The lower Wilson River was subjected to large-scale commercial gravel extraction during the early to mid 20th century for the purpose of providing gravel to the construction and landscaping industries. This caused progressive riverbed lowering and bank erosion, known as a 'knick point'. As this knick point moved upstream, the bed level lowered causing riverbanks to erode and riparian vegetation to collapse. Deep pool and riffle sequences that provide critical aquatic habitat were being in-filled and water quality had declined due to sedimentation.
Saving a significant habitat corridor
More than 14 km's of the Wilson River had been degraded by active knick point erosion which began to impact on intact reaches of the river, documented as having the highest fish diversity and aquatic condition of 80 NSW rivers. If the upstream migration of the knick point was to continue, large reaches of intact habitat would be degraded in the future. The remaining 26km of intact reaches of the Wilson River is identified as a regionally significant habitat corridor. Doing 'nothing' would reduce regional biodiversity and impede recovery through habitat loss. All bed and bank erosion leads to an increase in sediment load that deposits in the estuary. This can smother seagrasses, decrease water quality and also cause excessive shoaling reducing the amount of navigable waterway.
In a joint funding initiative between the Northern Rivers Catchment Management Authority (NRCMA) and Council, a series of in-stream works and riparian re-vegetation on private land, on two riffle zones within a one km reach of the Wilson River, was undertaken. These works were carried out to stop active knick point erosion from moving further upstream and protect 26km of intact river habitat and riparian vegetation from future erosion. The remedial works consisted of a placing a series of large woody debris (LWD) structures in the river to help restrict flows and enhance bed and bank stability.
Rehabilitation work included
- Several bank deflector log jams to deflect water away from sensitive eroding banks and the channel flows towards the elevated log sills.
- Two elevated log sills consist of three suspended logs that span the river at a height to hold up water during low to moderate flood events. The slowing and backflooding effect helps to reduce the risk of lowering the river bed any further which helps to keep the river banks stable.
- Four rock girdles - which are large rock spanning the width of the river to provide a physical barrier to further bed lowering.
The riparian vegetation within and around the access tracks is a mixture of 5-year native regrowth and woody weeds. Riparian widths range from 5 to 30 metres.Where soil was disturbed during work on the riverbank, dense plantings of Lomandra Hystrix was undertaken to ensure the riverbank is stabilised.
- Protection and rehabilitation of aquatic and riparian habitat in a 1km target reach of the Wilson River.
- Prevention of further knick point erosion of the upper Wilson River, protecting the remaining 26 kilometres of intact aquatic habitat from knick point induced erosion.
- A three-year study documenting the biotic and geomorphic response of the Wilson River to remedial works.
- Increased awareness of the value of rivers with local schools.
This project has been a joint initiative of Council, the Environmental Trust and the Northern Rivers Catchment Management Authority (NRCMA).
This page was last updated on: 12 April 2016