Witham Third District IDB is an English internal drainage board set up under the terms of the Land Drainage Act 1930. The Board inherited the responsibilities of the Witham General Drainage Commissioners, who were first constituted by an Act of Parliament of 1762. They manage the land drainage of an area to the north and east of the River Witham, between Lincoln and Dogdyke, which includes the valley of the River Bain to above Hemingby, and the valleys of Barlings Eau and most of its tributaries, to the north east of Lincoln.