river rising near Newbiggin, administrative county of Cumbria, historic county of Westmorland, England, and flowing 45 miles (72 km) westward and then southward to empty into the Irish Sea a few miles south of Heysham in Lancashire. The river drains part of the northern Pennines, and its entry to the sea at Sunderland Point is marked by extensive sand flats at low tide. Lancaster—an important crossing point—is the major town.