Even a quiescent canal is 20mm shallower at the end of an average day, due to evaporation, seepage and leakage. On a busy narrow canal, each lock cycle needs at least 100,000 litres. Broad locks go through 250,000 litres each time they are used, so maintaining constant, reliable water supplies is a top priority for BW engineers.
West Midland canals are particularly difficult, BW Midlands & South West regional engineering manager David Bligh reports. 'Birmingham is virtually the highest point in the UK canal network, at 120m above sea level, so everything drains away from it.
'To meet the need to constantly top up the system, we have to exploit all available sources.'
Disused local mine shafts now yield 35M litre a day and a backpumping station feeds the Grand Union Canal with 200 litre/second. But nearly all the water entering the canal system is from the original sources, and BW is investing heavily in upgrading such sources.
Bligh explains: 'These are mainly culverts and ditches fed by springs or small 'directable' streams whose use was authorised by the enabling legislation that set up the original canal company.
'Using these is significantly cheaper than pumping water into the system from other sources.'
However, the use of streams has its problems, Bligh says. Water from streams is usually diverted into the canal via small weirs or sluices, many of which need investment, but when the canal is full or overflowing, the surplus water 'defaults' into the water course downstream of the weir.
'These default channels are in riparian hands, ' Bligh points out. 'So when we repair or level the weirs we have to be very careful about water quality further downstream.'
In the canals proper, water quality is important to more than just the local wildlife. Several power stations draw cooling water from the network, and half Bristol's drinking water supplies comes from the Gloucester & Sharpness Canal.
A different municipal function is performed by the Staffordshire & Worcester canal, Bligh says. 'The 30km of feeders was in such bad condition we had to find another practical source. We found it in the local waste water treatment works, whose effluent is more than adequate for the purpose.'
Although the problems of supplying enough water to certain key sections of canal - notably the Caen Hill locks on the Kennet & Avon in the south - have been well-publicised, overall, says Bligh, the biggest day to day threat to all canal water supplies is vandalism.
'This is not actual damage - waterways structures are pretty robust - more the deliberate raising of paddles on locks overnight. This can drain down pounds and flood towpaths below.
Such losses can be minimised if BW staff get there quickly, however, and BW has speeded response times by installing a telemetry
system and, on the more vulnerable canals, closed circuit TV monitoring.