A not-so-obvious aspect of the Union Canal is its 2.4-mile long feeder canal from which around two million gallons of water per day flows into the Union Canal at the eastern end of the Almond Aqueduct.
This takes water from a large catchment area about half the size of West Lothian for which the main rivers are the River Almond and its tributaries. This also includes Cobbinshaw Reservoir which is 880 feet above sea level and seven miles south east of the start of the feeder canal. This was originally a small loch that was enlarged to 310 acres to supply the canal. Its outlet feeds the Bog Burn that becomes Murieston Water which joins the River Almond at Mid Calder.
800 yards to the east of this confluence is the weir which diverts some River Almond water into the feeder canal with a sluice to control the amount of water taken. From this point the feeder canal runs close to the river until it joins the Union Canal immediately east of the Almond Aqueduct which is two miles to the north-east.
From the weir, the feeder canal is on the north side of the river for 500 yards until it crosses the river on a cast iron aqueduct. This carries the feeder over the Almond at a height of 20 feet above the river in a cast iron trough six foot wide and three foot deep over which there are plates that enable the aqueduct to be also used as a footbridge.
300 yards north east of this aqueduct is a steep slope above the river underneath which the feeder canal runs in a six-foot wide 750-yard tunnel. Further downstream, the feeder canal was generally constructed on a step formed in the valley side or, where the valley slope is too steep, through further three further small tunnels between 140 and 175 yards long.
The end of the last tunnel is 100 yards from the Union Canal. Here the width of the feeder canal increases to form a junction with the canal at the east end of the Almond Aqueduct with landing stages. Unwary boaters heading west might go straight on into this dead end instead of turning right to continue on the canal over the aqueduct towards Falkirk.
Like the small feeder canal aqueduct, the Almond Aqueduct carries the canal over the river in a cast iron trough which is 420 feet long. The aqueduct is 75 feet above the river and has a sluice gate which can pour water back into the river when the canal water level is too high.
A walk along the feeder canal as it follows the steep sided valley getting ever higher above the river is an interesting experience. Seeing the river falling below can give the impression that the water in feeder canal is flowing uphill.