• River: Flathead River
• Length: 161 Miles
• Surface Area: 122,560 Acres
• Volume: 1,200,000 Acre Feet
• Drainage Area: 7,086 Square Miles
• Average Depth: 165 Feet
Flathead Lake is the largest natural freshwater lake west of the Mississippi River in the contiguous United States, taking Red Lake (Minnesota) and Lake of the Woods to be north of the Mississippi River, rather than west of the river. With a surface area of between 191.5 sq mi and 197 sq mi, Flathead Lake is slightly larger than Lake Tahoe. The lake is a remnant of the ancient, massive glacial dammed lake, Lake Missoula of the era of the last interglacial. Flathead Lake is 27.3 mi long and up to 15.5 mi wide. Flathead lake has a maximum depth of 370.7 ft, and an average of 164.7 ft . This makes Flathead lake deeper than the average depths of the Yellow Sea or the Persian Gulf. Polson Bay, at the lake's outlet was raised 10 ft by Kerr Dam. It is one of the cleanest in the populated world for its size and type.
Located in the northwest corner of the state of Montana, 7 miles south of Kalispell, it is approximately 30 miles long and 16 miles wide, covering 191.5 square miles, making it approximate in size to Minnesota's Mille Lacs Lake and half the area of San Francisco Bay (main bay). Flathead Lake is 30 miles southwest of Glacier National Park and is flanked by two scenic highways, which wind along its curving shoreline. On the west side is U.S. Route 93, and on the east, is Route 35.
The lake is bordered on its eastern shore by the Mission Mountains and on the west by the Salish Mountains. The Flathead valley was formed by the glacial damming of the Flathead River and sustains a remarkably mild climate for a region located this far north and inland; the Pacific Ocean is almost 400 miles to the west. The mild climate allows for cherry orchards on the east shore and vineyards for wine production on the west shore. There are also apple, pear, and plum orchards around the lake as well as vegetable, hay, honey, nursery tree, Christmas tree, sod/turf, and wheat production bordering or near the lake.