Comment by notahacker

Other relevant context: sections of UK canals being unintentional drained isn't particularly unusual, although the culprit is usually a paddle left open on a lock gate or a leaky culvert rather than a plug being pulled. Whether that inconveniences anyone for any length of time depends mostly on how full the reservoirs at the top end of the canal are...

Wouldn't have been that unusual in 1972 when nearly all the canals including that one had ceased commercial operations and many of them had been intentionally drained either. I suspect the transition from the canal being infrastructure maintained by locally-stationed full time professionals to a pleasure cruiseway which the new waterways board was willing to devote a bit of time to maintaining only after the previous one had spent several years trying to get it shut down probably had as much impact as the Blitz on the work crew having no idea about plugholes...