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Issue: Catchment scale influences - Land and vegetation condition - Erosion

This is an issue under the Inland waters theme of the Data Reporting System.

Why we need to know about this issue

Erosion of soil and sediment from hill slopes and riverbanks contribute greatly to increased sediment loads to rivers and estuaries. Soil erosion is a major source of nutrients in many waterways and their transport into inland waters increases the risk of eutrophication and algal blooms.

Soil erosion and sedimentation in inland waters can have other significant impacts such as siltation of river channels, infilling of wetlands, smothering of aquatic flora and fauna, and reduced light penetration and increasing the turbidity of waters.

Sediment loads have a strong influence on biodiversity and river function. Catchment erosion and consequent sedimentation can significantly alter the physical habitat of instream environments. This produces shallowing of pools and buries coarse bottom sediments leading to a loss of habitat and spawning sites for gravel bed dependent fish. This can produce flow on effects through food chain linkages.

Where the supply of sediment from upstream exceed a river's flushing capacity, this material will accumulate and smother stream bed habitats and fill deep pools. Sand and gravel are too unstable for growth of benthic organisms and loss of deep pools also means a loss of refuge and breeding grounds. Sediments also supply nutrients and change the food web structure of an ecosystem.

Suspended solids can block gills and filter feeders and smother sedentary aquatic plants, animals and their eggs. Additionally, nutrients are transported in suspended solids and these can have their own impacts as discussed in other issues.

Indicators

• LD-04 Area and change in area of exposed soil surface contributing to erosion 
  When unstable soil is eroded, it can lead to sedimentation, which can significantly alter the physical habitat of instream environments. Sediment loads in inland water catchments is one measure of the quantity of soil being eroded into waterways.
• IW-13 Catchment sediment load 
  Catchment erosion can lead to sedimentation, which can significantly alter the physical habitat of instream environments. Sediment loads in inland water catchments is one measure of the quantity of soil being eroded into waterways.
• IW-17 Exceedance of turbidity water quality triggers 
  Catchment erosion and consequent sedimentation can significantly alter the physical habitat of instream environments. This produces shallowing of pools and buries coarse bottom sediments leading to a loss of habitat and spawning sites for gravel bed dependent fish. This can produce flow on effects through food chain linkages.
  Turbidity is in part a measure of instream sedimentation. Number of exceedences of water quality triggers for turbidity is a measure of this pressure.
• IW-18 Exceedance of suspended solids water quality triggers 
  Suspended solids is in part a measure of instream sedimentation. Number of exceedences of water quality triggers for suspended solids is a measure of this pressure.
• IW-24 Extent of sedimentation (incl sand slugs) 
  Soil erosion has the potential for downstream impacts on creeks, rivers, reservoirs, lakes, and estuarine and marine environments. Water-borne erosion increases the supply of sediment to rivers. Extent of streams and water bodies affected by sedimentation is an indirect measure of the pressure of erosion on riparian vegetation.

Related issues

• Inland Waters - Habitat scale influences- Water Quality (for surface and groundwater)- Nutrients 
• Inland Waters - Catchment scale influences- Land and vegetation condition- Nutrients and sediments - sources and loads 
• Inland Waters - Habitat scale influences- In-stream habitat - woody debris and sand slugs