FAQ: How Was Flathead Lake Formed?

Did you know Flathead Lake is one of the largest natural freshwater lakes west of the Mississippi River? Flathead Lake was formed by powerful geological events that resulted in the clean water and thriving ecosystems that make the lake treasured today. Understanding how Flathead Lake was formed fosters a deeper appreciation for its extraordinary beauty.

How old is Flathead Lake?

Flathead Lake is estimated to be around 10,000 to 12,000 years old. It formed during the late Pleistocene epoch as glaciers retreated at the end of the last Ice Age.

How was Flathead Lake formed?

As the massive Flathead Lobe glacier moved through the area, it carved out the lake’s basin in the bedrock. Then, when the glacier melted, it dumped rocks and sediment, called a glacial moraine, which further shaped the landscape by creating natural dams that defined Flathead Lake’s shape and size. The basin eventually filled with water from the retreating glacier’s ice melt and runoff from nearby mountains and streams. 

How has the lake changed since it was formed? 

The Séliš Ksanka Qlispe̓ Dam, previously known as the Kerr Dam, has stabilized Flathead’s water levels which fluctuate naturally. The dam, as well as human development, climate change, and the introduction of non-native species have impacted the lake’s ecosystems. 

What makes Flathead Lake’s water so clear?

Flathead Lake is one of the cleanest lakes in the world. Its water has relatively low levels of phosphorus and nitrogen, which contribute to algae growth. Extensive conservation efforts have been undertaken in the region to prevent anthropogenetic changes. The Flathead Lake Biological Station closely monitors Flathead Lakes’ water quality, and regulations regarding land use, waste and stormwater management, boat decontamination, and wetland habitat protection have been successful in maintaining the ecological integrity of the lake and its watershed.

What evidence of glacial activity can be seen around Flathead Lake today?

Observant visitors to Flathead can notice geological signs of the glaciation that created the lake, rivers, and valleys. An easily identifiable feature is the moraines that form the lake’s borders, like the Polson Terminal Moraine. Also, the valleys around Flathead are U-shaped, which is a clear indication of glacial activity. Additionally, parallel striations can be found in rocks around the area that are evidence of glaciers scraping the bedrock with stones as they moved. Scattered around the valleys are large boulders, called glacial erratics, which were deposited by retreating glaciers, causing them to appear out of place.

The best way to admire all the fascinating attributes of Flathead Lake is to get out on its calm, clean water in a comfortable sailboat. Go Sail Flathead Lake offers sailing tours led by a knowledgeable captain who is passionate about the area, as well as bareboat charters for those who prefer to sail on their own. If you are interested in learning how to sail, we are the only ASA-certified sailing school in Montana, so check out our options for sailing courses.