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 , passes
Lincoln at and at
Boston, , 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 navigation 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.