Barlings Eau is a small river near Barlings, Lincolnshire, England. It is a tributary of the River Witham, joining it near Short Ferry. It acts as the central spine for a number of other small rivers, which drain the low-lying land to either side. Most of its course is within the area managed by the Witham Third District IDB, an Internal Drainage Board responsible for land drainage. One of their pumping stations is on the banks of the river.

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.