Dirty waters
Our wet winter has resulted in our rivers running brown and plumes of brown water where they enter the sea along the north-west coast. Increased siltation of our waterways poses a high risk to the Giant Freshwater Crayfish. Siltation or sedimentation can affect a crayfish’s ability to breathe oxygen through the gills, and this affects the animal’s health, particularly during its juvenile stage. Sedimentation can come from bed or bank erosion within a waterway, from stock access and in-stream stock watering points, from dirt roads and farm crossings, from exposed soil in or near waterways and from land clearing in the catchment. Vegetation, especially deep-rooting woody vegetation on the top of the river bank, is essential to hold the banks together and prevent erosion. It is also important to leave a well vegetated buffer zone between the paddock activities and the waterway, to filter out nutrients, soil and fine sediments carried downhill in wet seasonal conditions.
Inglis River in flood
Access tracks and farm crossings also need vegetated buffers (which can be grass and not just trees) between their runoff drainage and the waterway, to filter out the sediment and nutrients. Waterway crossings need careful design and installation, avoiding bends in rivers and creeks where water velocities are always higher. Pipes and culverts are easily blocked by sediment and woody debris and can lead to increased bank and bed erosion. Bridges are more expensive to install but often cost less in long term maintenance, whereas gravel crossings or causeways are cheaper to install but may lead to major bed erosion which is difficult and expensive to fix and often leads to loss of productive land when banks are undermined and collapse as a result of bed lowering. Giant Freshwater Crayfish do not cope well with crossings or culverts and often travel overland to avoid them. Soil erosion from farming and cropping activities can also turn the waterways brown. Reduce the risk through measures such as cover cropping, cultivating on the contour of the slope and maintaining vegetated buffers between production land and waterways.
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