From 1142 onwards, sluices were constructed to prevent flooding by the sea, and this culminated in the Great Sluice, which was constructed in 1766. It maintained river levels above Boston, and helped to scour the channel below it. The land through which the lower river runs has been the subject of much land drainage, and many drains are connected to the Witham by flood doors, which block them off if river levels rise rapidly. The river is navigable from Brayford Pool in Lincoln to Boston. Its locks are at Lincoln, Bardney and the Grand/Great Sluice. Passage through the latter is restricted typically to 4-hour intervals during daylight when the tidal levels are suitable. The river provides access for boaters to the Witham Navigable Drains, to the north of Boston, and to the South Forty-Foot Drain to the south, which was reopened as part of the Fens Waterways Link, a project to link the river to the Nene flowing through the city of Peterborough. From Brayford Pool the Fossdyke Navigation links to the Trent.
The Witham's course, which flows to the north and then to the south-east may be the result of glaciation (and possibly isostatic rebound) redirecting older rivers.[citation needed] The source of the river is on high ground near South Witham, Lincolnshire,[6] at around 340 feet (100 m) above ordnance datum (AOD). After briefly flowing to the east to reach South Witham, it flows generally north, passing through Colsterworth where it is crossed by the A1 road, which largely follows the line of the river to Newark on Trent. At Great Ponton, it is joined by the Cringle Brook on its left bank, and continues through Grantham, where it has already descended to 170 feet (52 m) AOD. After Barkston it turns to the west to pass through Marston. Foston Beck joins on the left bank, and at Long Bennington it resumes its northerly course. Beyond Claypole and near Barnby in the Willows it forms the border between Lincolnshire and Nottinghamshire for about 3 miles (4.8 km), before passing through an Army training area near Beckingham. The River Brant joins on the right bank, before it passes through North Hykeham to reach Lincoln, where it is only 16 feet (4.9 m) AOD.[7][6]
The upper waters are important for agricultural water extraction, and for coarse fish such as roach, common bream and pike; small mammals like water voles, and native crayfish.[5] A gap in the limestone scarp (see Lincolnshire Wolds) near Ancaster may represent an earlier course of the River Trent towards Boston, but is now occupied by the River Slea.[8]
In Lincoln, the river flows into Brayford Pool and exits along a narrow channel that passes under the medieval High Bridge.[9] The bridge not only restricts navigation due to its small size, but the volume of water that can pass through the gap is limited in times of flood. This is alleviated by the Sincil Dyke, which leaves the main channel at Bargate Weir and runs for 1.5 miles (2.4 km) through the industrial areas to the south of the main city centre. It used to rejoin the main channel at Stamp End, but was re-routed into the South Delph, a drainage ditch constructed by John Rennie in the early 19th century that joins the main channel below Bardney lock. The origins of the Sincil Dyke are unknown, but it is known to have been used as a drainage channel in the mid-13th century and is thought to be pre-medieval or even Roman. Parts of it were culverted in 1847 to allow the construction of Lincoln Central railway station.[10]
From Lincoln, the river again turns first east, then south, making a cut through a belt of upland known as the Lincoln Gap. This section has also been suggested as a lower course of the Trent during and before periods of glaciation.[8]
From Dogdyke near Coningsby to Boston, the north bank of the river was used by a section of the Great Northern Railway from Lincoln to Boston. A long-distance footpath, the Water Rail Way, follows the course of the river from Lincoln to Boston. The path uses sections of the river towpath and abandoned railway tracks, and has been opened in stages, with the final 2 miles (3.2 km) being completed in September 2008. The path is now part of Route 1 of the National Cycle Network and features a number of sculptures along its length, each commissioned from local artists.[11] They include Lincoln longwool sheep at Stixwould, Lincoln Red cows at Washingborough, and Lincoln curly pigs, which became extinct in 1972, at Southrey.