Erosion and sedimentation
Erosion is the transport by wind, water and ice of soil, sediment and rock fragments produced by the weathering of geological features. Sedimentation occurs when eroded material that is being transported by water, settles out of the water column onto the surface, as the water flow slows. Severe stream erosion
The sediments that form a waterway's bed, banks and floodplain have been transported from higher in the catchment and deposited there by the flow of water. Estuaries are shaped by the mixing of water and sediments from both a waterway and the ocean, creating complex sedimentary environments.
Waterways, and their estuaries, are dynamic landforms and deposited sediments will be moved again under the right conditions. The channel size, shape and bed material change, both over time and in response to changes in water flow and sediment load. A channel is considered to be relatively stable when its water flow and sediment flux are in balance over time. If there is a change in either of these factors, then the channel will adjust its slope, depth, width, meander pattern, bed composition and vegetation density accordingly.
While erosion and sedimentation are natural processes, inappropriate land-use and management practices in the catchment and direct damage to a waterway's channels or banks can accelerate these processes and stimulate adjustments in the channel (see River Restoration Manual No. 18: Stream channel and floodplain erosion and No. 19 Stream and Catchment Hydrology (section 7)).
Erosion is also caused by activities that interfere directly with waterways, such as straightening or deepening channels for flood control, building dams, bridges or crossings, uncontrolled livestock access and removing riparian (fringing) vegetation. Riparian vegetation plays a critical role in stabilising the sediments that form waterways. Living plants and large woody debris, such as fallen logs, slow water flow and reduce its erosive power, while root networks hold the sediments in place. See also Water Note 9 The importance of large woody debris.
When waterway banks are cleared of their vegetation, banks are more vulnerable to erosion. Floods are more likely to cause waterways to change their course and form new meanders or flood channels. Topsoil can also be stripped from the floodplain leading to the loss of valuable agricultural land.
Excess sediment can be damaging to the ecological health of waterways and reduce their environmental, social and cultural values. Mobilised coarse sandy sediment tends to accumulate in areas of slow-flow and may smother bottom-dwelling organisms and their habitats. Deep permanent river pools, that are valuable habitats for aquatic fauna and refuges for wildlife during summer and drought, may become filled by course sediments.
Large sediment accumulations can cause upstream flooding, or deflect the flow into the adjacent stream bank or even onto adjacent land, causing further erosion. In addition sediment can fill the deep permanent pools of rivers to ruin this critical refuge habitat.
Increased fine sediment suspended in the water column (turbidity) reduces the penetration of light and therefore the ability of algae and other aquatic plants to photosynthesize and clogs the gills of fish. Fine sediment and the nutrients it transports are also associated with seasonal blue-green algal blooms. As a consequence, of excess sediment, the abundance and distribution of aquatic plants and animals change, natural food webs are disrupted and aquatic diversity declines.
Once sediment has entered a waterway it is difficult and expensive to remove, requiring engineering solutions and heavy equipment. Techniques for sediment management are described in Water Note 17: Sediment in streams. See Water note WN38: Management of sediment in pools of the Avon River system for a case study that describes the restoration of deep permanent river pools filled by sediment.