Wanderlust World Wanderlust World The width of the River Thames varies significantly along its 215-mile course. At its narrowest, near Mapledurham Lock in Oxfordshire, it measures about 200 feet (61 meters). At its widest, near the Isle of Grain in Essex where it meets the North Sea, it spans approximately 1 mile (1.6 kilometers). On average, the river is about half a mile (0.8 kilometers) wide.