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Teton River near Driggs, Idaho

Teton Dam Site

Site of the former Teton Dam

Teton Dam near Rexburg, Idaho suddenly failed on first filling of the reservoir in 1976. Although several studies have been done on the catastrophic impacts to people and property downstream from the dam, the impacts of the dam failure on the 17-mile reach of the former reservoir area were never formally evaluated. To determine what impacts did occur in this upstream canyon reach (from the filling of Teton Reservoir and subsequent failure of Teton Dam), a geomorphology and river hydraulics study was completed by Reclamation during 1997-2000, more than 20 years since the dam failure (Randle and others, 2000; Bountry and Randle, 2001). When Teton Dam failed, the reservoir was 270 feet deep (at the dam) and drained in less than 6 hours. The filling and the subsequent rapid draining of the reservoir triggered more than 200 landslides in the river canyon that was inundated by the former reservoir. Debris material from these landslides covered river terraces and buried the river channel in several locations. In addition, many existing riffles were enlarged by landslide debris and, in some locations, new rapids were formed. A complex of 27 rapids now exists in the river canyon creating slow-velocity pools upstream of each rapid. In 1997, a 100-year flood resulted in only minor reworking of the landslide debris that had deposited in the river channel. The study concluded that it would take centuries of natural weathering to erode the large boulders that comprise the existing rapids. The annual mean flow of the Teton River is 843 ft3/s. With a total drainage area of 890 mi2, the mean annual runoff is 13 inches. Except for the dam failure, the highest peak flow recorded was 11,000 ft3/s on February 12, 1962 and the lowest mean-daily flow recorded was 199 ft3/s.

Drawdown Landslide

Drawdown after failure accelerated landslides

Predam aerial photographs and contour maps of the river canyon were compared with recent aerial photographs and a new survey of the existing river channel. This comparison was made to document the impacts of reservoir filling and dam failure and to put these impacts in the context of natural processes. The logistics of a bathymetric survey in this remote river canyon were very difficult, but the data were essential to the study. Historical photographs and analysis of soil samples showed that landslides have been occurring in the canyon for thousands of years, but that this natural process was greatly accelerated by the failure of Teton Dam. Several deep pools and riffles were present at the time of the dam failure, but have been enlarged due to the deposit of additional landslide debris. In two separate locations, the landslide debris has created new rapids with more than 16 feet of drop in the water surface. These rapid complexes have significantly raised the water surface of the river relative to predam conditions.

Hydrographic Survey

Hydrographic Survey on the Teton River

Bed-material data from the river channel was used to evaluate the impacts of landslides on sediment transport. It was determined that sand and gravel-sized sediments are being trapped in the four upstream most pools of the former reservoir. Hydraulic modeling was performed to predict water depth, velocity, and the travel time of water for a given discharge. Water-temperature data were collected concurrently throughout the river canyon to determine if summer flows were being warmed as they passed through downstream river pools. The temperature data indicated that river flows were being warmed by 1 to 2 °F. The hydraulic modeling results correlated this warming with an increase in the travel time of water through the deeper and slower river pools. Bathymetric surveys of the channel bottom, comparison of current and historic aerial photographs and maps, and the modeling of river hydraulics were very useful for documenting the impacts of dam construction and failure.

References

Bountry, J.A.; Randle, T.J. (March 2001), "Upstream Impacts After the 1976 Failure of Teton Dam," in Proceedings of the Seventh Federal Interagency Sedimentation Conference, in Reno, Nevada.

Randle, T.J.; Bountry, J.A.; Klinger, R.; and Lockhart, A., May 2000, Geomorphology and River Hydraulics of the Teton River Upstream of Teton Dam, Teton River, Idaho, U.S. Department of the Interior, Bureau of Reclamation, Denver, Colorado, 50 pages.

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