Select Committee on Environment, Transport and Regional Affairs Appendices to the Minutes of Evidence

Annex 2



Issue: Nutrient pollution from point discharges

  The diversity of plants and animals which are present in clean, mesotrophic conditions are the principal wildlife interest of canal systems—though canals which are naturally eutrophic also have a range of typical aquatic plants. Pollution occurs in the form of excess nutrients—nitrogen and particularly phosphorus—producing conditions where only a few aquatic plants can grow (often in abundance). These include fennel pondweed Potamogeton pectinatus, water milfoil Myriophylluum spicatum and rigid hornwort Ceratophyllum demersum. In nutrient-rich conditions, algae and duckweed will also begin to dominate the canal. Dense growths of weeds also create management problems for the navigation authority.

Suggested solutions

  Where canals are polluted from a sewage source, it is possible to install a phosphorus removal process at the sewage works. Reduction of phosphorus will also reduce the enrichment potential of nitrogen. English Nature identified problem discharges affecting the Pocklington Canal SSSI (Yorkshire) and the Chesterfield Canal SSSI (Nottingham), and the costs of phosphorus removal at the sewage works of Yorkshire Water and Severn-Trent Water have been included in the AMP 3 programme by OFWAT.

  Phosphorus levels in the canal should not exceed 100 µgl-1 for naturally eutrophic systems and 35 µgl-1 mesotrophic systems. Standard methods of phosphorus removal rarely achieve less than 1 mg/1 in the effluent (a x 10 reduction). If there is insufficient flow in the receiving canal to provide dilution, consideration should be given to diverting the discharge.

  All SSSI canals should be checked for sewage pollution, and improvement schemes included in AMP 4 where appropriate. Consideration should also be given to removing phosphorus from sewage discharges which are producing algal and weed growth problems in non-SSSI canals. It will also give these canals a chance to develop more natural flora.

Issue: Nutrient pollution from diffuse sources

  Nutrient pollution can also arise from run-off from agricultural land, though this is likely to occur only intermittently (eg at times of heavy rainfall). Effects will occur in plant and animal communities through eutrophication, though levels of phosphorus are unlikely to be as high as those caused by regular point discharges.

Suggested solutions

  Where there are recurrent problems, they should be addressed at source by improved farming practices or, where necessary, changes in land use (eg reversion to grassland from arable). Low-level run-off problems can be mitigated by the installation of buffer strips, though these will not be effective where there is underdrainage. Well-vegetated corridors of shrubs, hedgerows, tall grasses and wetland vegetation will be very effective in removing sediment loads, some phosphorus removal and 50 per cent-100 per cent nitrogen removal. They should be provided around header reservoirs, if the catchment is under intensive agricultural use. Stream water or ditch water entering the reservoir from an intensively used agricultural catchment needs to be treated using plugs of vegetation at the inflow point.


Issue: Low flows

  Many designated wildlife sites are found in the headwaters of the system, and/or are remainder canals, side arms or loops. In all cases, water flow may be low. Header reservoirs may have fallen into disrepair, the canal may leak and some sections may be dry or have very shallow water. The Pocklington Canal in South Yorks is, at its headwaters, a remainder canal of SSSI status. There is sufficient depth of water in the canal (1.0-1.25 metres), but little flow. A sewage treatment scheme is planned to reduce inputs of phosphorus. Algae proliferate at present. Some improvement in flow may also be needed for elimination of the algal problem. The Grantham Canal SSSI, Lincolnshire is also a remainder canal, algae dominate the canal SSSI. Nutrients arise from small point sources and diffuse run-off from agriculture and there is no obvious flow.

Suggested solutions

  These may entail restoring header reservoirs, repairing leaks and tackling nutrient sources. The wildlife interest should return, but some physical management also may be necessary eg removal of excessive silt loads and encroaching edge species (see below). We recommend that low-flow problems affecting SSSIs and causing algal and duckweed accumulation elsewhere in the canal system are identified and appropriate solutions developed. We recognise that, in some situations, there may be no available extra water. Restoration schemes may make new sources of water accessible or prevent leakage. Caution should be exercised, since increases in boat traffic are usually damaging to the characteristic aquatic plants.


Issue: Encroaching vegetation and siltation

  Canals are slow moving rivers—or linear lakes—with the whole canal able to sustain plant growth. Plants compete vigorously one with another for light and space. If they are not managed, marginal species will invade the canal as rooted or floating rafts. The canal may become blocked with vegetation leading to excessive siltation and reducing the space for aquatic plants. It is not best practice to remove marginal growth from long lengths of canals or repeatedly to do so. This leads not only to a loss of overall interest but also to a "vacuum" in the system, which is likely to be filled by colonising species, such as algae and duckweeds.


  Canals may need regular weed cutting and light dredging/desilting over some sections or be left to provide reeded sections for breeding birds. Even so, such sections need a clear through channel free from vegetation to provide an essential water flow. A "chequer-board" pattern of management every 3-10 years is ideal. Alternatively, one side can be managed one year and the other 3-10 years later. The interval of management will depend very much on the nutrient status of the canal. Mesotrophic canals will have a longer interval between management than those with higher nutrient status. There are many examples of good practice developed by British Waterways staff: these could be collated into an informative, instructive manual.


Issue: Impact of boat movements

  Boat movements at high densities can destroy the special aquatic interest found in an SSSI and impoverish the biodiversity of other canals, thus diminishing their aesthetic appeal to the visitor. British Waterways states that it is "part of our remit from government to develop waterways sustainably. Biodiversity is also one of the things that makes our waterways so attractive for people to visit. Our duty is to manage a balance sensitively" (A framework for waterway wildlife strategies, 2000).

  The conservation of aquatic plants can be compatible with light boat traffic. The use of boats as a management tool for conservation, by keeping the centre of the channel clear of encroachment from marginal plants is questionable. In the case of floating water-plantain Luronium natans, it has been suggested that boat movements are beneficial. However, the most flourishing populations, producing rosettes, floating leaves and flowers, are to be found on the non-navigated Rochdale Canal and Welsh section of the Montgomery Canal. In this case, further research on the ecology and physiology of the species may be desirable.

Suggested solutions

  Boats are not the ideal management tool, but their introduction in small numbers on designated wildlife sites can be a way of integrating their use with nature conservation management needs. Research by the University of Liverpool on boat numbers and nature conservation interest is due to be published shortly. This must be applied on a site-specific basis in devising management plans which are tailored to local conditions, including water quality and quantity.

  The control of boat numbers on SSSI stretches could be achieved by a quota system, with permits available from lock-keepers. On selected stretches, visitors could board an interpretative boat which would cruise the SSSI canal section a limited number of times, depending on the wildlife interest. Alternatively, public access to the towpath may be a more appropriate option.

  For some sites, particularly those in the wider countryside, there may be other management options such as widening the canal, chaining off sections of canal, allowing traffic one-boat-width only, or creation of off-line reserves. In such cases, boat numbers would not be restricted, but aquatic wildlife interest may only survive in zoned or protected areas.

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