Sedimentation has the potential to impact on river and estuary health through a number of different ways.
Firstly, many contaminants can bind to sediment and so be transported along a river and into estuaries. Where the sediment settles (generally where the speed of the water movement is slower such as in river pools) it can cause pockets of contamination within a waterway.
Secondly, if there is a large amount of sediment present in the water it will reduce the amount of light that is able to penetrate through the water. This can limit the ability of aquatic plants to grow as less light reaches them as well as settling on their leaves smothering them.
Thirdly, fine sediment can clog the gills of fish and macroinvertebrates making it difficult for them to breathe. Lastly, accumulated sediment can smother the available habitat for aquatic animals. Where there is little sedimentation there is usually lots of nooks and crannies in which animals can hide and make their homes. If there has been a lot of sedimentation it will fill these crevices, reducing the amount of space available for animals to live in.
Some sediment movement and sedimentation is natural in waterways however human intervention has increased the amount of sediment that is being moved about and the amount of sedimentation that is occurring. The vegetation that is found naturally around rivers and estuaries helps to stabilise the banks by binding together the soil with their roots. Where this vegetation is removed soils will usually erode more quickly, flushing sediment into the nearby waterways. Allowing stock, such as cattle, unrestricted access to rivers also increases the amount of erosion and hence sediment in the water, as they like to trample around in the water, causing banks to collapse and stirring up the bottom.
The construction of dams and the abstraction of water directly from rivers and from groundwater bores close to waterways also has an impact on the amount of sediment present. In a natural system sedimentation will occur however periodic high flow events will flush the accumulated sediment out. Where the pattern of flow in a river has been changed and these episodic high flow events become less frequent or are lost, the accumulated sediment will no longer be periodically flushed out.