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 stretches for  long and has two main tributaries, the North Fork and East Fork. It originates in the Pioneer Mountains of the Salmon-Challis National Forest, flowing northeast before making a turn southeast, with the Lost River Range separating its valley from that of the Little Lost River.