significant portion of the eastern slope of the Rocky Mountains and the central Alberta prairie. The Peace contributes the majority of the water, about 66 km3 (54 million acre-feet) per year, and the Athabasca contributes 25 km3 (20 million acre-feet). The Peace and Athabasca meet at the Peace-Athabasca Delta, a vast inland delta at the western end of Lake Athabasca, which also takes runoff from the northern third of Saskatchewan. The Slave River is formed by the confluence of the two rivers and flows due north into Great Slave Lake, at Fort Resolution, Northwest Territories. The Slave is by far the largest river flowing into the lake, with an annual flow of 108 km3 (87 million acre-feet). It contributes about 77% of the overall inflow, and forms a large delta where it enters the lake. Other rivers entering Great Slave Lake are the Taltson, Lockhart and Hay Rivers, the latter of which also extends into Alberta and BC. Main stem The Mackenzie River issues from the western end of Great Slave Lake about south-west of Yellowknife. The channel is initially several kilometres wide but narrows to about at Fort Providence, which was historically an important ferry crossing in the summer, and used as an ice bridge in the winter for traffic along the Yellowknife Highway. In 2012 the Deh Cho Bridge was completed at a point about upstream, providing a safer permanent crossing. It is the only bridge across the main stem of the Mackenzie. West of Fort Providence the Mackenzie widens considerably, resembling a shallow, swampy lake more than a river; one large widening here is known as Mills Lake. After heading west for about the Mackenzie narrows and turns northwest through a long stretch of fast water and rapids, past the village of Jean Marie River. At Fort Simpson it is joined by the Liard River, its biggest direct tributary, from the west. The Liard drains a large area in the southern Yukon and northern BC and carries a large amount of sediment during the summer melt – which does not fully mix with the clear water in the Mackenzie for almost downstream, with the resulting phenomenon of a clear current on the east bank and muddy water on the west bank. The river continues west-northwest until its confluence with the North Nahanni River, where it turns north towards the Arctic. It flows through open taiga with its