| Big Lost River | |
| Map: | Lostidahomap.png |
| Subdivision Type1: | Country |
| Subdivision Name1: | United States |
| Subdivision Type2: | State |
| Subdivision Name2: | Idaho |
| Subdivision Type5: | Cities |
| Subdivision Name5: | Mackay, Arco, Atomic City |
| Length: | 135miles |
| Discharge1 Location: | near Arco[1] |
| Discharge1 Min: | 0cuft/s |
| Discharge1 Avg: | 86.3cuft/s |
| Discharge1 Max: | 2500cuft/s |
| Source1: | Confluence of North Fork and East Fork Big Lost River |
| Source1 Location: | Custer County, Idaho |
| Source1 Coordinates: | 43.9336°N -114.1106°W[2] |
| Source1 Elevation: | 6860feet |
| Mouth: | Big Lost River Sinks |
| Mouth Location: | Butte County, Idaho |
| Mouth Coordinates: | 43.7942°N -112.8406°W |
| Mouth Elevation: | 4790feet |
| Basin Size: | 1867sqmi[3] |
| Tributaries Left: | North Fork Big Lost River, Thousand Springs Creek |
| Tributaries Right: | East Fork Big Lost River, Antelope Creek |
The Big Lost River is a major river in the U.S. state of Idaho, about long.[4]
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.[3] 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.