Pair 1 is not correctly matched: Originating in the icy headwaters of the Tibetan highlands, the Mekong River flows through the steep canyons of China, known as the upper basin, through lower basin countries Myanmar, Laos, Thailand, and Cambodia, before fanning across an expansive delta in Vietnam and emptying into the South China Sea. Pair 2 is not correctly matched: The River Thames is the longest river in England, flowing 215 miles from the Cotswolds to the North Sea.