Information and Advisory Note Number 22                                                Back to menu

Rivers and their catchments: causes and effects of turbid water

1. Introduction

1.1 The aim of this Information and Advisory Note is to address the nature and causes of turbid water in rivers, it includes recommendations for methods of mitigating turbid water for a variety of human activities and a list of relevant legislation. This information and Advisory Note should be
read in association with 'Rivers and their Catchments: an overview: (Information and Advisory Note No. 18)

1.2 Turbidity refers to the amount of light that is scattered or absorbed by a fluid, it is measured in turbidity units (FTU) based on the comparison of the scattering of light by a water sample with that of a standard suspension of formazin. Turbidity in streams refers to the cloudiness of the water due to the presence of suspended particles of silt, clay, waste effluents and other particulate materials. These can originate from natural sources such as peaty waters from upland areas, as well as from human activities.

1.3 The supply of suspended sediment and coloured water to a river system is controlled by the drift deposits and soils in the catchment and the erosion and transport mechanisms in operation. Erosion is a function of physical processes and the resistance of the materials being eroded (for example the impact of rainfall on bare soils); transport is a function of the surface flows generated and the proximity of a watercourse. The sediment load and turbidity of the water are determined by the particle size distribution of the soils and drift deposits, and will tend to be higher in areas of predominantly clay soils than in areas of sandier soils. Activities which cause an increase in turbidity are summarised Table 1.

2. Nature of impacts on rivers and their catchments

2.1 Physical Impacts

2.1.1 Fine sediment can affect the stream channel by:

• altering the substrate composition, clogging channel bed interstices and reducing habitat space for small fish and invertebrates;
• causing marginal changes to the instream channel morphology, and general habitat availability;
• reducing the permeability of the bed material.

2.1.2 High suspended sediment concentrations can also affect water resources by:

• damaging turbines in hydroelectric plants, and increasing requirements for water treatment procedures;
• reducing reservoir and diversion dam storage capacity.

2.2 Chemical Impacts

2.2.1 High turbidity levels can cause a decline in the intergravel concentration of dissolved oxygen (DO).

2.3 Biological impacts

2.3.1 An increase in stream water turbidity can cause a reduction in the depth of light penetration into the water column. This effectively decreases rates of photosynthetic activity and thus primary productivity in submerged plants (a basic food source for aquatic animals). A reduction in the food source at the primary level may then have a knock-on effect upon higher trophic levels.

2.3.2 High turbidity levels can also cause physical damage to leaf surfaces by abrasion and by smothering.

2.3.3 High turbidity levels can adversely affect invertebrate populations, interfere with the behaviour, feeding and growth of salmonids and other fish species. It can also cause damage to fish gills by abrasion (hyperplasia), and clogging. Research indicates that fine particles between 2-3(p.m are mostly responsible for gill abrasion in salmonid populations.

2.3.4 High suspended sediment concentrations may also increase the susceptibility of fish to disease. Mucus secreted by fish in response to high concentrations of suspended solids attracts bacteria and fungus.

2.3.5 Turbidity can increase surface water temperature and lead to thermal stratification, a problem not only in rivers, but also for downstream reservoirs and lakes.

2.3.6 Deposition of suspended sediment during high flood events can replenish nutrients and soil materials on the flood plain.

2.4 Speed of recovery

2.4.1 Streams and rivers have different levels of sensitivity to increases in suspended sediment concentrations, depending on the natural background levels. Some of the controlling factors are gradient of channel-bed, the adaptability of the flora and fauna and the particle size distribution of the sediment. Occasional periodic increases in both suspended and deposited sediment are
a natural phenomenon, and stream and river habitats have adapted to cope with a range of sediment concentrations resulting from natural events. If the rate and magnitude of sediment loading resulting from human activities exceed those of natural events, this can put serious stresses on watercourses and associated habitats. The key controlling factors that determine the speed of stream recovery are:

• the spatial and temporal extent of damage due to the increased sediment concentrations;
• elimination of the sediment source;
• the ability of the river to flush out the suspended and deposited material.

3. Mitigation of impacts on rivers and their catchments

3.1 Forestry

3.1.1 The following actions can reduce the impacts of forest roads:

• do not locate close to watercourses;
• prevent direct discharge from forest roads to watercourses;
• prevent sediment and debris entering streams wherever possible; the use of buffer strips may help to impede the direct flow of runoff into stream channels, and encourage sediment deposition outwith the stream channel; buffer strips should be at least 5 metres wide, but up to 10 times the stream width for larger watercourses;
• avoid unnecessary maintenance;
• timing of any road construction and maintenance should avoid wet periods and fish spawning periods;
• soil exposed by construction activities should be revegetated quickly;
• drains and culverts should be used alongside road edges to direct water away from road surfaces.
• Minimise mechanical disruption of stream beds and banks.
• Protect upland riparian vegetation.

3.1.2 To reduce the impacts of ploughing:

• do not plough unnecessarily;
• wherever possible use scarifying, mounding or ripping as an alternative to ploughing on steep ground;
• drains leading to a watercourse/stream should be tapered in depth and stopped 15-20 metres short of the channel, so that water discharged from the drain can filter through ground vegetation and deposit suspended sediment before reaching the channel;
• cleaning of the forest drains should be avoided between mid-October and mid-May in order to reduce the chances of suffocating gravel spawning grounds and salmonid eggs through sedimentation.

3.1.3 To reduce the impacts of thinning, felling and extraction:

• trees should be felled away from streams and care taken not to break the soil surface when moving logs;
• mechanical disruption of stream beds and banks should be avoided;
• extraction in sensitive areas should be timed to coincide with dry periods;
• clearance of woody debris from forest streams should be undertaken so as to avoid fish spawning periods; woody debris also forms part of nutrient cycling and instream habitats.

3.2 Mining

3.2.1 The following actions can be taken to reduce the impacts of mining:

• provide improved settling facilities, such as tanks or pools;
• retain topsoil removed from a new mining site, and replace it on completion of mining activities.

3.3 Agriculture

3.3.1 The following measures will reduce farming impacts:

• ensure adequate dilution of suspended solids;
• leave an uncultivated buffer strip alongside watercourses;
• ploughing should be undertaken in spring and ground re-seeded immediately;
• spreading of animal manures should be undertaken in dry periods whenever possible;
• avoid manure spreading within 10 metres of watercourses, and when ground is frozen or waterlogged;
• use fences to keep cattle/sheep from damaging stream and river banks;
• dean field ditches from one bank only to prevent excessive erosion.
   

3.4 Fish Farming

3.4.1 The following measures will reduce the impacts of fish farming:

• provide a second pool/setting pond which can be used during periods of pond maintenance; sludge removal would help to avoid re-suspension of fine particles and subsequent release into watercourses.
• frequent sludge removal from settling ponds and careful design of pond inlets would aid settling of suspended solids within the settling pools.
   

For more detailed guidelines refer to Forests & Water Guidelines, and Prevention of Environmental Pollution from Agricultural Activity.

4. Legislation and Government guidelines

4.1 Legislation and guidelines applicable to this advice note include:

• The Water Resources Act (1991);
• The Water Industry Act (1991);
• Environmental Protection Act (1990);
• Water (Prevention of Pollution) (Code of Practice) (Scotland) Order 1992 (SI 395);
• Control of Pollution Act (1974);
• Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) (Scotland) Regulations 1991 (S11991 No. 346 S.35);
• EC Directive 78/659/EEC - on the quality of fresh waters needing protection or improvement in Order to support fish life.

4.2 In Scotland SEPA are required to 'promote the cleanliness of inland waters'. Each authority establishes its own water quality objectives, but each also has to comply with national water quality standards and objectives as specified by the Government/EC Directives. SEPA Is responsible for enforcing this legislation.

4.3 Further information and advice are available from:

• River Purification Boards/SEPA;
• Forestry Commission;
• Farming and Wildlife Advisory Group;
• Agricultural Development Advisory Service (ADAS);
• Institute of Hydrology.

Further reading

English Nature 1994. Roads and Nature Conservation - Guidance on Impacts, mitigation and enhancement. Peterborough, English Nature.

Forestry Authority 1993. Forests and Water Guidelines. Third edition. London, HMSO.

FWAG (no date). FWAG Handbook for Environmentally Responsible Farming. The Environment Research Fund (available from Farming and Wildlife Advisory Group, National Agricultural Centre, Stoneleigh, Kenilworth, Warwickshire, CV8 2RX).

Mackay, D.W. 1994. Pollution control. In Maitland, P.S., Boon, PJ. & McLusky, D.S. (eds) The Fresh Waters of Scotland: a National Resource of International Significance. Chichester, John Wiley, 517-531.

Maitland, P.S., Boon. PJ. & McLusky, D.S. 1994. The Fresh Waters of Scotland: a National Resource of International Significance. Chichester, John Wiley.

Shaw, E.M. 1988. Hydrology in Practice. Second edition. London. Chapman & Hall.

Scottish Office 1992. Prevention of Environmental Pollution from Agricultural Activity. HMSO, Dd. 8218115 (available from SOAEFD).

Werritty. A. 1995. integrated Catchment Management: a Review and Evaluation. Scottish Natural Heritage Review No. 58.

Authors: Rachel Bronsdon, Institute of
Hydrology, Stirling. Jonathan Clark, Earth
Sciences Branch. Scottish Natural Heritage,
Edinburgh, .- -

 

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