river in the historic county of Yorkshire in north-central England. It rises in the Pennines in the administrative county of North Yorkshire and then flows 60 miles (97 km) southeast to become an important tributary of the River Ouse (which drains into the Humber, an estuary of the North Sea) a few miles south of York. Parts of the river form the boundary between West Yorkshire and North Yorkshire. Its upper reaches, in the Pennines, lie in a steep, troughlike valley known as Wharfedale.