SWAN FALLS SECTION

(MILE 28-14)

The Swan Falls section is a rock-walled gorge bounded by steep cliffs 600-700 feet high. The adjoining upland rises even higher, and all the floodwater necessarily had to pass this gap. The narrowest places are at miles 26 (fig. 13, section A-A') and 28, where the canyon is about 2,200 feet wide at the top and 650 feet wide at the bottom. These constrictions are cut into solid basalt - a series of thick lava flows at mile 26 and the core of a massive basalt plug at mile 28. At a flood altitude of 2,625 feet, the constrictions at miles 26 and 28 are, respectively, 1,700 feet and 1,900 feet wide. These constrictions acted as the hydraulic dam that impounded floodwater in the Grand View basin. The scrubbed appearance of this stretch of canyon and the lack of flood debris indicate that these constrictions probably acquired their present size during peak discharge. The floodwater no doubt widened the canyon at the narrowest places by removing lava blocks, but it probably did not appreciably deepen the canyon. Downcutting would be evident as a discontinuity in the profile of Sinker Creek where it joins the canyon at mile 28, but this tributary is at grade with the Snake River, and no significant knickpoint can be discerned in its profile upstream. (Topographic details for this part of the flood path are shown by the U.S. Geol. Survey map of the Oreana quadrangle, Idaho, scale 1: 62,500.)

Melon Gravel does not occur in important amounts in the Swan FaIls section except downstream from a point several miles below the constriction at mile 26. The part upstream, between miles 28 and 26, is swept entirely clean of gravel. The first gravel deposit below the gap is at the west abutment of Swan Falls, 250 feet above the river (mile 231/2). This deposit, which includes 6-foot boulders, is at an altitude of 2,575 feet and was used by Jenkins (p. 12) to establish a tailwater altitude in his calculations of a probable discharge of 15 million cfs through the canyon neck upstream. For the next several miles, gravel occurs as patches hugging the canyon walls and distributed through a vertical range of 275 feet. Large amounts of gravel begin to appear only below mile 19, which marks the head of a large centrally located bar that extends another 4 miles downstream. This bar is built where the canyon begins to widen. No doubt the first several miles below Swan Falls, where Melon Gravel is so meager and patchy, was a turbulent fast stretch of water. As will be seen, it must have also been a place of considerable erosion, as shown by the amount of gravel dumped in the Walters basin immediately downstream.

FIGURE 13. Sections of Snake River canyon at principal constrictions, showing maximum height of Bonneville Flood. Section G-G’ shown in figure 17.