major tributary of the Fraser River, in southern British Columbia, Canada. The North Thompson (210 miles 【340 km】) rises in the Cariboo Mountains east of Wells Gray Provincial Park and follows an easterly then southwesterly course to Kamloops; the South Thompson (206 miles) emerges from Shuswap Lake and flows northwesterly to Kamloops (see photograph-->), where the two streams unite for about 100 miles (160 km) and merge with the Fraser at Lytton. The river was explored by Simon Fraser in 1808 and was named for David Thompson, an explorer and geographer erroneously credited with having reached its upper course.