a large river of British North America, N.W. territory, rises in the Great Slave Lake, flows in a N.W. direction, and falls into the Arctic Ocean by numerous mouths, after a course of about 900 miles. It is generally about 3 miles in width, and the delta at its mouth, formed by flat alluvial islands, is 90 miles in length, by from 15 to 40 in width. The principal affluents are—the Mountain and Peel Rivers, and the Great Bear Lake River, which brings down the surplus waters of that lake. Mackenzie River is named after Alexander Mackenzie, by whom it was discovered and navigated in 1789.