Bear River has its source in lakes on the north slope of Hayden Peak, 12,485 feet in elevation, near the western limit of Utah's high Uintas. The horseshoe shaped river follows a course 500 miles long, but its mouth is only 90 miles from its source.
Within the last 500,000 years, during middle Pliestocene time, many small basalt volcanoes and cinder cones such as Ice Cave Knoll near Niter Ice Cave, erupted to form the Gem Valley Lava Field. These same flows diverted the Bear River from its previous northward drainage into the Snake River system and instead into the Great Basin and. thus, the Great Salt Lake. The Bear River cut the narrow, straight-sided canyon across young lava flows. South of here, the Bear River enters Oneida Narrows, a bedrock canyon cut through ancient quartzite of the Precambrian Brigham Group. This major canyon may have been first 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 are about 5,480 feet in elevation, 330 feet above the Lake Bonneville shoreline, which is the level of the mouth of Black Canyon. The Bear River first flowed into Lake Thatcher 140,000 years ago, and again 50,000 years ago, preceding the overflow of Lake Thatcher into Lake Bonneville to the south. The northernmost arm of Lake Bonneville
reached into flat, irrigated Gentile Valley south of here. Black Canyon is cut down to the base level of Lake Bonneville shoreline. Bear River cut deeper into the basalt lava to meet the lowering lake levels.
Before the river was altered for power or irrigation purposes, it had significant flow. An early photograph of the river near Grace clearly shows water depth within a few feet of the canyon rim. The photo caption illustrates the river's size. "Those who lived here will remember the roar of the river could be heard anywhere in the valley, night or day."