River Witham, main river of Lincolnshire, England, with a total length of about 80 miles (130 km). It flows from the northeastern Midlands, first northward past Grantham to Lincoln, where it cuts through the Lincoln Edge (a limestone ridge) in a steep-sided gap, and then eastward and later southeastward across the drained Fens to enter The Wash, a shallow embayment of the North Sea, at Boston. It is a sluggish stream, now embanked throughout its course below Lincoln, and locally straightened for drainage in its upper course.