| Boulder River | |
| Pushpin Map: | Montana |
| Pushpin Map Caption: | Location of the mouth of the southwestern Boulder River in Montana |
| Subdivision Type1: | Country |
| Subdivision Name1: | United States |
| Subdivision Type2: | State |
| Subdivision Name2: | Montana |
| Subdivision Type4: | County |
| Subdivision Name4: | Jefferson |
| Length: | 77.3miles[1] |
| Discharge1 Location: | Boulder |
| Discharge1 Avg: | 119 cu/ft. per sec.[2] |
| Source1: | Continental Divide |
| Source1 Location: | Boulder Mountains, Beaverhead-Deerlodge National Forest |
| Source1 Coordinates: | 46.1894°N -112.5317°W |
| Source1 Elevation: | 7339feet[3] |
| Mouth: | Jefferson River |
| Mouth Location: | Cardwell |
| Mouth Coordinates: | 45.8669°N -111.9456°W[4] |
| Mouth Elevation: | 4278feet |
The Boulder River is a 77miles tributary of the Jefferson River in southwestern Montana in the United States.[1]
It rises in the Rocky Mountains at the continental divide in the Beaverhead-Deerlodge National Forest in western Jefferson County. It flows east and southeast through the mountains past Boulder, then south to join the Jefferson near Cardwell.[5]
Game fish in the river include brook, brown, and rainbow trout, and mountain whitefish.[6] Brown trout are most prevalent in the last 2miles, near the mouth, and the other three species are more prevalent in the reach upstream of the town of Boulder.[6] The lowermost 12miles of the river is affected by irrigation withdrawals, and the reach below the community of Basin is affected by seepage from old mines and tailings.[6]