The Bear River is the longest river in North America not draining into an ocean. It circles through three states, Wyoming, Utah and Idaho, but
starts and ends in Utah (See map of Bear River/ Bear Lake at right). The past movements of the Bear River are important to our understanding of Bear
Lake, in part because the volume of water in the river can rapidly change lake levels, and because the river water chemistry is very different
than the lake chemistry. Today, the Bear River enters the Bear Lake Valley north of Bear Lake. It flows northward, following the Bear Lake fault
along the eastern edge of the Bear Lake Valley and no longer flows directly into Bear Lake, except through a man-made channel created in AD 1912.
But it used to....About 140,000 years ago, lava flows from near Soda Springs, Idaho blocked the Bear River channel and diverted the river into the Great Basin (see the Bear River Images below). Erosion of the lava dam probably allowed the Bear River to drain oceanward again but another lava flow about 80,000 years ago returned the river to the Great Basin where it remains today (Bouchard et al, 1998). Some time after that, a large lake formed in Bear Lake Valley, possible due to landslides damming the north end of the valley near Georgetown, Idaho. The Bear River then flowed into the larger lake, changing the chemistry of the water and creating beaches and other shoreline features 25-200 ft above the level of today's lake.
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Image reconstructions by Darrell Kaufman
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