Bear River rises in lakes on the north slope of Hayden Peak, near the western limit of Utah's high Uintas, at 12,485 feet in elevation. Although the horseshoe-shaped river follows a course 500 miles long, its mouth lies only 90 miles from its source. Within the last 500,000 years, during middle Pliestocene time, small basalt volcanoes and cinder cones, including Ice Cave Knoll near Niter Ice Cave, erupted to form the Gem Valley Lava Field. These lava flows diverted Bear River from its earlier northward drainage into the Snake River system and sent it instead into the Great Basin and the Great Salt Lake. The river then cut the narrow, straight-sided Black Canyon Gorge across the young lava flows. South of this point, Bear River enters Oneida Narrows, a bedrock canyon cut through ancient quartzite of the Precambrian Brigham Group, and this major canyon may first have been cut by a stream flowing northward into Idaho. As long as 2 million years ago, lakes began forming in the Thatcher basin. The highest shorelines there are about 5,480 feet in elevation, 330 feet above the Lake Bonneville shoreline at the level of the mouth of Black Canyon. Bear River first flowed into Lake Thatcher 140,000 years ago, and again 50,000 years ago, before Lake Thatcher overflowed southward into Lake Bonneville. The northernmost arm of Lake Bonneville extended into flat, irrigated Gentile Valley south of here. Black Canyon was cut down to the base level of the Lake Bonneville shoreline, and Bear River cut deeper into the basalt lava as lake levels fell. Before the river was altered for power or irrigation, it carried a significant flow, and an early photograph near Grace shows water only a few feet below the canyon rim; people who lived here remembered that its roar could be heard anywhere in the valley, night or day.