Big Lost River

The Big Lost River is a major river in the U.S. state of Idaho, about 135 mi long.

Description
The river starts in the Rocky Mountains and flows in a generally southeast direction into the Snake River Plain. True to its name, the Big Lost River's surface flow does not reach any larger river but vanishes into the Snake River Aquifer at the Big Lost River Sinks, giving the river its name. The river is one of the Lost streams of Idaho, several streams that flow into the plain and disappear into the ground.

It rises at the confluence of the North Fork and East Fork Big Lost River deep in the Pioneer Mountains, a subrange of the Rockies, in Custer County, south-central Idaho. It flows northeast, then turns sharply southeast at the confluence with Thousand Springs Creek, which comes in from the left and into Butte County. The river is dammed to form Mackay Reservoir near the town of Mackay, then continues south through an agricultural valley, passing Arco. After Arco, the river begins flowing east, northeast, and finally due north. The river terminates at the Big Lost River Sinks, a patch of marshland where its water drains into the ground.

Even though its surface flow is lost (hence its name) a short distance out of the mountains, the river is hydrologically connected to the Snake River, the largest river of Idaho by discharge, via the Snake River Aquifer and various springs along the course of the Snake in its journey through the plain.