Searching for the Manchester and Salford Junction Canal

Pennine Waterways explains:

The Manchester and Salford Junction Canal is Manchester's almost forgotten waterway. It was built in 1839 to link the River Irwell and the Manchester, Bolton and Bury Canal to the Rochdale Canal. 

The canal was less than a mile long, with 4 locks and a tunnel below the Deansgate area. It was built to let boats get between the Irwell and the Rochdale Canal without having to pass through the Bridgewater Canal's Hulme link, the tolls for which were high. 

The canal was never a great success as it never carried as much traffic as anticipated, partly because the Bridgewater company retaliated by reducing their own tolls. Loading wharves were constructed in the tunnel section with goods being hoisted through shafts to and from the railway warehouse later built above. 

A unique feature of the canal was that the two top locks were constructed as parallel staircase locks (i.e. with a total of 4 broad lock chambers). This suggests an anticipation of a high level of traffic that never materialised. The site of these staircase locks lies under what was Manchester Central railway station.