the witham's course is one of the strangest of british rivers, the result of glaciation (and possibly isostatic rebound) redirecting older rivers. the source of the river is on high ground near south witham, lincolnshire, from whence it flows generally north, very close to and almost parallel with the trent around the outskirts of newark, before turning east towards lincoln. the upper waters are important for agricultural water extraction, and for coarse fish such as roach, common bream and pike; small mammals like water voles, and native crayfish. a gap in the limestone scarp (see lincolnshire wolds) near ancaster may represent an earlier course of the river trent towards boston, but is now occupied by the river slea.