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The Mountain Rapids of the Slave River

The Mountain Rapids are one of a group of four rapids, collectively called the Slave River Rapids. This spectacular section of the Slave River is one of the finest examples of rapids in North America and has been designated as a National Historic Site because of its significance as a transportation barrier that affected the development of Canada's north. Today, these violent and dangerous rapids have become a tourist destination for expert kayakers and river rafters.

For most of its course, the Slave River follows the boundary between the igneous and metamorphic rocks of the Cambrian Shield to the east and the sedimentary rocks of the Interior Plains to the west. The Slave River discharges over 80 per cent of all of Alberta's combined river flow. June's average monthly discharge at Fitzgerald is a remarkable 6000 cubic metres per second, which is about 11.5 times the flow of the North Saskatchewan River at Edmonton. Between Fitzgerald and Fort Smith, a distance of 27 kilometres, the Slave River drops 33.2 metres over four sets of rapids: the Cassette, Pelican, and Mountain rapids, and the Rapids of the Drowned. All four of these rapids are formed by granite ridges of the Canadian Shield which cross the river channel in a northeast/southwest direction. These ridges are the only easily accessible outcrop of Canadian Shield west of Slave River.

The Mountain Rapids are immediately east of a long granite peninsula which cuts the river channel in half. Before the construction of the portage road on the west side of the river, the other three sets of rapids were bypassed on the east using narrow side-channels. The Mountain Rapids, however, could not be passed on the east, so the Mountain Portage was established at the narrowest neck of the peninsula. Scows were winched to the summit and allowed to slide down the other side until they reached the river.

Several trails parallel the powerline and lead to the end of the peninsula. The roar of turbulent water can be heard long before you reach the viewpoint that overlooks the rapids. The pink granite islands in the channel are an extension of the peninsula which has been breached in several areas by the river. The islands just below the first rapids are the world's most northerly nesting area of American white Pelicans.

Most of the granite exposed at the rapids has been smoothed, polished, and striated, not by the river but by a southward advancing continental glacier over 10,000 years ago. These elongated rounded glacial landforms are called "whalebacks" and their shape indicates the direction of ice flow. The low, rounded ends point in the direction from which the ice came, while the down-ice end is rough and angular where the glacier plucked off blocks of rock as it passed over.

Reprinted from A Traveller's Guide to Geological Wonders in Alberta by Ron Mussieux and Marilyn Nelson with permission of the authors and the Provincial Museum of Alberta.


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