THE EFFECT OF TETON DAM AND ITS FAILURE ON THE GEOMORPHOLOGY OF THE TETON RIVER: 25 YEARS LATER

Dramatic geomorphic changes are apparent along the Teton River as a result of the Teton Dam failure in 1976. In addition to the obvious modifications to the channel downstream of the dam the most enduring change is the numerous landslides that failed into the canyon upstream of the damsite. Landslides began failing upon initial filling of the reservoir and culminated with the rapid draw down associated with the dam failure. An inventory of the more than 200 landslides made immediately following the dam failure provides an important baseline for the location, extent, and approximate volume of material deposited into the river canyon from the adjacent slopes. Comparison of recent aerial photography (2001) to photography taken prior to the dam construction (1972) and immediately following the dam failure (1976) provides a clear depiction of geomorphic changes imposed on the river by the dam and reservoir, a measure of the extent of geomorphic recovery, and a means for estimating the rate of recovery and possible magnitude of future changes.

Landslides and rockfalls have been the dominant geomorphic agent responsible for forming the Teton River Canyon during the last several hundred thousand years. However, in a relative instant, up to 3.6 million cubic yards of landslide debris buried the river channel, flood plain, and terraces in the reservoir basin, locally blocked or deflected the course of the river, and formed 27 new rapids. In general, this movement of sediment from the canyon walls to the canyon floor increased the water surface profile and decreased stream flow velocities through the canyon. It was determined that the largest landslides originated exclusively from the north-facing slopes, which were blanketed by thick fine-grained colluvium, and that the character of the deposits controlled the overall size and extent of the landslides. During the last 25 years, most of the landslides that once impinged on the river channel have been significantly modified.