Lakes, Rivers, and Reservoirs of Montana

Montana’s "Big Sky" country holds abundant freshwater resources across its mountain and prairie regions.

Natural Lakes

• Flathead Lake – Largest natural freshwater lake west of the Mississippi (197 sq mi).
• Lake McDonald – Glacier National Park’s largest, iconic turquoise waters.
• Lower Red Rock Lake – Remote, in Beaverhead‑Deerlodge NF.
• Lake Helena – Urban reservoir near capital.

Major Rivers

• Missouri River – Longest in North America; ~500 mi in MT.
• Yellowstone River – Longest undammed river in lower 48; ~600 mi in MT.
• Clark Fork River – ~300 mi, Clark Fork Basin.
• Flathead River – ~200 mi, Flathead Lake outlet.
• Kootenai River – ~100 mi in NW MT.

Key Reservoirs (Missouri River System)

• Fort Peck Lake (largest in MT, 5th largest US).
• Canyon Ferry Lake.
• Hauser Lake.
• Holter Lake.
• Lake Koocanusa (Libby Dam, Kootenai River, shared with BC).

Recreation & Ecology

These waters support trout fisheries, irrigation, hydropower, and world‑class boating/fishing tourism.