Built between 1963 and 1967, this enormous earth-filled dam wedges the upper end of the canyon, creating a reservoir, Williston Lake. Measuring 362km/225mi in length, the lake encompasses part of the valleys of the Parsnip and Finlay rivers, which meet in the Rocky Mountain trench. The project is felt to be an ecological failure by some as some of the springs, which formerly replenished the marshland around the Peace-Athabasca delta in springtime, have disappeared.