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Then it is Rock River, of which the continuation is called Slave
River until it loses itself in Great Slave Lake. From its escape
from this lake to its mouth it is known as Mackenzie River.
The river is navigable, if not from its source, at least from Jas-
per House to its mouth, a distance of about 2,000 miles. In thi»
long line, navigation in boats of the country, is interrupted at only
two places : by the group of rapids in the rivière d la Biche, and
one in Slave River. The latter rapids, at about 1200 miles from
the Arctic Ocean, present the first obstacle to vessels going up
stream. Vessels of less draught could easily navigate from above
these rapids to the foot of river à la Biche rapids, but not at all
seasons of the year, as when the water is low there are numerous
sand banks in che way. From the latter rapids to Jasper House
the current is exceedingly strong, and the water generally shallow ;
so that here, navigation i3 very difficult and possible only in boat»
of the country when powerfully propelled.
The breadth of the river—at first only about a quarter of a mile—
gradually increases, but irregularly. In some places it is two
miles broad ; and, in short, as regards its length and its volume of
water, is one of the finest rivers in the world.
From its source to Lake Athabaka the water is muddy, being
strongly charged with clay and sand, that form shifting banks
difficult to become familiar with and to avoid.    The turbidness of
the water increases the inconvenience of these obstructions.
During July this river, like all others rising in the Rocky Moun-
tains, experiences a sudden increase, due to the melting of snow.
In its upper part particularly, it then becomes an impetuous tor
rent, and navigation is very difficult, and often dangerous. Thi»
happens frequently, when there is intense heat of several days*
duration in the snowy region. The phenomenon lasts generally
inversely as its intensity.
The delta of Athabaska River at its entrance into the lake of the
same name is remarkable, and all the more so as it is also acted
upon by a great stream, Peace River, whose mouth is quite close to
it. These two powerful streams carry with them, besides sand and
clay, a great quantity of débris, and this heaped against the south-
western shore of the lake, forms the tongue of land that separate»
the two great souroes of Mackenzie River. The tongue is not yet
completed.    Rivers d'Embarras and d'Upinettes, Lake Mamawee,