Of the two great affluents of the Yukon, the Tanana is for the most part unnavigable, while the Koyukuk is navigable for more than 450 m.
In the farther interior, in the valleys of the Yukon, the Tanana, the Copper and the Sushitna the summers are much the same in character, the winters much more severe.
The gravest problem of mining in the interior country, even graver than that presented by the climate, is transportation; in 1900 the Tanana fields, for example, were provisioned from Circle City, about 125 m.