The Big Lost River is about  long and has two main tributary forks, the North Fork Big Lost River and the East Fork Big Lost River. The river originates in the Pioneer Mountains in Salmon-Challis National Forest, flows northeast then turns southeast, with the Lost River Range to the east separating the Big Lost River Valley from the Little Lost River Valley.
The Big Lost River is roughly  long and has two main tributaries, the North Fork and East Fork. It begins in the Pioneer Mountains within the Salmon-Challis National Forest, flowing northeast before turning southeast, with the Lost River Range dividing its valley from that of the Little Lost River Valley.