page_title: Flathead Lake
section_title: Geology
hierarchical_section_title: Flathead Lake / Geology
caption_attribution_description: Clouds above Flathead lake in Polson Montana
context_page_description: Flathead Lake is a large natural lake in northwest Montana, and is the largest natural freshwater lake by surface area that is west of the source of the Missouri River in the contiguous United States.
  The lake is a remnant of the ancient, massive glacial dammed lake, Lake Missoula of the era of the last interglacial. Flathead Lake is a natural lake along the mainline of the Flathead River. It was dammed in 1930 by Kerr Dam at its outlet on Polson Bay, and the lake level was raised by 10 feet. It is one of the cleanest lakes in the populated world for its size and type.
context_section_description: Flathead Lake lies at the southern end of a geological feature called the Rocky Mountain Trench. The trench, which formed with the Rocky Mountains, extends north into the southern Yukon as a straight, steep valley, which also holds the headwaters of the Columbia River. During the last ice age this trench was filled by an enormous glacier. As the glacier moved southward it carved out the trench. The Polson Moraine, near present-day Polson, Montana, marks the southernmost extent of the glacier during the last ice age and thus is the site of the glacier's terminal moraine.
  The large size of the Polson Moraine indicates that the glacier stalled here for many years before retreating. As the climate warmed, a portion of the glacier in the Mission Valley receded more slowly than the main body, which kept the lake basin from being filled with sediment. Eventually this ice also melted, forming a lake behind the moraine. Once the water reached the top of this moraine dam, it began to cut a channel through it. Most moraine dammed lakes drain quickly because water cuts entirely through the moraine. However, Flathead Lake remains because a bedrock hill buried underneath the Polson Moraine prevented the moraine from being completely cut through so the meltwater never completely drained.
  At one time, probably when the valley was partially filled by a glacier, the level of Flathead Lake was about 500 feet (150 m) higher and drained through the valley west of Elmo, Montana, which is at the end of Big Arm Bay, bottom center in the aerial photo above. Water carved out a wide, flat-bottomed pass with a deeper, narrow channel at the south edge of the pass. The deeper channel and traces of the dry riverbed are still visible from Route 28.
caption: Some clouds over Flathead Lake in Polson, Montana, as the sun is setting
exif: {"Image Make": "Hewlett-Packard", "Image Model": "hp photosmart 733", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Ver 1.09Lite", "Image YCbCrPositioning": "Co-sited", "Image ExifOffset": "338", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail Orientation": "Horizontal (normal)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "1780", "Thumbnail JPEGInterchangeFormatLength": "13570", "EXIF ExposureTime": "63/50000", "EXIF FNumber": "123/25", "EXIF ExposureProgram": "Program Normal", "EXIF ISOSpeedRatings": "100", "EXIF ExifVersion": "0220", "EXIF DateTimeOriginal": "2004:08:29 19:03:33", "EXIF DateTimeDigitized": "2004:08:29 19:03:33", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "307/100", "EXIF ShutterSpeedValue": "963/100", "EXIF ApertureValue": "459/100", "EXIF BrightnessValue": "461/50", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "159/25", "EXIF SubjectDistance": "9999/100", "EXIF MeteringMode": "CenterWeightedAverage", "EXIF LightSource": "D55", "EXIF Flash": "Flash did not fire, auto mode", "EXIF FocalLength": "119/20", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "2048", "EXIF ExifImageLength": "1536", "Interoperability InteroperabilityIndex": "R98", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "EXIF InteroperabilityOffset": "1750", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF DigitalZoomRatio": "0", "EXIF SceneCaptureType": "Standard", "EXIF Saturation": "Normal", "EXIF Sharpness": "Normal", "EXIF SubjectDistanceRange": "0"}