the witham is navigable from brayford wharf in lincoln to boston. there are two locks between boston and lincoln – one at bardney and the other in lincoln itself, the stamp end lock which is unusually a guillotine lock. the main obstruction to navigation is the high bridge or glory hole in lincoln, a medieval structure which is only about wide and high at normal river levels. in times of flood it is unnavigable. the bridge spans the river for , and consists of an arch built in c1160, with extensions added in 1235, 1540 to 1550 and 1762/3. it is the only british bridge which still has secular medieval buildings standing on it, and is believed to be the second oldest masonry arch bridge in the country. it is currently a scheduled ancient monument and a grade i listed building.