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 12-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.