The River Witham is a river almost entirely in the county of Lincolnshire in the east of England. It rises south of Grantham close to South Witham at SK8818, passes through the centre of Grantham, passes Lincoln at SK9771 and at Boston, TF3244, flows into The Haven, a tidal arm of The Wash, near RSPB Frampton Marsh. The name "Witham" seems to be extremely old and of unknown origin. Archaeological and documentary evidence shows the importance of the Witham as a navigable river from the Iron Age onwards. From Roman times it was navigable to Lincoln, from where the Fossdyke was constructed to link it to the River Trent. The mouth of the river moved in 1014 following severe flooding, and Boston became important as a port.

The Trent is the third longest river in the United Kingdom. Its source is in Staffordshire, on the southern edge of Biddulph Moor. It flows through and drains the North Midlands into the Humber Estuary. The river is known for dramatic flooding after storms and spring snowmelt, which in the past often caused the river to change course.