of time until this deposition of fine sediment makes the pollution sensitive fauna of the upper Styx extinct, as is the case in the other rivers of the city. A sediment working group was formed by the Trust in collaboration with ECan, CCC and local experts to investigate how to best manage sediment in the Styx.
The Styx Living Laboratory Trust is working collaboratively with Council and Environment Canterbury to address sediment issues within the Pūharakekenui via this project. The Trust's contribution to this project is funded by the Waitaha Action to Impact Fund.
The Christchurch River Environment Assessment Survey (CREAS) is a GIS-based survey of the habitat and basic ecology of waterways undertaken by the Christchurch City Council. The purpose of the CREAS monitoring is to capture the current state of waterways within and across catchments and to investigate any spatial and temporal changes in habitat and ecology through repeated surveys in these catchments.
These surveys will allow prioritisation of waterways for management, such as restoration and remediation. Surveys have been carried out since the early 2000s (original sites), and the programme will continue to be developed to assess waterways across Christchurch City and Banks Peninsula. The CREAS assessments are undertaken at 50 m intervals along wadeable reaches of entire waterways / whole catchments. Numerous measurements are made at each site, relating to bank attributes, channel attributes, river cover, riparian vegetation and other variables. CREAS was identified as one tool to identify sediment hotspots within the catchment. As CREAS has been repeated at sites throughout the catchment and over numerous years, the research will allow for a better understanding of variations in space and time; the data also provided insights into contributing environmental factors to sediment hotspots.
This project will have two parts. An initial report and a final report. The findings from the initial report are below and the final report is expected to be available shortly
MAP A
'Pūharakekenui / Styx River catchment’ shows the main river catchments in Ōtautahi / Christchurch, with the Pūharakekenui / Styx River catchment boundary highlighted (indicative only)
MAP B
‘CREAS sites’ shows all sites surveyed in the Pūharakekenui / Styx River catchment as part of the CREAS project. This includes sites that were surveyed between 2006 and 2009 (orange circles) and sites that were surveyed in 2018 (red circles). Some sites have been surveyed twice and at these sites points will overlap.
MAP C
'Sediment depth overview (2018)’ shows the sediment depth at all sites surveyed in 2018, with a scale from deep sediment (>100 cm) to no sediment (0 cm). Most sites across the Pūharakekenui / Styx River catchment had sediment deeper than 25 cm.
MAP D
'Sediment cover and deep sediment sites’ shows the 2018 sites that meet or exceed the CSNDC Target Attribute Level of 25%, respectively. We have also included hotspots of extremely deep (>100 cm) sediment. Sites with deep sediment typically also have high sediment cover.
MAP E
'Original sites with major sediment depth increase’ shows sites that were found to have shallow sediment depth (<5 cm) when first surveyed between 2006 and 2009 (orange circles) overlaid with sites that had sediment depth of at least 50 cm in 2018 (black triangles). These sites where sediment depth has markedly increased may warrant further investigation.
Map F
'Macrophyte cover and deep sediment sites’ shows the 2018 sites that meet or exceed the Council’s Comprehensive Stormwater Network Discharge Consent (CSNDC) Target Attribute Level of 100 cm)sediment. There appears to be no spatial relationship between high macrophyte cover and where deep sediment occurs.
MAP G
'Unstable and unvegetated sites’ shows where there are unstable banks and how this is related to areas where riparian cover (on the lower and immediate upper banks) is low. Low unvegetated cover does not mean that the remaining cover is vegetated as this area may consist of unvegetated impervious surfaces. This may provide a tool for prioritising riparian planting and subsequently reducing erosion and sediment inputs. Unstable banks are often spatially related to lower riparian vegetation cover.
Want to learn more about why sediment is a major threat to the Styx?
Check out our video: https://fb.watch/inJPV1Vq6A/