Excess fine sediments

What is the issue?

A key issue of concern to the S&TCUK is the increasing loads of fine sediments being delivered to our water systems. Healthy freshwater ecosystems require sediment inputs, but human activities, such as intensification of arable production and poor agricultural practises, are increasing this supply at a rate which is proving detrimental to fish, aquatic invertebrate and plants.

The effect of sediment on fish and invertebrates is complex, and will depend on the amount of fine sediment, the time of exposure to fine sediment, and the sediment composition, but impacts can include;

  • Fish and invertebrate mortality through smothering or suffocation
  • Reduced fish reproduction and growth through the degradation of spawning habitat/redds
  • Fish behavioural changes, such as impeding movement and altering feeding behaviour

Fine sediments entering freshwaters also bring bound up within them agricultural and industrial contaminants, such as phosphorus and/or heavy metals and organic pollutants (See: Water Pollution), which can also severely impact fish, invertebrate and plant assemblages.

The current UK standard for fine sediment is set by the EU Freshwater Fish Directive (FFD), and stipulates that suspended solid concentrations should not exceed a guideline annual mean of 25mg/l. This is only a ‘guideline’ standard which should be achieved where possible. In August 2006, UKTAG made the decision to continue running with the guideline threshold until the FFD is repealed in 2013. However, in order for sediment management to progress in England and Wales, better informed sediment targets are urgently required for compliance testing.

What S&TCUK has achieved so far

  • S&TCUK invested time and resources into a project assessing the efficiency of bankside fencing at reducing fine sediments reaching spawning redds. The outcomes of this survey were published and used to better inform sediment management.
  • We are also involved in a scientific project researching sediment impact on river systems and sources of sediment input. Both of these projects are in conjunction with partners including ADAS, and are geared at better informing the 2nd cycle of the Water Framework Directive.

What still needs to be done?

Urgent action is required by Defra;

  • to identify and use a suitable framework for establishing revised sediment targets for catchment compliance across England and Wales, taking better account of the impacts of sediment on ecology
  • to monitor and quantify the efficacy of sediment mitigation options currently being used, such as those under Catchment Sensitive Farming initiatives. If quantifiable improvements cannot be shown, The Government should provide clear guidance on when and how regulatory tools, such as Water Protection Zones, can be used if the voluntary approach fails to deliver.


Other information