The lakes within the Crown of the Continent tend to be cold, clear, and deep. Of the over 700 lakes within Glacier National park, 131 are named. The largest is Lake McDonald at nearly ten miles long and 464 feet deep. Upper Waterton Lake in Waterton Lakes National Park in Alberta is the deepest lake in the Rocky Mountains at 482 feet.
Geologic activity played a critical role in the development of these lakes. Glacial moraines dammed the outlets of long glacial valleys, forming most of the lower elevation lakes. The higher elevation lakes, known as “tarns,” are formed as water fills ice-scoured cirques.
The cold temperatures of the tarns tend to limit the growth of plankton and other microorganisms, which in turn limits the abundance of fish and other organisms. Despite the lack of fish, tarns still contain quite complex ecosystems made up of smaller plants and animals. The clear waters reflect blue and green wavelengths of light, resulting in the breathtaking clarity or striking bluish-green appearance of the water.