INVERTED VALLEY DEVELOPMENT AND GEOMORPHIC RESPONSE OF THE HENRYS FORK OF THE SNAKE RIVER TO EPISODIC BASALT DAMMING, MESA FALLS RECREATION AREA, CARIBOU-TARGHEE NATIONAL FOREST, IDAHO

The Mesa Falls Recreation Area is located approximately 16 km northeast of Ashton, Idaho and consists of Pleistocene volcanic units, including the Mesa Falls and Lava Creek Tuffs of the Yellowstone Group and the Gerrit Basalt. Mesa Falls Tuff was deposited at 1.3 Ma and was overlain by the 600 ka Lava Creek Tuff. The Henrys Fork of the Snake River formed on top of these tuffs and was episodically dammed by Gerrit Basalt flows as it incised its valley, preventing the Henrys Fork from reaching equilibrium. Valley incision followed by basalt damming and further incision have created inverted valleys represented by basalt terraces that become progressively younger at lower elevations.

The widespread and thin nature of the uppermost basalt flow and the presence of scattered glacial tills suggest that early in its history, the Henrys Fork flowed over a broad, glacial outwash plain. The sources for individual flows are uncertain; however, a thick succession of basalt on the eastern side of the Henrys Fork indicates that one flow might have traveled by way of a paleo-tributary, damming the river at that location. Active incision of the lowermost flow is presently taking place on the canyon floor.

Mapping and correlation of remnant basalt terraces on the Snake River Butte and Lookout Butte quadrangles help to better understand the geomorphic response of river systems to concurrent volcanic activity.