Sediment discharge and channel change in the North Fork Teton River, 1977-78, Fremont and Madison counties, Idaho

The Teton Dam failure flood of June 5, 1976, severely disrupted the geomorphic character of North Fork Teton River in Idaho. Extensive channel restoration was required to contain expected normal spring flows. Six principal sites were established on the 17-mile reach of the river to study sediment transport and channel change during 1977-78. During April 1 to September 30, 1977, total water discharge at Teton Island bridge was 97,530 acre-feet; 4,360 tons of total sediment were transported. Total water discharge, April 1 to September 30, 1978, was 191,940 acre-feet; 10,680 tons of total sediment were transported. Analyses of data indicated several trends of erosion and deposition. Minimal channel change in the upper 7 miles of the river indicated equilibrium may temporarily exist between hydraulic-flow properties and channel shape. Streambed profiles indicated little change in streambed elevations. Erosional tonnage at mid-study reaches was 4,260 tons. One-half mile downstream, an increase of 4,150 tons of suspended and 1,050 tons of bedload sediment probably was partly derived from upstream bank erosion. An estimated 5,870 tons was deposited within the next subreach downstream. Virtually the entire bedload was redeposited before the last subreach, 4.4 miles downstream measured bedload was 91 tons. Suspended-sediment discharge transported past the last site was 16,470 tons. Lateral erosion and deposition in the lower 10 miles of the river indicate that subreaches now shortened by manmade channel alinements may begin to meander. Future deposition of coarse material at upstream gravel and concrete impoundments may trigger instability in the entire river. (Kosco-USGS)