ORIGIN OF BASALT TERRACES AND HISTORY OF RIVER LOCATION, HENRYS FORK RIVER, ISLAND PARK, IDAHO

The Henrys Fork River (HFR) of eastern Idaho is one of the famous trout streams of the United States. The upper reaches of HFR flow through Island Park, which is located in the southwest portion of the Yellowstone I caldera and contains the entire Yellowstone II caldera. Rhyolite lava domes and flows formed near the margins of, and within, the Yellowstone I & II calderas. Later, basalt flows, glacial outwash, alluvium, rhyolite lava flows from the Yellowstone III caldera to the east, and loess resurfaced the Island Park area. These events influenced the course of HFR.

In southern Island Park the river forms a deepening canyon that contains Upper and Lower Mesa Falls. This canyon contains four basalt terraces formed when local flows entered the canyon, solidified, and were incised by the river. We refer to these terraces as top, upper middle, lower middle, and bottom terrace. Based on field relations, petrography, composition, and remanent magnetization, we correlate basalt terraces in the canyon to basalt flows in Island Park, as follows: the North Antelope Flat II flow produced upper middle terrace; the Pyroxene Andesite flow at produced lower middle terrace; the Hatchery Butte flow at produced top terrace; and the Pinehaven flow at ka produced bottom terrace.

HFR in Island Park can be divided into five sections based on channel age, from north to south these are: 1) From Island Park Reservoir dam to the end of Box Canyon, and from there south and west to near Coyote Gulch; 2) From section 1 to the southern end of Silver Lake; 3) From section 2 to the Highway 20 entrance to Riverside Campground; 4) From section 3 to Lookout Butte; and 5) From section 4 to Bear Gulch. Through most of its length in Island Park, HFR is confined between lava flows. We use field relations and Ar-Ar dates of basalt flows to infer the history of HFR in Island Park, as follows: section 5 is oldest, followed by 4, 2, and 1; section 3 is youngest.

We use field observations and terrace ages to infer other canyon properties. We observed basalt pillows in top terrace, suggesting that the Hatchery Butte flow dammed the river. The topography of bottom terrace suggest that Upper and Lower Mesa Falls existed—in approximately their current locations—prior to eruption of the Pinehaven flow. We estimate river incision rates of 0.1 – 0.5 mm/yr (100-500 m/My).