Flathead Lake is a remnant of Glacial Lake Missoula, which covered much of Western Montana until roughly 15,000 years ago. The Lake's major tributaries are the Flathead and Swan Rivers. There are numerous small streams that flow into the Lake, particularly on the wetter East Shore. Maximum river flow in the Flathead generally occurs between May 15 and June 15 during peak snowmelt.

Flathead Lake has a maximum depth of 113 m (370.7 ft), and an average depth of 50.2 m (164.7 ft). The lake has an irregularly shaped shoreline and a dozen small islands, the largest of which is Wild Horse Island at 876 hectares (2,164 acres).

The lake is 48 km (30 miles) long, 26 km (16 miles) wide, covering 510 square kilometers (197 square miles). It is larger in surface area than Lake Tahoe, but it is much smaller in volume due to Tahoe’s depth.

Flathead Lake is one of the cleanest lakes in the populated world for its size and type.

More Info: flbs.umt.edu