The Birmingham Canal emerges from the short Broad Street Tunnel to reach its terminus at the Gas Street Basin
Link . The basin turns sharply left to end before reaching Bridge Street just out of sight to the left. It used to continue for a short distance under Bridge Street to end in two parallel basins at Old Wharf.
The narrowboats at the centre are moored perpendicular to the Worcester Bar, a narrow barrier separating the Birmingham Canal Navigations (BCN) side of Gas Street Basin from that belonging to the Worcester and Birmingham Canal (W&B) beyond. Both canals were built by rival companies, and the older BCN (opened 1773) insisted that a physical separation was provided when the W&B opened in 1795. Since Birmingham lies significantly higher than Worcester, a direct connection would've meant that the BCN would lose significant amounts of water, which they couldn't afford given that they were struggling to keep up the required water levels at the high point of their network near Smethwick
Link anyway. However, the inconvenience of transshipping at Worcester Bar led to the opening of a stop lock (under the footbridge on the right) to connect the two systems. A stop lock is a lock between two water bodies at the same level which prevents a continuous flow of water from one into the other. After the New Main Line of the BCN was opened in 1829, the Smethwick summit was bypassed by a deep cut
Link , further reducing the problem of water loss through the connection.
At some point after nationalisation of the entire canal network in 1948, the stop lock was no longer needed and the lock gates have since been removed. However, the problem of water loss towards Worcester remains an issue to the present day - signs at the Tardebigge flight of locks
Link remind boaters to conserve water and avoid unnecessary lock operations.