EVOLUTION OF SALMON FALLS CREEK CANYON AND ITS TECTONIC IMPLICATIONS, SOUTHERN IDAHO AND NORTHERN NEVADA

Salmon Falls Creek (SFC) flows 135 mi from its source near the ID-NV border to the Snake River, with a 33 ft/mi gradient. The three major divisions of SFC are an upper section that flows south and middle and lower sections that flow north. The lower section extends 51 mi from Salmon Dam to the stream's mouth, through which the canyon is continuous, narrow and deep, has been cut in Pliocene basalt and underlying rhyolite, and has a 39 ft/mi gradient. The middle section extends 49 mi from the west side of Henry Valley to Salmon Dam. Here, the discontinuous canyon is wider than, but not as deep as, in the lower section. In this section's north part SFC cut a meandering canyon in the Greys Landing rhyolite (~7.6 Ma) and older rhyolite units in the Rogerson graben. Farther south SFC flows through alluvium-filled valleys and cuts a Jurassic granitic pluton. At 12 ft/mi the middle section gradient is less than in the lower and upper sections. The 35 mi long upper section extends from the source area in the uplands west of Rogerson graben to Henry Valley. In its upper section SFC, with a 54 ft/mi gradient, is mainly in a narrow canyon cut into various volcanic units, but locally it flows in narrow, fault-bounded valleys.

Several lines of evidence suggest both the middle and upper SFC sections initially flowed south away from the central Snake River Plain (SRP) topographic bulge, joined in Henry Valley and flowed on south into the Great Basin. After subsidence of the Yellowstone-like central SRP topographic bulge, basalt erupted in the region where the lower canyon section was later cut. Here, basalt from about 40 Pliocene shield volcanoes flowed north toward the middle of the SRP. This northward land tilt was established before ~6.5 Ma, the age of the uppermost basalt flow, from Hill 5384, at Salmon Dam. This tilt reversal resulted from subsidence of the central SRP bulge. The reversal of the SFC flow direction in the middle section likely occurred during the million years between the Greys Landing rhyolite and Hill 5384 basalt eruptions. Most of the canyon cutting in the lower SFC section likely occurred after about 3 Ma, as Lake Idaho's draining lowered the base level hundreds of feet.