MASS MOVEMENT EFFECTS ON THE MIDDLE FORK OF THE SALMON RIVER, IDAHO

The Middle Fork Salmon is a well-known whitewater river of the central Idaho mountains. Its canyon is cut mostly in resistant plutonic and metamorphic rocks. Relief of 800-1500 m is common on sideslopes and in small, steep tributary basins. Vegetation ranges from dense high-elevation conifer forest to sparse semiarid grasslands. A variety of mass movement processes have impacted the Middle Fork in latest Pleistocene to present time, including a large rock avalanche that dammed the river at Cutthroat Cove. Associated lake sediments have a ¹⁴C age of 14.5 cal ka and show that the landslide dam remained stable long enough for a delta to prograde well into the lake. The river has now downcut ~50 m below the rock avalanche deposit crest, and there is only a slight convexity in the current river profile in the dam area. Younger lake sediments indicate that another rockfall dammed the river at 1.8 cal ka, where the large boulders of Weber Rapids (mile 82.6) exist as a remnant.

The majority of other rapids were formed by large debris flows, many from high-intensity convective storms on steep erodible slopes and (or) recently burned areas. Sulphur Slide in 1936 and Cannon Creek (tree-ring dated to ca. this time) were not associated with fires, but historic accounts implicate a very heavy thunderstorm. Both still form pools 100s of meters long in the river above. A series of debris flows since 1997 has created or strongly altered at least 8 rapids on the Middle Fork and Main Salmon River just below their confluence. The Lake Creek, Bernard Creek, and Cramer Creek flows were associated with areas burned in 2000 and 2003, whereas the 1997 Pole Creek-Haystack Rapid debris flow was generated from unburned but sparsely vegetated and grussy slopes. Debris-flow episodes in the 1930s and the last decade are both associated with unusually warm summers, prompting the speculation that convective storm intensity increases markedly with rising summer temperatures in this area. Finally, Earthquake Rock rapid was created by rockfall during the 1983 Borah Peak quake. Viewed on 10⁶ yr timescales, these various mass movements stem fundamentally from canyon incision at an estimated 0.1-0.2 m/kyr and associated relief generation, which in this area has no clear tectonic driver. Kilometer-scale river profile convexities may relate to reaches with frequent long-term mass-movement input (e.g., Powerhouse Rapids).