United States to the
south; and of the southern parts of the Rocky Mountains and the western
prairie provinces drained by the great Saskatchewan river. The parallel
of 49 deg. approximately separates the Saskatchewan waters from the
streams going south to the Missouri, though a few small tributaries of
the latter river begin on Canadian territory.

The northern part of Alberta and Saskatchewan and much of northern
British Columbia are drained through the Athabasca and Peace rivers,
first north-eastwards towards Athabasca Lake, then north through Slave
river to Great Slave Lake, and finally north-west through Mackenzie
river to the Arctic Ocean. If measured to the head of Peace river the
Mackenzie has a length of more than 2000 m., and it provides more than
1000 m. of navigation for stern-wheel steamers. Unfortunately, like
other northward-flowing rivers, it does not lead down to a frequented
sea, and so bears little traffic except for the northern fur-trading
posts. The Mackenzie forms a large but little-known delta in lat. 69
deg., and in its flood season the head-waters pour down their torrents
before the thick ice of the lower part with its severer climate has yet
given way, piling up the ice in great barriers and giving rise to
widespread floods along the lower reaches. Similar flooding takes place
in several other important northward-flowing rivers in Canada, the St
Lawrence at Montreal affording the best-known instance. Second among the
great north-western rivers is the Yukon, which begins its course about
18 m. from tide-water on an arm of the Pacific, 2800 ft. above the sea
and just within the Canadian border. It flows first to the north, then
to the north-west, passing out of the Yukon territory into Alaska, and
then south-west, ending in Bering Sea, the northward projection of the
Pacific, 2000 m. from its head-waters. Of its course 1800 m. are
continuously navigable for suitable steamers, so that most of the
traffic connected with the rich Klondike gold-fields passes over its
waters. The rest of the rivers flowing into the Pacific pass through
British Columbia and are much shorter, though the two southern ones
carry a great volume of water owing to the heavy precipitation of snow
and rain in the Cordilleran region. The Columbia is the largest, but
after flowing north-west and then south for about 400 m., it passes into
the United States. With its expansions, the narrow and deep Arrow lakes,
it is an importan