Although coal mining was one of the main reasons for the success of the navigation, it has also brought problems, caused by subsidence. In March 1988, the bank near Lemonroyd lock collapsed into St Aidan's opencast mine, which then flooded. A significant factor was the presence of excavations below the opencast workings where lower coal seams had previously been mined. The failure resulted in some of material, including the banks of the river and the canal, slipping into the workings, which then flooded to a depth of , creating a lake which covered . An act of Parliament was obtained to allow of new waterway to be constructed. The original locks at Kippax and Lemonroyd were replaced by a single lock at Lemonroyd. Opencast mining finally resumed ten years later, after the site had been pumped out, but the coal reserves are now exhausted, and the site forms a nature reserve, with wet grasslands, reedbeds and open water covering .