Total traffic on the river had fallen to 18,548 tons in 1905, and averaged 5,870 tons during the years of the First World War, mainly general merchandise and agricultural produce. With the nationalisation of the waterways following the Second World War, the navigation eventually became the responsibility of British Waterways as a result of the Transport Act 1962, and since 2 July 2012 has been managed by the Canal & River Trust. Today, commercial traffic, apart from tour boats, has ceased above the port of Boston (The Haven) and only pleasure craft carry on through the lock at the Grand Sluice into the Witham. Although the lock is only long, it is possible for longer boats to pass through it at certain states of the tide. Unlike many such sea locks, the reverse-facing gates close on every tide, as the normal high tide water level is higher than the level of the river, while at low tides there is insufficient water in the Haven to allow exit from the lock. Passage is therefore restricted to a brief period approximately two hours before or after high water. It is still possible to navigate many of the drains in small vessels, and a new lock, completed in December 2008, provides entry to the South Forty-Foot Drain from below the Grand Sluice so that vessels will be able to reach the Fens without venturing out to the Wash, as part of the Fens Waterways Link. 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. There are traffic lights on the short section between Brayford Pool and Stamp End Lock, which are used to indicate the state of the river in times of flood. Green indicates that flows are low or normal, and boats can proceed along this stretch. Red indicates that flows are high and that great care is required.