LATERAL AND VERTICAL CHANNEL MOVEMENT AND BED-MATERIAL ENTRAINMENT POTENTIAL ON THE MADISON RIVER DOWNSTREAM FROM EARTHQUAKE LAKE, MONTANA

The 1959 Hebgen Lake earthquake caused a massive landslide that dammed the Madison River and formed Earthquake Lake. The U.S. Army Corps of Engineers excavated a spillway through the slide to permit outflow from Earthquake Lake. In June 1970, flows higher than 5,170 cubic feet per second severely eroded the channel through the slide and washed out roadway embankments downstream. Flooding and erosion also occurred in 1971 and 1986 (flows greater than 3,250 and 5,000 cubic feet per second, respectively). Subsequent studies concluded that flows greater than 3,500 cubic feet per second could cause substantial erosion. Thus, since about 1975 releases from Hebgen Reservoir (just upstream from Earthquake Lake) have been managed to minimize the possibility of flows exceeding 3,500 cubic feet per second at the outlet of Earthquake Lake.

The Madison River is a blue-ribbon trout fishing stream, and the operation of Hegben Reservoir to keep flows below 3,500 cubic feet per second could increase the accumulation of fine sediments on salmonid spawning beds downstream from Earthquake Lake. Thus Montana Fish, Wildlife and Parks requested the U.S. Geological Survey to investigate the lateral and vertical movement and potential for bed-material movement of the Madison River downstream from Earthquake Lake, to help determine whether the 3,500 cubic feet per second threshold could be increased.

Comparison of channel geometry data obtained in 1971 and 2006, along with 14 sets of aerial photographs taken between 1970 and 2006, showed that up to 12 feet of channel incision, up to 5 feet of aggradation, and formation of large meander bends occurred in the channel through the slide. Incision and aggradation, as well as up to 500 feet of lateral movement, were observed downstream from the landslide. Bed-material sampling was only possible in two locations due to extremely high and fast river flows, but at those locations boundary shear stresses generated by the 3,500 cubic feet per second threshold fall within the ranges of critical shear stresses necessary to move bed material. These comparisons, based on limited data and conservative Shields parameter estimates, indicate that some channel materials could be moved at 3,500 cubic feet per second, but that the current threshold would not likely result in the large channel movements observed in the past.