MANAGING CANALS FOR WILDLIFE
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 systemsthough canals which are naturally
eutrophic also have a range of typical aquatic plants. Pollution
occurs in the form of excess nutrientsnitrogen and particularly
phosphorusproducing 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.
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
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.
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
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
Issue: Encroaching vegetation and siltation
Canals are slow moving riversor linear
lakeswith 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,
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.
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.